Saturday, October 31, 2015

Global nutrient transport in a world of giants

The past was a world of giants, with abundant whales in the sea and large animals roaming the land. However, that world came to an end followingmassive late-Quaternarymegafauna extinctions on land and widespread population reductions in greatwhale populations over the past few centuries. These losses are likely to have had important consequences for broad-scale nutrient cycling, because recent literature suggests that large animals disproportionately drive nutrient movement. We estimate that the capacity of animals to move nutrients away from concentration patches has decreased to about 8% of the preextinction value on land and about 5% of historic values in oceans. For phosphorus (P), a key nutrient, upward movement in the ocean by marinemammals is about 23%of its former capacity (previously about 340 million kg of P per year). Movements by seabirds and anadromous fish provide important transfer of nutrients from the sea to land, totalling ~150 million kg of P per year globally in the past, a transfer that has declined to less than 4% of this value as a result of the decimation of seabird colonies and anadromous fish populations. We propose that in the past, marine mammals, seabirds, anadromous fish, and terrestrial animals likely formed an interlinked system recycling nutrients from the ocean depths to the continental interiors, with marine mammals moving nutrients from the deep sea to surface waters, seabirds and anadromous fish moving nutrients from the ocean to land, and large animals moving nutrients away from hotspots into the continental interior.

There were giants in the world in those days.
Genesis 6:4, King James version

The past was a world of giants, with abundant whales in the oceans and terrestrial ecosystems teeming with large animals. However, most ecosystems lost their large animals, with around 150 mammal megafaunal (here, defined as ≥44 kg of body mass) species going extinct in the late Pleistocene and early Holocene (1, 2). These extinctions and range declines continued up through historical times and, in many cases, into the present (3). No global extinctions are known for any marine whales, but whale densities might have declined between 66% and 99% (4-6). Some of the largest species have experienced severe declines; for example, in the Southern Hemisphere, blue whales (Balaenoptera  musculus) have been reduced to 1% of their historical numbers as a result of commercial whaling (4). Much effort has been devoted to determining the cause of the extinctions and declines, with less effort focusing on the ecological impacts of the extinctions. Here, we focus on the ecological impacts, with a specific focus on how nutrient dynamics may have changed on land following the late-Quaternary megafauna extinctions, and in the sea and air following historical hunting pressures.

These are excerpts from the original pdf that can be found here


1. Sandom C, Faurby S, Sandel B, Svenning JC (2014) Global late Quaternary megafauna extinctions linked to humans, not climate change. Proc Biol Sci 281(1787):20133254.
2. Barnosky AD, Koch PL, Feranec RS, Wing SL, Shabel AB (2004) Assessing the causes of late Pleistocene extinctions on the continents. Science 306(5693):70–75.
3. Dirzo R, et al. (2014) Defaunation in the Anthropocene. Science 345(6195):401–406.
4. Christensen LB (2006) Marine mammal populations: Reconstructing historical abundances at the global scale. Fisheries Centre Research Reports 14(9):1–161.


Thursday, October 22, 2015

Life on Earth likely started at least 4.1 billion years ago

UCLA geochemists have found evidence that life likely existed on Earth at least 4.1 billion years ago — 300 million years earlier than previous research suggested. The discovery indicates that life may have begun shortly after the planet formed 4.54 billion years ago.

The research is published today in the online early edition of the journal Proceedings of the National Academy of Sciences.

“Twenty years ago, this would have been heretical; finding evidence of life 3.8 billion years ago was shocking,” said Mark Harrison, co-author of the research and a professor of geochemistry at UCLA.

Reed Hutchinson/UCLA.
Mark Harrison at UCLA.

“Life on Earth may have started almost instantaneously,” added Harrison, a member of the National Academy of Sciences. “With the right ingredients, life seems to form very quickly.”

The new research suggests that life existed prior to the massive bombardment of the inner solar system that formed the moon’s large craters 3.9 billion years ago.

“If all life on Earth died during this bombardment, which some scientists have argued, then life must have restarted quickly,” said Patrick Boehnke, a co-author of the research and a graduate student in Harrison’s laboratory.

Scientists had long believed the Earth was dry and desolate during that time period. Harrison’s research — including a 2008 study in Nature he co-authored with Craig Manning, a professor of geology and geochemistry at UCLA, and former UCLA graduate student Michelle Hopkins — is proving otherwise.

“The early Earth certainly wasn’t a hellish, dry, boiling planet; we see absolutely no evidence for that,” Harrison said. “The planet was probably much more like it is today than previously thought.”

The researchers, led by Elizabeth Bell — a postdoctoral scholar in Harrison’s laboratory — studied more than 10,000 zircons originally formed from molten rocks, or magmas, from Western Australia. Zircons are heavy, durable minerals related to the synthetic cubic zirconium used for imitation diamonds. They capture and preserve their immediate environment, meaning they can serve as time capsules.

The scientists identified 656 zircons containing dark specks that could be revealing and closely analyzed 79 of them with Raman spectroscopy, a technique that shows the molecular and chemical structure of ancient microorganisms in three dimensions.

Bell and Boehnke, who have pioneered chemical and mineralogical tests to determine the condition of ancient zircons, were searching for carbon, the key component for life.

One of the 79 zircons contained graphite — pure carbon — in two locations.

“The first time that the graphite ever got exposed in the last 4.1 billion years is when Beth Ann and Patrick made the measurements this year,” Harrison said.

How confident are they that their zircon represents 4.1 billion-year-old graphite?

“Very confident,” Harrison said. “There is no better case of a primary inclusion in a mineral ever documented, and nobody has offered a plausible alternative explanation for graphite of non-biological origin into a zircon.”

The graphite is older than the zircon containing it, the researchers said. They know the zircon is 4.1 billion years old, based on its ratio of uranium to lead; they don’t know how much older the graphite is.

The research suggests life in the universe could be abundant, Harrison said. On Earth, simple life appears to have formed quickly, but it likely took many millions of years for very simple life to evolve the ability to photosynthesize.

The carbon contained in the zircon has a characteristic signature — a specific ratio of carbon-12 to carbon-13 — that indicates the presence of photosynthetic life.

“We need to think differently about the early Earth,” Bell said.

Wendy Mao, an associate professor of geological sciences and photon science at Stanford University, is the other co-author of the research.

The research was funded by the National Science Foundation and a Simons Collaboration on the Origin of Life Postdoctoral Fellowship granted to Bell.

