Chapter 6. Evolution of the Brain and Behavior
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By Christie Aschwanden Maybe it’s their famously protruding brow ridge or perhaps it’s the now-discredited notion that they were primitive scavengers too dumb to use language or symbolism, but somehow Neanderthals picked up a reputation as brutish, dim and mannerless cretins. Yet the latest research on the history and habits of Neanderthals suggests that such portrayals of them are entirely undeserved. It turns out that Neanderthals were capable hunters who used tools and probably had some semblance of culture, and the DNA record shows that if you trace your ancestry to Europe or Asia, chances are very good that you have some Neanderthal DNA in your own genome. The bad rap began when the first Neanderthal skull was discovered around 1850 in Germany, says Paola Villa, an archaeologist at the University of Colorado. “The morphological features of these skulls — big eyebrows, no chin — led to the idea that they were very different from us, and therefore inferior,” she says. While the majority of archaeologists no longer believe this, she says, the idea that Neanderthals were inferior, brutish or stupid remains in popular culture. Neanderthals first appeared in Europe and western Asia between 300,000 and 400,000 years ago. They are our closest (extinct) relative, and their species survived until 30,000 to 40,000 years ago, when they vanish from the fossil record, says Svante Paabo, director of the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany, and author of “Neanderthal Man: In Search of Lost Genomes.” Why these relatives of ours thrived for so long and then ended their long, successful run about the same time that modern humans began to spread remains a point of debate and speculation.
Link ID: 20498 - Posted: 01.20.2015
// by Jennifer Viegas Researchers eavesdropping on wild chimpanzees determined that the primates communicate about at least two things: their favorite yummy fruits, and the trees where these fruits can be found. Of particular interest to the chimps is the size of trees bearing the fruits that they relish most, such that the chimps yell out that information, according to a new study published in the journal Animal Behaviour. The study is the first to find that information about tree size and available fruit amounts are included in chimp calls, in addition to assessments about food quality. "Chimpanzees definitely have a very complex communication system that includes a variety of vocalizations, but also facial expressions and gestures," project leader Ammie Kalan of the Max Planck Institute for Evolutionary Anthropology told Discovery News. "How much it resembles human language is still a matter of debate," she added, "but at the very least, research shows that chimpanzees use vocalizations in a sophisticated manner, taking into account their social and environmental surroundings." Kalan and colleagues Roger Mundry and Christophe Boesch spent over 750 hours observing chimps and analyzing their food calls in the Ivory Coast's Taï Forest. The Wild Chimpanzee Foundation in West Africa is working hard to try and protect this population of chimps, which is one of the last wild populations of our primate cousins. © 2015 Discovery Communications, LLC
Helen Fisher, a biological anthropologist at Rutgers University responds: Several years ago I embarked on a project to see if the seven-year itch really exists. I began by studying worldwide data on marriage and divorce and noticed that although the median duration of marriage was seven years, of the couples who divorced, most did so around their fourth year together (the “mode”). I also found that divorce occurred most frequently among couples at the height of their reproductive and parenting years—for men, ages 25 to 29, and for women, ages 20 to 24 and 25 to 29—and among those with one dependent child. To try to explain these findings, I began looking at patterns of pair bonding in birds and mammals. Although only about 3 percent of mammals form a monogamous bond to rear their young, about 90 percent of avian species team up. The reason: the individual that sits on the eggs until they hatch will starve unless fed by a mate. A few mammals are in the same predicament. Take the female fox: the vixen produces very thin milk and must feed her young almost constantly, so she relies on her partner to bring her food while she stays in the den to nurse. But here's the key: although some species of birds and mammals bond for life, more often they stay together only long enough to rear their young through infancy and early toddlerhood. When juvenile robins fly away from the nest or maturing foxes leave the den for the last time, their parents part ways as well. Humans retain traces of this natural reproductive pattern. In more contemporary hunter-gatherer societies, women tend to bear their children about four years apart. Moreover, in these societies after a child is weaned at around age four, the child often joins a playgroup and is cared for by older siblings and relatives. This care structure allows unhappy couples to break up and find a more suitable partner with whom to have more young. © 2015 Scientific American
By Viviane Callier In the deep sea, where light is dim and blue, animals with bigger eyes see better—but bigger eyes are more conspicuous to predators. In response, the small (10 mm to 17 mm), transparent crustacean Paraphronima gracilis has evolved a unique eye structure. Researchers collected the animals from 200- to 500-meter deep waters in California’s Monterey Bay using a remote-operated vehicle. They then characterized the pair of compound eyes, discovering that each one was composed of a single row of 12 distinct red retinas. Reporting online on 15 January in Current Biology, the researchers hypothesize that each retina captures an image that is transmitted to the crustacean’s brain, which integrates the 12 images to increase brightness and contrast sensitivity, adapting to changing light levels. Future work will focus on how images are processed by the neural connections between the retinas and the brain. © 2015 American Association for the Advancement of Science.