Wednesday, October 21, 2015

The Cave of Homo naledi, or A Textbook Example of How to Do Science

Recently, a magnificent new hominin fossil was announced: Homo naledi, known from a single chamber in an almost-inaccessible cave in South Africa. NCSE’s Stephanie Keep nicely explains the significance of this “weird as hell” fossil in posts here, here, and here.
Why is it so weird? In short, the problem is this: H. naledi had a very small, orange-sized brain, but there is strong evidence that it went to great efforts—extremely difficult movement of corpses perhaps requiring group cooperation and the use of fire as a light source—to place its dead in this almost-inaccessible chamber. We don’t see obvious signs of burial until the relatively big-brained Neanderthals appear on the scene. The idea of something with such a small brain conceiving of and executing the complex behaviors required to repeatedly dispose of their dead in a super-hard-to-get-to cave blows open a lot of our assumptions about cognition. I think the scientific term that best describes this is “cray-cray.”
“How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?” —Sherlock Holmes

One of the set of papers released as part of the announcement of this fossil involved the geology of the cave. Dirk et al.’s “Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa” provides a brilliant description and explanation of the cave environment in which at least fifteen individual Homo naledi found themselves, however that happened. It also explains what happened to the bodies once they were in there (that’s the taphonomy part). But perhaps best of all, the paper provides a fabulous example of how scientists think about a difficult problem. Teachers could base an entire lesson plan on this one paper.

When I heard initial reports about this fossil find, I was shaving and I was so startled by what I heard that I cut myself, exclaiming, “Occam!” Occam’s razor is a good first step in thinking about scientific claims; the elevator-pitch explanation of Occam’s razor is that given two possible explanations, the simpler one is probably the right one. So when the initial news reports came out, I anticipated that very soon someone would find a flaw and blow open the unlikely, improbable, seemingly-impossible claim that Homo naledi had been put its dead in the cave deliberately.

After all, in 2010 when NASA announced the discovery of bacteria using arsenic in its DNA, it seemed to take only days before people identified severe methodological problems with this research. (Turns out the bacteria, like every other known living thing, use phosphorus, not arsenic.) Surely the same thing would happen here with H. naledi. It hasn’t yet.

One of the keys to the strangeness of the H. naledi find is that there seems to be only one entrance to the cave. Surely if there were another, easier entrance that would really change the significance of this find. So as I ruminated on this, I thought:
  1. There’s got to be another entrance, perhaps on the cave ceiling, meaning that Homo naledi could have dropped into this isolated spot; maybe subsequent carbonate growth (flowstone) covered and hid this entrance
  2. If there was another cave entrance, then there should be other critters in the cave deposits, not just H. naledi
  3. If there was an easier cave entrance, then there should also be outside debris—rocks different from the cave rocks, twigs, and leaves from local plants, etc.
  4. If any of those three things were true, I reasoned, then there could be another explanation for how H. naledi got into this spot. So then I read the Dirk et al. paper I was pleased to find that each of these considerations had been addressed—and refuted.

1. Other entrance

My initial assumption that the team had not thoroughly addressed the issue of a second entrance, perhaps on the ceiling, perhaps now covered with carbonate deposits, was discussed in the paper this way:
An exhaustive search by a professional caving team and researchers has failed to find any other plausible access points into the Dinaledi Chamber, and there is no evidence to suggest that an older, now sealed, entrance to the chamber ever existed. Furthermore, detailed surface mapping of the landscape overlying the Rising Star cave system illustrates that no large flowstone-filled fractures occur in the region above the Dinaledi Chamber.
Okay, so strike one for my skepticism. The researchers seem to have anticipated this problem and done due diligence—looking not only inside but outside and above—for signs of another way into this remote cave.

2. Other critters

If there was a second entrance, or if the cave was in some way more accessible in the past, then one would expect to find significant evidence of animals other than H. naledi. All manner of animal—rodents, bats, birds—take up residence in accessible caves. Here’s how this paper addressed this issue:
The lack of other contemporaneous fauna in the assemblage, and complete lack of surface modifications by vertebrates (carnivores, scavengers or rodents) further suggests that the Dinaledi Chamber remained undisturbed by other animals, which could not reach the chamber.
In all, six birds and several rodents were found in cave, but this is hardly the wealth of animals fossils one would expect if this cave were easily open to the outside. Moreover, the lack of disturbance suggests that the birds and rodents were rare; certainly if there was a food source lying on the cave bottom, we should expect heavy disturbance of scavengers. Instead we find many fossils that are well articulated.

The lack of disturbance touches on another possibility: maybe other H. naledi did not move these remains into the cave. Could large predators have dragged them in? Here again the researchers have anticipated the argument:
Nor have we found any trace of carnivore remains or the remains of other likely prey animals. Thus, the predator would have had to select a single prey species—H. naledi—carrying into the chamber all age and size categories (Berger et al., 2015) without leaving a trace of its own presence. We consider this very unlikely.
Strike two for my skepticism.

3. Outside debris
Sediment inside a cave can tell you a lot about what’s going on in the cave—is there regular traffic from outside? Does water flood the cave, perhaps transporting and trapping individuals inside the cave? Most cave deposits have measurable debris from the outside—rocks, branches, leaves, or litter from modern humans. Nothing of the sort was found.

Analysis of the sediment shows it to be fragments of the surrounding rock—and little else. No leaves, no branches, no anomalous pebbles carried in on the toe of some scavenger. It’s as if the cave has been sealed from the outside except for a few enterprising H. naledi. To have a cave so cut off is unusual:
The sedimentary deposits in the Rising Star cave system that host the remains of the new hominin species of Homo naledi (Berger et al., 2015) are anomalous when compared to all other deposits of hominin remains in the Cradle of Humankind in a number of significant ways.
The presence of clay in the sediment suggest periodic flooding, leading to the idea that maybe these fossils were somehow drowned in this cave system and swept into this remote location. But even here the researchers have anticipated this idea:.
..the remains of H. naledi could have accumulated as a result of a catastrophic event during which a large group of animals was trapped in the cave. This could have happened either during a single event when a large number of hominin individuals were in the chamber, or in a death trap scenario over a period of time as individuals repeatedly entered the Dinaledi Chamber and died. Either scenario would have to explain why the animals chose to penetrate this deep into the cave, into the dark zone, moving away from all entrance points into the cave system. The sedimentological evidence presented suggests that accumulation of the fossils occurred over a period of time during deposition and reworking of Units 2, and 3, which refutes a single event hypothesis. Apart from this, and noting that the assemblage recovered to date represents only a small part of the total fossil content in the chamber, the sheer number of remains encountered in the Dinaledi Chamber, is hard to explain as the result of a single calamity.
Strike three. After reading this, I’m impressed and convinced.

The work on H. naledi is amazing science, and this section on geology and taphonomy by Dirk et al. is an excellent example of how scientists think. There are a whole bunch of possible alternate explanations, and they get whittled down one-by-one, until the only thing left is the improbable, counter-intuitive conclusion that H. naledi individuals were deliberately dropped into this cave. Dirk et al. have addressed virtually every other explanation and demonstrated why they don't fit with the evidence.

One thing they found particularly intrigued me. Among all of the fossils, they found old survey pegs left behind in this chamber, and evidence that some of the fossils on the surface had been moved. Apparently, the cavers that discovered the chamber were not the first ones to have stumbled upon it. Yet until quite recently, no one knew this cave existed; whoever left those survey pegs did not recognize the importance of this find and didn’t bother to note it on a map.

So the history of this cave, like the history of the fossils it contains, shows us another important aspect of scientific discovery: sometimes important findings are seen, but not understood. Science is far from finished; latent discoveries abound, waiting for new eyes. We do not find the revolutionaryH. naledi except in this one cave—what else are we missing?


Sunday, October 18, 2015

The Myth of Erk

The story goes that in the beginning, when the Great Force that gave form to the world caused the first animals to arise, one of them, called Erk, after observing the things that were happening around him, turned in anguish to the Great Force and put these questions to her: "Why does everything have to be so hard, so difficult, so painful…? Why must some suffer and die so that others may live? Why must we struggle against one another? Why is it necessary to expend so much effort in order to get what we need to stay alive? Why didn’t you make a simpler, an easier world in which everything would be more accessible, comfortable, and agreeable? What is the sense of so much suffering, so much discomfort, so much death?"

The Great Force knew the answer, but she also knew that Erk would understand it only through direct experience. Therefore, after listening to Erk’s complaints and even though the Great Force knew that she had made no mistake in designing the world, she changed it so that Erk and his companions should discover the sense of things for themselves.

At first the Great Force thought that a few little changes would be enough to make them recognize the truth. So she softened the conditions of life for living things: She moderated the climate so that it would be more kindly to life, made access to food easier for the animals, prevented the deaths of the youngest individuals, reduced the number of accidents, diseases, and catastrophes as well as the suffering the victims had to endure. But Erk and his companions, after a brief period of euphoria, began again to complain that in this new version of the world there was still pain, that death still existed, that it was still a hostile world, a place that was too hard and too difficult for life.

Upon seeing this the Great Force decided to take drastic action in order to open the eyes of Erk and his companions. From that moment, everything was easy, comfortable, simple, and agreeable. No one suffered or died, or had to make any effort to procure the necessities of life. What was needed was obtained instantly and without effort. Nor was it necessary to be alert, since there were no dangers from which to protect oneself and no harm to fear. There were no conflicts, no aggression, and no confrontations of any kind among the animals or between them and their surroundings. The world was at peace. It seemed marvelous.

But after the initial rejoicing, a new and very disagreeable sensation began to arise, a symptom of a great problem where there were no problems, a profound malaise in the midst of that well-being: Boredom. Due to the lack of motivation, of initiative, of goals, of incentives, of challenges, of activities…because of the prevailing indolence in those idyllic conditions the animals were bored. Since they didn’t need to make an effort for anything, worry about anything…they had nothing to do, nothing that was worth the trouble, nothing to motivate them, nothing that would push them to get up out of their lethargy and act. All the same, they were still animals and therefore felt within themselves an imperious need to act. And for this reason it happened that after a little while the animals grew so bored and so nauseated with this state of inactivity that, just in order to be able to do something and find an outlet for their instinctive need of action, they began to develop absurd behaviors that had nothing to do with those they had developed in the beginning before Erk had spoken to the Great Force. Because they had everything they needed, they began to desire other things that they did not need, just in order to be able to act and to exert themselves in getting those things. Thus they began to build, to destroy, to dig, to eat, to copulate, to run, to attack one another…compulsively and frenetically, and as a result many found their capacity for action seriously impaired, they suffered grave injuries, and they profoundly altered their habitat; but they did not stop acting that way, because they simply preferred to suffer all those consequences rather than endure the boredom of having nothing to do; at least these negative effects provided them with stimulation and sensations that kept their bodies and minds in working order and served in turn as a spur to act again under the pretext of palliating the same effects.

Upon seeing all of that, Erk finally understood. He turned to the Great Force again and said to her: "I have come to understand what was the sense of the world just as you created it in the beginning. I have come to understand that it should be that way and not the way that seemed to me more pleasant, because that is really the best way it can be. I have come to understand that when I saw no sense in the world it was because of my own weakness and ignorance, and because I didn't look at it in the right way or thinking properly. I allowed myself to be carried away by a mirage, and I disowned by real nature and the world to which I really belong. Now I know it, now I am stronger and I will never again fall into that error. Thank you, I have learned the lesson, but now please return the world to what is was originally."

Seeing that she had now achieved her objective, the Great Force made everything return to what it had been in the beginning.

So it was that Erk came to understand what was his place in the world and in life, as well as the sense of the world and of life. Since then we wild animals, generation after generation, have kept the memory of those events alive so that, like Erk, we will remember what is our place and what is our function, and so that we will not fall into the same error.

And even so, in spite of everything, many human beings have completely forgotten the story of Erk and live blinded by the same error, trying to create Paradise and immersing themselves and the world ever deeper in Hell.

"El Mito de Erk," Copyright 2004 by E=m.c2.
"The Myth of Erk" (English transl.), Copyright 2005 by Theodore John Kaczynski

Friday, October 16, 2015

125-million-year-old mammal fossil reveals the early evolution of hair and spines

The discovery of a new 125-million-year-old fossil mammal in Spain has pushed back the earliest record of preserved mammalian hair structures and inner organs by more than 60 million years.

The specimen, named Spinolestes xenarthrosus, was fossilized with remarkably intact guard hairs, underfur, tiny hedgehog-like spines and even evidence of a fungal hair infection. The unusually well-preserved fossil also contains an external ear lobe, soft tissues of the liver, lung and diaphragm, and plate-like structures made of keratin known as dermal scutes. The microscopic structures of hair and spines in Spinolestes are the earliest-known examples in mammalian evolutionary history.

Cretaceous mammal Spinolestes(life reconstruction)
in its natural environment of the Las Hoyas wetland.
Credit: Oscar Sanisidro
The findings are described by scientists from the Autonomous University of Madrid, University of Bonn and the University of Chicago in a study published inNature on Oct. 15.