By Michael Balter If there’s one thing that distinguishes humans from other animals, it’s our ability to use language. But when and why did this trait evolve? A new study concludes that the art of conversation may have arisen early in human evolution, because it made it easier for our ancestors to teach each other how to make stone tools—a skill that was crucial for the spectacular success of our lineage. Researchers have long debated when humans starting talking to each other. Estimates range wildly, from as late as 50,000 years ago to as early as the beginning of the human genus more than 2 million years ago. But words leave no traces in the archaeological record. So researchers have used proxy indicators for symbolic abilities, such as early art or sophisticated toolmaking skills. Yet these indirect approaches have failed to resolve arguments about language origins. Now, a team led by Thomas Morgan, a psychologist at the University of California, Berkeley, has attacked the problem in a very different way. Rather than considering toolmaking as a proxy for language ability, he and his colleagues explored the way that language may helps modern humans learn to make such tools. The researchers recruited 184 students from the University of St. Andrews in the United Kingdom, where some members of the team were based, and organized them into five groups. The first person in each group was taught by archaeologists how to make artifacts called Oldowan tools, which include fairly simple stone flakes that were manufactured by early humans beginning about 2.5 million years ago. This technology, named after the famous Olduvai Gorge in Tanzania where archaeologists Louis and Mary Leakey discovered the implements in the 1930s, consists of hitting a stone “core” with a stone “hammer” in such a way that a flake sharp enough to butcher an animal is struck off. Producing a useful flake requires hitting the core at just the right place and angle. © 2015 American Association for the Advancement of Science.
by Clare Wilson Could a lopsided gap help set us apart from our primate cousins? Our brains and chimps' are built differently in the areas that give us our social skills and language. The human brain has a 4.5-centimetre-long groove running deeper along the right side than the left. Chimp brains lack this asymmetry, as François Leroy of the French National Institute of Health and Medical Research in Saclay, and colleagues, have discovered. The groove's function is unknown, but its location suggests it played a role in the evolution of our communication abilities. "One day this will help us understand what makes us tick," says Colin Renfrew of the University of Cambridge, who was not involved in the study. Although our brain is about three times the size of a chimp's, anatomical features that only the human brain possesses are surprisingly hard to find. One known difference is in a region called Broca's area, which is also involved in speech and is larger in humans than chimps. The asymmetrical groove in humans was also known, but the new study, in which 177 people and 73 chimps had brain scans, revealed it is almost completely absent in the other primates. In humans, the deeper groove in the right brain lies in the region that controls voice and face recognition and working out what other people are thinking – our so-called theory of mind. The shallower groove on the left is at the heart of the areas associated with language. The lack of symmetry could signify that tissue layers in the right brain have been reorganised, says Leroy. © Copyright Reed Business Information Ltd.