“Spinolestes is a spectacular find. It is stunning to see almost perfectly preserved skin and hair structures fossilized in microscopic detail in such an old fossil,” said study co-author Zhe-Xi Luo, PhD, professor of organismal biology and anatomy at the University of Chicago. “This Cretaceous furball displays the entire structural diversity of modern mammalian skin and hairs.”

The Las Hoyas Quarry in east-central Spain was once a lush wetland with a thriving diversity of life around 125 million years ago during the early Cretaceous period. Spanish paleontologists have studied the site since 1985 and found hundreds of fossils, including important birds and dinosaurs. In 2011, the first mammal fossil at the site was discovered by a team led by Angela D. Buscalioni, PhD, professor of paleontology at the Autonomous University of Madrid, who partnered with collaborators including Luo and Thomas Martin, PhD, professor of paleontology at the University of Bonn, to study the rare specimen.

Cretaceous furball
Spinolestes xenarthrosus lived in the Cretaceous period and belonged to an extinct lineage of early mammals known as triconodonts. The specimen measured roughly 24 cm in length and is estimated to have weighed around 50 to 70 grams, about the size of a modern-day juvenile rat. Its teeth and skeletal features indicate it was a ground-dweller that ate insects. Its soft tissues, with discernable microscopic structures, were preserved through a rare process known as phosphatic fossilization. Individual hair follicles and bulbs, as well as the composition of individual hair shafts, could be identified using an electron scanning microscope.

Spinolestes had remarkably modern mammalian hair and skin structures, such as compound follicles in which multiple hairs emerge from the same pore. It had small spines around a tenth of a millimeter in diameter on its back, similar to modern hedgehogs and African spiny mice, which appeared to be formed by the fusion of filaments at follicles during development. The team even found abnormally truncated hairs that are evidence of a fungal skin infection known as dermatophytosis, which is widely seen among living mammals.

“Hairs and hair-related integumentary structures are fundamental to the livelihood of mammals, and this fossil shows that an ancestral, long-extinct lineage had grown these structures in exactly the same way that modern mammals do,” Luo said. “Spinolestes gives us a spectacular revelation about this central aspect of mammalian biology.”

Spinolestes is also the first example of a Mesozoic mammal in which soft tissues in the thoracic and abdominal cavities are fossilized. The team noted microscopic bronchiole structures of the lung, as well as iron-rich residues associated with the liver. These areas were separated by a curved boundary that is thought to be a muscular diaphragm for respiration. This represents the earliest-known record of mammalian organ systems.

The fossil of Spinolestes contains a large external ear, the earliest-known example in the mammalian fossil record, as well as dermal scutes—plate-like structures made of skin keratin. A more developed form of scutes can be seen in modern armadillos and pangolins.

Skeleton of the Cretaceous mammal Spinolestes with preserved fur shadows.
The outer ear can be seen at the upper edge of the photo (arrow).Credit: Georg Oleschinsk 

Spinolestes had extra articulations between vertebrae, which strengthened its spinal column—modern-day mammals such as armored shrews and armadillos possess similar articulations. The authors speculate that this might provide a clue as to the lifestyle of Spinolestes. Armored shrews, for example, use their exceptional vertebral strength to push apart logs or dead palm leaves to feed on insects within.

“With the complex structural features and variation identified in this fossil, we now have conclusive evidence that many fundamental mammalian characteristics were already well-established some 125 million years, in the age of dinosaurs,” Luo said.


The study, “A Cretaceous eutriconodont and integument evolution in early mammals,” was supported by Spanish MINECO and Junta de Comunidades de Castilla-La Mancha. Additional authors include Jesus Marugan-Lobon and Hugo Martın-Abad of the Autonomous University of Madrid, Romain Vullo of Université de Rennes in France.

Wednesday, October 14, 2015

Introduction to Evolution

An Intro to Evolution
Species evolve. There is no debate. There is no disagreement, at least not among those aware of facts. Yes, the mechanics of evolution are still being worked out. Does it happen slowly over time or in fits and spurts? How does epigenetics factor in? Regardless, the jury is in, ladies and gentlemen. It’s no longer a question of if evolution happens but rather how.

Resources About Evolution

Evolution works on individuals. More specifically, it works on the genome. Every organism on this planet contains massive strings composed of different iterations of the same bases—adenine, thymine, guanine, and cytosine. Together these form our DNA, a self-replicating molecule that codes for proteins, and these proteins string together into life as we know it. Some combinations are better at getting themselves replicated (unconsciously of course) and these go on to make more copies like themselves. As this process builds in complexity it is easy to mistake it as being directed or "for the good of the group." However, it does not work this way. Just as individual fish or birds can come together, each following their own set of rules to form massive schools or flocks that seem to take on complex animations, so too the individual genes within each organism function together as a whole to direct the myriad shapes, behaviors, and ecological interactions we have on this planet.
Darwin may be the most famous person in history to champion this idea of slow change over time but he most certainly was not the first. Lamarck, whose name you might be familiar with as well, believed that any trait an individual acquires during its lifetime will be passed on to its offspring. Giraffes stretched their necks to reach higher trees and therefore their offspring were born with longer necks. This has since been scoffed at for the silly idea that it is, however, advances in our understanding of things such as the immune system and epigenetics may actually give some credit to such ideas. It is an exciting time to be an evolutionary geneticist.
There was another person who, operating independently of Darwin, was developing an idea of evolution that was very similar to what we know today. His name is Alfred Russell Wallace. Unlike Darwin, Wallace was a poor man and had to schmooze his way into the right circles in pursuit of natural history. Working both in South America and Malay Archipelago and facing some devastating setbacks, Wallace converged on the idea of the mutability of species in time (evolution) in an eerily similar fashion as Darwin. As Wallace and Darwin were putting their ideas together, they found out about one another and exchanged notes. Wallace soon realized that Darwin had developed the idea much further than his own, ceded his ideas to Darwin, and by so doing gave up his spotlight in the theory of evolution. How amicable this transfer of ideas really was is a matter debate.

On a more personal level, I study plants, and nowhere is evolution more apparent. One of my favorite professors once said "When it comes to evolution, plants can do anything. Hell, you can pretty much get a new species overnight." He was being a bit facetious of course, but in a sense, he was on to something.

Life on Earth iTunes Course and Ebook

From Open Culture: Created under the direction of Pulitzer Prize-winning author and Harvard naturalist Edward O.Wilson, Life on Earth can be downloaded in 7 units on iTunes. The free book also comes with a free iTunesU course.
Learn More
All of evolution can all be boiled down to one simple question: is it adaptive or not? In evolutionary terms "adaptive" should be taken to mean a trait or suite of traits that incurs an advantage to an organism. In evolution there is no hierarchy and there is certainly no plan, so this is not a value judgment nor is it one of anything resembling choice.
This brings to mind the orchids. Quite possibly the most diverse group of plants on the planet, these marvels of evolution have captured the minds of people throughout history, and much of this fascination has to do with their bizarre floral morphologies. There seems to be no end to the variety of shapes, sizes, and colors, which gives each species its own unique look. The reason lies in the genetics of those flowers. Back when orchids were just starting to diverge, the genes for flower development were duplicated and decoupled from one another. Each new set of genes influences different sepals and petals. Because of this, orchids are able to avoid the pleiotropic effects that most other plants face and thus the sepals and petals can literally follow their own evolutionary trajectory. The result of this wacky evolutionary trajectory is the myriad orchid species we see today.
If it’s wacky evolution that you are interested in, look no further than evolution via sexual selection. In this case, organisms evolve based on the traits that are sexually appealing to the opposite sex. In most cases, sexual evolution is driven by female choice. This can be seen in the plumage of many birds. Take, for instance, the gaudy appearance of a group of birds endemic to a few islands surrounding New Guinea. Known as birds of paradise, the males of these relatives of the crow are decked out in some of the most fanciful and functionless feather adornments in all of the bird world. Colors range from yellows, to reds, to iridescent blues. Forget blending in, these males want to be seen. Their end goal in all of their puffery is to mate with a female. The females choose the males with the most outlandish adornments. Obviously, any male that can evade being eaten with all of that outlandish plumage must be fit enough to father the next generation. The result of this sexual selection are fancier and fancier males.  
You see, the genetics of an organism translate into physical and chemical reactions that, in the face of environmental pressures, either allow it to survive or doom it to death. Take, for instance, the case of rye (Secale cereale). It is easy to look at a cultivated crop of rye and assume we intentionally selected better and better varieties of this grass until we ended up with something so useful. Indeed, this is how it has gone with most plants and animals humans have domesticated. However, the case of rye may be a bit more interesting. Through a mechanism coined Vavilovian mimicry (sometimes referred to as crop mimicry), wild rye managed to escape death in ancient fields and evolve into an edible crop that now enjoys a worldwide distribution.
Wild rye (Secale montanum) was not intentionally grown for food. It was a weed in the fields of other crops like wheat and barley. Both wheat and barley are annual plants, producing their edible seeds at the end of their first growing season. Wild rye, however, is a perennial and does not produce seed until at least its second season. Therefore, most wild rye plants growing in wheat or barley fields are killed at the end of the season when the field gets tilled. However, some mutant rye plants occasionally pop up and produce seeds in their first year. Thus, it is believed that these mutant annual rye were harvested unintentionally and reseeded season after season. Over time, other traits likely developed to help push rye into the spotlight for these early farmers. Like many wild grasses, wild rye has weak spindles (the part that holds the seed to the plant). In the wild, this allows for efficient seed dispersal. On the farm, this is not a desirable trait as you end up quickly losing the seeds you want to harvest. Again, by accidently selecting for mutants that also had thicker spindles and thus held on to their seeds, farmers were unintentionally domesticating rye to parallel other cereal crops. It is believed that oats (Avena sterilis) also originated in this manner. Sure, this is a type of artificial selection, albeit unintentional, but the point remains the same.
At the core of evolution is heritable change. If it survives, there is good chance it will pass its DNA to its offspring and thus, evolution. The idea that an organism is "perfectly tuned" to its environment is a bit off. Evolution is a constant arms race. The world is constantly in flux and there has never been a time when things were static. There is no point in which the Earth could "return to" that wouldn’t be a completely arbitrary point in time. As such, life needs to constantly keep up.

Edward Wilson's "Diversity of Life" Lecture"

E.O. Wilson presents at the Nicholas School of the Environment.
As the saying goes, diversity is the spice of life and that is very true for life on this planet. In general, the more diverse the genome, the more likely a group of individuals of a given species will be able to adapt to changes in their environment. Summers are growing drier throughout much of the globe. Whether or not any species can adapt has to do with the different codes of DNA in their cells. It is likely that at least a few individuals of a species will have the right complement to keep up following a disturbance. That is, unless we wipe most of them out. The reason our current biodiversity crisis is so scary is that we are losing the genetic insurance for the future. We don’t have to kill every last member of a species to ensure its extinction. There is a critical mass in all of this. If the genes aren’t there and a species cannot adapt to change they simply won’t.
While we are on the subject, there seems to be a lot of confusion over where we humans fit into this story. Some believe our technology has stopped our evolution. Certainly our technology has buffered us over the past few centuries. We are no longer victims of the same selection processes that plagued our nomadic ancestors. Instead, we are biding our time until hard selection rears its apocalyptic head. We are changing our environment so rapidly that very soon we won’t be able to keep up. The "civilized" world is living on borrowed time, and we are upright walking apes full of ape-like tendencies. We are brutally tribal. We wage war and drive our natural resources to near or complete extinction.
It would seem that based on ideas such as the Male Warrior Hypothesis, we are simply acting on deep seeded evolutionary anachronisms. It could very well be that our tribal inclinations and war-like tendencies are simply animalistic throwbacks to a time in which our hominid ancestors lived in a much more brutal world than we do. Alas, the major difference is that today we have drones and nukes instead of spears.
For example, modern humans exhibit a wildly overblown form of kin selection. This was useful when our ancestors banded together into small tribes that mostly consisted of family related by both mate selection as well as blood. We are a social primate after all. We are full of specialized neurons called "mirror neurons" that allow us to feel what others are feeling. In other words, they allow us to empathize. In today’s overly connected society, our tribal boundaries can become blurred and our need to empathize with others can extend further than it ever has before. Add in countless forms of superstition and an almost anti-adaptive need for glory and legacy and suddenly you have people jumping in front of bullets to save a stranger.
Our brains only ever evolved to contemplate the short term. Our abstractions of philosophy and compassion are just that, abstractions. In due time, natural selection will catch up and it will most definitely be a hard selection. Evolution has no end game. It has no end. As long as life exists on this planet, biotic and abiotic forces will continue to shape it. Life will continue to adapt. That’s how evolution works. Something will survive. The question remains whether or not we will be around long enough to adapt as well.


Biologist finds “we are on pace to create a mass extinction” of frogs worldwide

When John Alroy, a professor of biology at Australia’s Macquarie University, was asked by a reporter how many of the species populating Earth had already gone extinct, he realized he had no simple answer.

So he checked the scientific literature on the subject, and found that lacking, too.