By Virginia Morell Animals that live in larger societies tend to have larger brains. But why? Is it because a larger group size requires members to divide up the labor on tasks, thus causing some individuals to develop specialized brains and neural anatomy? (Compared with most humans, for instance, taxicab drivers have brains that have larger areas that are involved with spatial memory.) Or is it because the challenges of group living—needing to know all the foibles of your neighbors—cause the brains of all members to grow larger? Scientists tested the two hypotheses with wild colonies of acacia ants (Pseudomyrmex spinicola), which make their nests in the hollow spines of acacia trees in Panama. Ant workers at the base of the tree wait to attack intruders, while workers foraging on the leaves (as in the photo above), aren’t as aggressive but are faster at managing the colony’s brood. This division of labor is most marked in larger colonies (those found on larger trees), while workers in smaller colonies do both jobs. The scientists studied 17 colonies of ants and measured the brain volumes of 29 of the leaf ants and 34 of the trunk ants. As the colony size increased, the leaf ants showed a marked increase in the regions of the brain concerned with learning and memory, the scientists report today in the Proceedings of the Royal Society B. But the same neural areas decreased in the trunk ants. Thus, larger societies’ need for specialized workers, some strictly for defense, others for foraging and brood tending—rather than for social masters—seems to be the key to the expanding brain, at least in ants.
Link ID: 20457 - Posted: 01.08.2015
|By Joshua A. Krisch There is a mystery on Tiwai Island. A large wildlife sanctuary in Sierra Leone, the island is home to pygmy hippopotamuses, hundreds of bird species and several species of primates, including Campbell’s monkeys. These monkeys communicate via an advanced language that primatologists and linguists have been studying for decades. Over time, experts nearly cracked the code behind monkey vocabulary. And then came krak. In the Ivory Coast’s Tai Forest Campbell’s monkeys (Cercopithecus campbelli) use the term krak to indicate that a leopard is nearby and the term hok to warn of an eagle circling overheard. Primatologists indexed their monkey lexicon accordingly. But on Tiwai Island they found that those same monkeys used krak as a general alarm call—one that, occasionally, even referred to eagles. “Why on Earth were they producing krak when they heard an eagle,” asks co-author Philippe Schlenker, a linguist at France’s National Center for Scientific Research and professor at New York University. “For some reason krak, which is a leopard in the Tai Forest, seems to be recycled as a general alarm call on Tiwai Island.” In a paper published in the November 28 Linguistics and Philosophy Schlenker and his team applied logic and human linguistics to crack the krak code. Their findings imply that some monkey dialects can be just as sophisticated as human language. In 2009 a team of scientists travelled to Tai Forest with one mission—to terrify Campbell’s monkeys. Prior studies had collected monkey calls and then parsed vague meanings based on events that were already happening on the forest floor. But these primatologists set up realistic model leopards and played recordings of eagle screeches over loudspeakers. Their field experiments resulted in some of the best data available about how monkeys verbally respond to predators. © 2014 Scientific American
Jason G Goldman We humans don’t typically agree on all that much, but there is at least one thing that an impressive amount of us accept: which hand is easiest to control. If you use one hand for writing, you probably use the same one for eating as well, and most of us – around 85% of our species – prefer our right hands. In fact, "there has never been any report of a human population in which left-handed individuals predominate", according to archaeologist Natalie Uomini at the University of Liverpool in the UK. Lateralisation of limb use – that is, a bias towards one side or the other – usually begins in the brain. We know that some tasks are largely controlled by brain activity in the left hemisphere, while the right hemisphere governs other tasks. Confusingly, there is some crossing of nerves between the body and the brain, which means it’s actually the left side of the brain that has more control over the right side of the body and vice versa. In other words, the brain’s left hemisphere helps control the operation of the right hand, eye, leg and so on. Some argue that this division of neurological labour has been a feature of animals for half a billion years. Perhaps it evolved because it is more efficient to allow the two hemispheres to carry out different computations at the same time. The left side of the brain, for instance, might have evolved to carry out routine operations – things like foraging for food – while the right side was kept free to detect and react rapidly to unexpected challenges in the environment – an approaching predator, for instance. This can be seen in various fish, toads and birds, which are all more likely to attack prey seen in the right eye. © 2014 BBC.