It’s difficult to determine how many species have gone extinct in the past because of how hard it is to prove that an animal that hasn’t been observed recently is actually gone forever, Alroy writes in a study published this week in the Proceedings of the National Academy of Sciences.

  • John Alroy, a professor of biology at Australia’s Macquarie University, says "a runaway train of extinction is now likely to produce what would be seen as a global mass extinction."
  • A large majority of the 200 extinct frog species were probably lost in the past few decades, just as extinctions and severe population crashes began accelerating in the 1970s and 1980s.
  • The current extinction rate for frogs is four orders of magnitude higher than the long-term background average, Alroy found.

It may be difficult to estimate how many species have truly disappeared from Earth forever, but that doesn’t mean it’s altogether impossible.

Alroy consulted lists of specimens in museum collections and published field surveys to compile data that he then analyzed using a highly conservative approach known as the Bayesian statistical method to infer the number of extinct amphibian and reptile species across the world.

The results are alarming. “It suggests that about 200 frog extinctions have occurred and hundreds more will be lost over the next century, so we are on pace to create a mass extinction,” Alroy wrote in the report, noting that the current extinction rate is four orders of magnitude higher than the long-term background average.

A large majority of those 200 frog species probably went extinct in the past few decades, just as extinctions and severe population crashes began accelerating in the 1970s and 1980s, Alroy told Mongabay in an email.

“However, there may have been quite a few species that went extinct before that point and never were described scientifically,” he added. “This is one reason of many that I think the estimate I’ve given is very conservative. The real toll could be two or three times higher.”

An adult female streamside rainfrog (Craugastor aurilegulus) in Honduras. Photo by Joe Townsend.

If so, Alroy’s findings are troubling indeed, given that frogs are a good indicator of ecosystem health — so much so that they’re often described as a proverbial “canary in a coal mine.”

Frogs are just as vulnerable or more vulnerable to threats like climate change, habitat destruction, pollution, invasive species and introduced pathogens as any other species. “In other words, the important thing about frogs, other than that they are interesting and beautiful and ancient, is that they are a good indicator of what’s probably going on with many other groups that aren’t as well documented,” Alroy said.

Based on the average extinction rate from 1971–2000, Alroy concluded that nearly seven percent of frog species may be lost within the next century (currently we’ve lost a little over three percent). He cautions in the report that while that rate may seem slow on a human time scale, it is a conservative minimum estimate — and human impacts are intensifying pressure on frog populations the world over.

“Thus, the data suggest that a runaway train of extinction is now likely to produce what would be seen as a global mass extinction on the ultimately more important landscape of geological time,” Alroy writes.

These findings would seem to be consistent with a variety of previous research that has concluded human activity is causing species to die off at a rate at least 100 times faster than historical levels and that the Earth has entered a sixth mass extinction period.

Calder found frog extinctions were particularly high in Central America, which was not surprising given the epidemic of a deadly pathogenic fungus called Batrachochytrium dendrobatidis that has been tied to major population declines. Frog populations in Australia have also been hit hard by the lethal fungus.

The geographic pattern of frog extinctions did hold some surprises for Alroy, however. Extinctions occurred in Brazil, Madagascar and New Guinea, places where there have not been well-documented population losses, suggesting there might be mass extinctions happening in the tropics that are going largely unnoticed.

The good news in Alroy’s report, if there is any, is that now that we know more clearly the extent of the problem, we can begin to discuss how to address it. Alroy writes, “Mitigating this crisis will require strong ongoing support of monitoring by field ecologists and museum scientists.”

Alroy, J. (2015). Current extinction rates of reptiles and amphibians. Proceedings of the National Academy of Sciences: 10.1073/pnas.1508681112.

Monday, October 12, 2015

Ecotourism may be doing more harm than good say researchers

Ecotourists may be putting wildlife at risk by changing the behaviour of the creatures they flock to see, researchers have warned.

Animals that become accustomed to large numbers of visitors are likely to lose some of their instinct for self preservation, US experts said.

The “taming” effect is said to run the risk of leaving them more at the mercy of predators.

Lead researcher Dr Daniel Blumstein, from the University of California at Los Angeles, said: “When animals interact in ‘benign’ ways with humans, they may let down their guard.

Tourists snorkelling surrounded by fish in a tributary of the Cuiaba in Brazil. Image by Cell Press/PA Wire

“As animals get used to feeling comfortable with humans nearby, they may become bolder in other situations.

“If this boldness transfers to real predators, then they will suffer higher mortality when they encounter real predators.”

Ecotourism is booming, with protected areas around the world receiving eight billion visitors a year, the team pointed out.

“This massive amount of nature-based ecotourism can be added to the long list of drivers of human-induced rapid environmental change,” Dr Blumstein said.

Writing in the journal Trends in Ecology & Evolution, the researchers compare the effects of ecotourism with that of animal domestication and urbanisation.
Foxes in urban areas less likely to flee from danger. Image by Steve K / CC BY 2.0

In each case, interactions between people and animals could lead to habituation – described as “a kind of taming”.

Evidence from domesticated silver foxes to goldfish had shown that animals living in close proximity to humans become less wary of predators.

Foxes, squirrels and birds living in urban areas were also bolder and less likely to flee from danger.
Researchers are warning that ecotourism might be doing more harm than good. Image by Daryl Wallace / CC BY-SA 2.0 

In some cases, the presence of humans could discourage predators and create safe havens, the researchers added.

With humans around, vervet monkeys were less bothered by leopards, for instance.

But the scientists questioned what might happen to these animals when the visitors leave.

They wrote: “We know that humans are able to drive rapid … change in other species.

“If individuals selectively habituate to humans – particularly tourists – and if invasive tourism practices enhance this habituation, we might be selecting for or creating traits or syndromes that have unintended consequences, such as increased predation risk.

“Even a small human-induced perturbation could affect the behaviour or population biology of a species and influence the species’ function in its community.”

(Press Association)

Sunday, October 11, 2015

Species extinction is a great moral wrong

Nearly three decades ago, conservation biologist Michael Soulé published an article titled "What is Conservation Biology?" Its strong and enduring influence stemmed partly from Soulé's success in articulating an appealing ethical vision for this new field. At its heart was the belief that the human-caused extinction of other species is a great moral wrong.

"The diversity of organisms is good," Soulé wrote, and "the untimely extinction of populations and species is bad." Other species have "value in themselves," he asserted – an "intrinsic value" that should motivate respect and restraint in our dealings with them.

In a recent article published in the journal BioScience titled "What is Conservation Science?" Peter Kareiva and Michelle Marvier attempt to update Soulé's conservation philosophy, but lose sight of this moral commitment.

Update: Dr. Michelle Marvier has responded to this article in the comment section below.

Specifying the ethical principles that they believe should guide conservationists, they give a prominent place to increasing human wealth and "working with corporations," while recognition of the right of other species to continue to flourish is nowhere to be found. In fact, the article's rhetoric serves to normalize extinctions and make readers more comfortable with them. For example, it describes concern for the local extinctions of wolves and grizzly bears in the United States as "nostalgia" for "the world as it once was" and suggests that people need not keep other species on the landscape when their continued presence is incompatible with our economic goals.
Unfortunately this position does not appear to be an aberration in this one article, but rather an essential part of the authors' view that conservationists should accommodate ourselves to the new realities of the Anthropocene Epoch (so named due to the pervasive impact that human activities now have on Earth's ecosystems).
An earlier essay that they published with Robert Lalasz, "Conservation in the Anthropocene," also contemplates mass extinction with equanimity – because such extinctions will not necessarily change whole ecosystems or inconvenience human beings. There, the authors argue that:

  … Ecologists and conservationists have grossly overstated the fragility of nature … In many circumstances, the demise of formerly abundant species can be inconsequential to ecosystem function. The American chestnut, once a dominant tree in eastern North America, has been extinguished by a foreign disease, yet the forest ecosystem is surprisingly unaffected. The passenger pigeon, once so abundant that its flocks darkened the sky, went extinct, along with countless other species from the Steller's sea cow to the dodo, with no catastrophic or even measurable effects.

Presumably these extinction events were indeed catastrophic for the species in question! And also, perhaps, for other species that preyed on or otherwise interacted with them. But such catastrophes do not appear to count morally for the authors; they are not real catastrophes as long as the "ecosystem functions" that benefit humans remain intact. This is shortsighted. There is an extensive body of ecological research showing that even though there is often redundancy in biological communities, as species are lost, ecosystems start to lose functionality and become more prone to collapse. Leaving aside the scientific absurdity that some of the most abundant tree and bird species in North America could disappear with "no measurable effects," there is an ethical blindness here that is even more troubling.

According to recent studies, humanity could extinguish one out of every three species on Earth during the next few centuries if we continue on our current habitat-destroying, resource-hogging path. In one sign of the times, in 2008, the US Fish and Wildlife Service listed the polar bear as threatened with extinction due to the effects of global climate change. Those of us who love wild nature receive such news with lumps in our throats. Yet in response to this threat Kareiva, Marvier and Lalasz had this to say:

 Even that classic symbol of fragility — the polar bear, seemingly stranded on a melting ice block — may have a good chance of surviving global warming if the changing environment continues to increase the populations and northern ranges of harbor seals and harp seals. Polar bears evolved from brown bears 200,000 years ago during a cooling period in Earth's history, developing a highly specialized carnivorous diet focused on seals. Thus, the fate of polar bears depends on two opposing trends — the decline of sea ice and the potential increase of energy-rich prey. The history of life on Earth is of species evolving to take advantage of new environments only to be at risk when the environment changes again.
Such a glib statement ("seemingly stranded on a melting ice block") is both scientifically unjustified and morally obtuse. As Kierán Suckling, Executive Director of the Center for Biological Diversity, correctly points out, "no credible scientist believes that polar bears, who hunt from sea-ice platforms, will rapidly evolve to sustain themselves hunting harbor seals in open water." And equating past extinctions due to natural causes with the possible extinction of the polar bear due to human-caused climate change fails to acknowledge the human responsibility for this threat. Karieva and Marvier suggest that the polar bear's fate depends on "two opposing trends" as "the environment changes," — when it really depends on whether or not humanity substantially reduces our greenhouse gas emissions.

Extinguishing species through the continued expansion of human economic activities appears to be morally acceptable to Kareiva, Marvier and some other Anthropocene proponents, as long as this destruction does not harm people themselves. But this view is selfish and unjust. Human beings already control more than our fair share of Earth's resources. If increased human population and economic demands threaten to extinguish the polar bear and many other species, then we need to limit our population and economic demands, not make excuses that will just lead to greater ecological damage.

Conservation biologists, with our knowledge and appreciation of other species, are the last people who should be making excuses for their displacement or making light of their extinction. It is particularly inappropriate for Peter Kareiva to do so, given his position as chief scientist at the Nature Conservancy, an organization dedicated to preserving biodiversity. TNC's fundraising rests in part on appeals to a strong and widely shared moral view that other species have a right to continued existence. Much of the conservation value of TNC's easements and land purchases depends on society-wide moral and legal commitments to preserve threatened and endangered species and their habitats. Kareiva and Marvier state that they "do not wish to undermine the ethical motivations for conservation action," or presumably, conservation law. Yet their articles do precisely that, with potentially disastrous implications for practical conservation efforts, particularly in the long term.
To be clear: We do not think there is anything wrong with people looking after our own legitimate needs. This is an important aspect of conservation. Kareiva and Marvier are right to remind us that protecting ecosystem services for human beings is important. They are right that concern for our own wellbeing can sometimes motivate significant biodiversity preservation. We believe that people should preserve other species both for their sakes and for ours.

But it is a mistake to reduce conservation solely to concern for our own well-being, or to assume that it is acceptable to extinguish species that do not benefit humans. Such an overly economistic approach to conservation leads us astray morally. It makes us selfish, which is the last thing we want when the very existence of so many other life forms is at stake. Fairly sharing the lands and waters of Earth with other species is primarily a matter of justice, not economic convenience.

Natural species are the primary expressions and repositories of organic nature's order, creativity and diversity. They represent thousands of millions of years of evolution and achievement. They show incredible functional, organizational and behavioral complexity. Every species, like every person, is unique, with its own history and destiny. When humans take so many resources or degrade so much habitat that another species is driven extinct, we have taken or damaged too much and have brought a meaningful story to an untimely end.

At its core, the science of conservation biology affirms that knowledge about the living world should go hand in hand with love and respect for it. Biologist Colin Tudge put it well in his book The Variety of Life:
The prime motive of science is not to control the Universe but to appreciate it more fully. It is a huge privilege to live on Earth and to share it with so many goodly and fantastical creatures.
From this perspective, even one human-caused extinction is one too many. From this perspective, the goodness of the human career on Earth depends as much on how well we appreciate and get along with other species as on how well we do so with other people.
Michael Soulé is right: other species have value in themselves and a right to continued existence. Human beings should preserve them whether or not it is convenient or economically beneficial for people.