by Colin Barras It's not just great minds that think alike. Dozens of the genes involved in the vocal learning that underpins human speech are also active in some songbirds. And knowing this suggests that birds could become a standard model for investigating the genetics of speech production – and speech disorders. Complex language is a uniquely human trait, but vocal learning – the ability to pick up new sounds by imitating others – is not. Some mammals, including whales, dolphins and elephants, share our ability to learn new vocalisations. So do three groups of birds: the songbirds, parrots and hummingbirds. The similarities between vocal learning in humans and birds are not just superficial. We know, for instance, that songbirds have specialised vocal learning brain circuits that are similar to those that mediate human speech. What's more, a decade ago we learned that FOXP2, a gene known to be involved in human language, is also active in "area X" of the songbird brain – one of the brain regions involved in those specialised vocal learning circuits. Andreas Pfenning at the Massachusetts Institute of Technology and his colleagues have now built on these discoveries. They compared maps of genetic activity – transcriptomes – in brain tissue taken from the zebra finch, budgerigar and Anna's hummingbird, representing the three groups of vocal-learning birds. © Copyright Reed Business Information Ltd.
Carl Zimmer For thousands of years, fishermen knew that certain fish could deliver a painful shock, even though they had no idea how it happened. Only in the late 1700s did naturalists contemplate a bizarre possibility: These fish might release jolts of electricity — the same mysterious substance as in lightning. That possibility led an Italian physicist named Alessandro Volta in 1800 to build an artificial electric fish. He observed that electric stingrays had dense stacks of muscles, and he wondered if they allowed the animals to store electric charges. To mimic the muscles, he built a stack of metal disks, alternating between copper and zinc. Volta found that his model could store a huge amount of electricity, which he could unleash as shocks and sparks. Today, much of society runs on updated versions of Volta’s artificial electric fish. We call them batteries. Now a new study suggests that electric fish have anticipated other kinds of technology. The research, by Kenneth C. Catania, a biologist at Vanderbilt University, reveals a remarkable sophistication in the way electric eels deploy their shocks. Dr. Catania, who published the study on Thursday in the journal Science, found that the eels use short shocks like a remote control on their victims, flushing their prey out of hiding. And then they can deliver longer shocks that paralyze their prey at a distance, in precisely the same way that a Taser stops a person cold. “It shows how finely adapted eels are to attack prey,” said Harold H. Zakon, a biologist at the University of Texas at Austin, who was not involved in the study. He considered Dr. Catania’s findings especially impressive since scientists have studied electric eels for more than 200 years. © 2014 The New York Times Company
Link ID: 20400 - Posted: 12.06.2014
Ewen Callaway A shell found on Java in the late 1800s was recently found to bear markings that seem to have been carved intentionally half a million years ago. The photograph is about 15 millimetres wide. Expand A zigzag engraving on a shell from Indonesia is the oldest abstract marking ever found. But what is most surprising about the half-a-million-year-old doodle is its likely creator — the human ancestor Homo erectus. "This is a truly spectacular find and has the potential to overturn the way we look at early Homo," says Nick Barton, an archaeologist at the University of Oxford, UK, who was not involved in the discovery, which is described in a paper published online in Nature on 3 December1. By 40,000 years ago, and probably much earlier, anatomically modern humans — Homo sapiens — were painting on cave walls in places as far apart as Europe2 and Indonesia3. Simpler ochre engravings found in South Africa date to 100,000 years ago4. Earlier this year, researchers reported a 'hashtag' engraving in a Gibraltar cave once inhabited by Neanderthals5. That was the first evidence for drawing in any extinct species. But until the discovery of the shell engraving, nothing approximating art has been ascribed to Homo erectus. The species emerged in Africa about 2 million years ago and trekked as far as the Indonesian island of Java, before going extinct around 140,000 years ago. Most palaeoanthropologists consider the species to be the direct ancestor of both humans and Neanderthals. © 2014 Nature Publishing Group
Link ID: 20390 - Posted: 12.04.2014
Katie Langin In the first couple of years after birth, sea lion sons seem to be more reliant on their mothers—consuming more milk and sticking closer to home—than sea lion daughters are, according to a study on Galápagos sea lions published in the December issue of the journal Animal Behaviour. The young males venture out to sea on occasion, but their female counterparts dive for their own food much more often. The curious thing is, it's not like the young males aren't capable of diving. As one-year-olds, males can dive to the same depth as females (33 feet, or 10 meters, on a typical dive). It's also not like their mother's milk is always on hand. Sea lion moms frequently leave their growing offspring for days at a time to find food at sea. (Watch a video of a Galápagos sea lion giving birth.) And yet, despite all this, for some reason sons are far less likely than daughters to take to the sea and seek out their own food. "We always saw the [young] males around the colony surfing in tide pools, pulling the tails of marine iguanas, resting, sleeping," said Paolo Piedrahita, a Ph.D. student at Bielefeld University in Germany and the lead author of the study. "It's amazing. You can see an animal—40 kilograms [88 pounds]—just resting, waiting for mom." © 1996-2014 National Geographic Society.
By Sarah C. P. Williams Craving a stiff drink after the holiday weekend? Your desire to consume alcohol, as well as your body’s ability to break down the ethanol that makes you tipsy, dates back about 10 million years, researchers have discovered. The new finding not only helps shed light on the behavior of our primate ancestors, but also might explain why alcoholism—or even the craving for a single drink—exists in the first place. “The fact that they could put together all this evolutionary history was really fascinating,” says Brenda Benefit, an anthropologist at New Mexico State University, Las Cruces, who was not involved in the study. Scientists knew that the human ability to metabolize ethanol—allowing people to consume moderate amounts of alcohol without getting sick—relies on a set of proteins including the alcohol dehydrogenase enzyme ADH4. Although all primates have ADH4, which performs the crucial first step in breaking down ethanol, not all can metabolize alcohol; lemurs and baboons, for instance, have a version of ADH4 that’s less effective than the human one. Researchers didn’t know how long ago people evolved the more active form of the enzyme. Some scientists suspected it didn’t arise until humans started fermenting foods about 9000 years ago. Matthew Carrigan, a biologist at Santa Fe College in Gainesville, Florida, and colleagues sequenced ADH4 proteins from 19 modern primates and then worked backward to determine the sequence of the protein at different points in primate history. Then they created copies of the ancient proteins coded for by the different gene versions to test how efficiently each metabolized ethanol. They showed that the most ancient forms of ADH4—found in primates as far back as 50 million years ago—only broke down small amounts of ethanol very slowly. But about 10 million years ago, the team reports online today in the Proceedings of the National Academy of Sciences, a common ancestor of humans, chimpanzees, and gorillas evolved a version of the protein that was 40 times more efficient at ethanol metabolism. © 2014 American Association for the Advancement of Science.
By John Edward Terrell We will certainly hear it said many times between now and the 2016 elections that the country’s two main political parties have “fundamental philosophical differences.” But what exactly does that mean? At least part of the schism between Republicans and Democrats is based in differing conceptions of the role of the individual. We find these differences expressed in the frequent heated arguments about crucial issues like health care and immigration. In a broad sense, Democrats, particularly the more liberal among them, are more likely to embrace the communal nature of individual lives and to strive for policies that emphasize that understanding. Republicans, especially libertarians and Tea Party members on the ideological fringe, however, often trace their ideas about freedom and liberty back to Enlightenment thinkers of the 17th and 18th centuries, who argued that the individual is the true measure of human value, and each of us is naturally entitled to act in our own best interests free of interference by others. Self-described libertarians generally also pride themselves on their high valuation of logic and reasoning over emotion. The basic unit of human social life is not and never has been the selfish and self-serving individual. Philosophers from Aristotle to Hegel have emphasized that human beings are essentially social creatures, that the idea of an isolated individual is a misleading abstraction. So it is not just ironic but instructive that modern evolutionary research, anthropology, cognitive psychology and neuroscience have come down on the side of the philosophers who have argued that the basic unit of human social life is not and never has been the selfish, self-serving individual. Contrary to libertarian and Tea Party rhetoric, evolution has made us a powerfully social species, so much so that the essential precondition of human survival is and always has been the individual plus his or her relationships with others. © 2014 The New York Times Company
Link ID: 20371 - Posted: 12.01.2014