The authors adapted this article from an editorial they wrote for Biological Conservation.

What is the Anthropocene Epoch?

According to some scientists, we live at a rare change of geological epochs: the shift from the Holocene (which began about 12,000 years ago when the last ice age ended) to a new epoch tentative named the Anthropocene, because of the immense and increasing human influence on the Earth. Human-dominated ecosystems now cover more of our planet's land surface than do wild ecosystems, while agriculture, construction and mining may move more earth than the natural processes of rock uplift and erosion The key question is whether we should reduce those impacts for our own good and the good of other species, or accept increased human domination of Earth.

The Authors

Philip Cafaro, PhD Richard Primack, PhDDr. Philip Cafaro ( is Professor of Philosophy at Colorado State University, an affiliated faculty member with CSU's School of Global Environmental Sustainability and Book Review Editor of Elsevier's Biological Conservation journal. His main research interests are in environmental ethics, consumption and population issues, and wild lands preservation. He is the author of Thoreau's Living Ethics and Life on the Brink: Environmentalists Confront Overpopulation, among other books.
Dr. Richard B. Primack ( is Professor of Biology at Boston University and Editor-in-Chief of Biological Conservation, an Elsevier journal focusing on the protection of biodiversity.  His research concerning the effects of climate change on the plants and animals of Massachusetts is the focus of a new book coming out in March titled Walden Warming: Climate Change Comes to Thoreau's Woods.


Wednesday, October 7, 2015

The World Heard by Hominins 2 Million Years Ago

Two million years ago, hominins heard the world very differently from modern humans, according to a new study.

Researchers at Binghamton University analyzed the fossilized ear bones of two early hominins from South Africa, A. africanus and P. robustus, which lived 3.3 million and 1.8 million years ago. With computerized tomography scans, they were able to virtually reconstruct the ears and estimate the hominins’ auditory abilities.

       A skull from the species Paranthropus robustus. Some species of early
          hominins could not hear as well as humans at most frequencies.

 Darryl de Ruiter

“They didn’t hear as well as humans, and they are more like chimps,” said Rolf Quam, an anthropologist at Binghamton University and one of the study’s authors.

There was one exception. From one to three kilohertz, the hominins had better hearing than modern humans and chimpanzees.

“They were almost like superhumans in this range,” Dr. Quam said. “They could hear softer sounds than we could.” The findings appear in the journal Science Advances.

Dr. Quam and his colleagues also looked at the auditory abilities of early humans in northern Spain who lived about 400,000 years ago. Their hearing was very similar to that of modern humans, Dr. Quam said.

“We’re on the trail to reconstruct how our hearing changed from a chimplike hearing pattern to a human hearing pattern,” he said.


Crows May Learn Lessons From Death

In recent years, a peculiar sort of public performance has taken place periodically on the sidewalks of Seattle.

It begins with a woman named Kaeli N. Swift sprinkling peanuts and cheese puffs on the ground. Crows swoop in to feed on the snacks. While Ms. Swift observes the birds from a distance, notebook in hand, another person walks up to the birds, wearing a latex mask and a sign that reads “UW CROW STUDY.” In the accomplice’s hands is a taxidermied crow, presented like a tray of hors d’oeuvres.

A new study investigated what crows might understand about death.
Credit Art Wolfe/Science Source

This performance is not surreal street theater, but an experiment designed to explore a deep biological question: What do crows understand about death?

Ms. Swift has been running this experiment as part of her doctoral research at the University of Washington, under the guidance of John M. Marzluff, a biologist. Dr. Marzluff and other experts on crow behavior have long been intrigued by the way the birds seem to congregate noisily around dead comrades. Dr. Marzluff has witnessed these gatherings many times himself, and has heard similar stories from other people.

“Whenever I give a talk about crows, there’s always someone who says, ‘Well, what about this?’ ” he said.

Dr. Marzluff and Ms. Swift decided to bring some scientific rigor to these stories. They wanted to determine whether a dead crow really does prompt a distinctive response from living crows and, if so, what the purpose of the large, noisy gatherings might be.

To run the experiment, Ms. Swift began by delivering food to a particular spot each day so the crows learned to congregate there to eat. Then one of her volunteers would approach the feast with a dead crow, and Ms. Swift observed how the birds reacted.

Almost every time, the crows mobbed the corpse-bearing volunteers. Ms. Swift is eternally grateful to her volunteers that they didn’t abandon the research at that point.

“If you’ve ever been divebombed by a crow, it’s really terrifying,” she said.

If the volunteer carried a dead pigeon, however, the crows mobbed the person only about 40 percent of the time. And if the volunteer stepped forward empty-handed, the crows just moved away until the coast was clear and then returned to the food.

Ms. Swift then ran more tests to see how much of an impression the dead crows made on the live ones. Because crows can tell individual humans apart by their faces, she had her volunteers wear latex masks. Even though she used a rotating crew of volunteers, each group of crows would see the same face throughout the trial. She had them return to the feeding site once a week to see how the crows responded.

“It’s a very Hannibal Lecter thing — it looks like you cut someone’s face off and are wearing it,” said Ms. Swift, who spent a lot of time reassuring Seattle residents that she was actually doing science. “A lot of people would say, ‘I don’t care what you say, I’m calling the police.’ ”

Up to six weeks later many birds still scolded the visitors even when they approached with nothing in their hands. Volunteers wearing unfamiliar masks, on the other hand, were scolded significantly less often.

Ms. Swift found more signs that dead crows left a strong impression on living ones. In the days after seeing a volunteer with a dead crow, birds took significantly longer to approach food. The sight of a dead pigeon had no such effect.

In their report, which appears in the November issue of Animal Behaviour, Ms. Swift and Dr. Marzluff propose that crows pay careful attention to their dead as a way to gather information about threats to their own safety. “It’s a long-term learning opportunity,” Ms. Swift said. “Knowing that you need to be wary in a particular place — that’s valuable.”

The presence of a dead crow could tell other crows that a particular place is dangerous and should be visited with caution. The loud calls the birds make could be a way to share information with the rest of their group.

“Work like this helps to remind us of the cognitive complexity that exists in animals other than humans,” said Teresa Iglesias, an evolutionary biologist affiliated with Australian National University who was not involved in the study.

That’s not to say every animal pays attention to its dead, however. In fact, the club is fairly exclusive, including species such as chimpanzees, elephants, dolphins and relatives of crows known as scrub jays.
“It’s pretty consistently animals that live in social groups and are known for having more advanced cognitive skills,” Ms. Swift said. “It’s amazing to think a crow — a bird — is doing something like this that so few other animals are doing that we know.”