|By Jason G. Goldman A sharp cry pierces the air. Soon a worried mother deer approaches the source of the sound, expecting to find her fawn. But the sound is coming from a speaker system, and the call isn't that of a baby deer at all. It's an infant fur seal's. Because deer and seals do not live in the same habitats, mother deer should not know how baby seal screams sound, reasoned biologists Susan Lingle of the University of Winnipeg and Tobias Riede of Midwestern University, who were running the acoustic experiment. So why did a mother deer react with concern? Over two summers, the researchers treated herds of mule deer and white-tailed deer on a Canadian farm to modified recording of the cries of a wide variety of infant mammals—elands, marmots, bats, fur seals, sea lions, domestic cats, dogs and humans. By observing how mother deer responded, Lingle and Riede discovered that as long as the fundamental frequency was similar to that of their own infants' calls, those mothers approached the speaker as if they were looking for their offspring. Such a reaction suggests deep commonalities among the cries of most young mammals. (The mother deer did not show concern for white noise, birdcalls or coyote barks.) Lingle and Riede published their findings in October in the American Naturalist. Researchers had previously proposed that sounds made by different animals during similar experiences—when they were in pain, for example—would share acoustic traits. “As humans, we often ‘feel’ for the cry of young animals,” Lingle says. That empathy may arise because emotions are expressed in vocally similar ways among mammals. © 2014 Scientific American
By Virginia Morell When we listen to someone talking, we hear some sounds that combine to make words and other sounds that convey such things as the speaker’s emotions and gender. The left hemisphere of our brain manages the first task, while the right hemisphere specializes in the second. Dogs also have this kind of hemispheric bias when listening to the sounds of other dogs. But do they have it with human sounds? To find out, two scientists had dogs sit facing two speakers. The researchers then played a recorded short sentence—“Come on, then”—and watched which way the dogs turned. When the animals heard recordings in which individual words were strongly emphasized, they turned to the right—indicating that their left hemispheres were engaged. But when they listened to recordings that had exaggerated intonations, they turned to the left—a sign that the right hemisphere was responding. Thus, dogs seem to process the elements of speech very similarly to the way humans do, the scientists report online today in Current Biology. According to the researchers, the findings support the idea that our canine pals are indeed paying close attention not only to who we are and how we say things, but also to what we say. © 2014 American Association for the Advancement of Science.
Christopher Stringer Indeed, skeletal evidence from every inhabited continent suggests that our brains have become smaller in the past 10,000 to 20,000 years. How can we account for this seemingly scary statistic? Some of the shrinkage is very likely related to the decline in humans' average body size during the past 10,000 years. Brain size is scaled to body size because a larger body requires a larger nervous system to service it. As bodies became smaller, so did brains. A smaller body also suggests a smaller pelvic size in females, so selection would have favored the delivery of smaller-headed babies. What explains our shrinking body size, though? This decline is possibly related to warmer conditions on the earth in the 10,000 years after the last ice age ended. Colder conditions favor bulkier bodies because they conserve heat better. As we have acclimated to warmer temperatures, the way we live has also generally become less physically demanding, which overall serves to drive down body weights. Another likely reason for this decline is that brains are energetically expensive and will not be maintained at larger sizes unless it is necessary. The fact that we increasingly store and process information externally—in books, computers and online—means that many of us can probably get by with smaller brains. Some anthropologists have also proposed that larger brains may be less efficient at certain tasks, such as rapid computation, because of longer connection pathways. © 2014 Scientific American
Link ID: 20345 - Posted: 11.24.2014
James Gorman Evidence has been mounting for a while that birds and other animals can count, particularly when the things being counted are items of food. But most of the research is done under controlled conditions. In a recent experiment with New Zealand robins, Alexis Garland and Jason Low at Victoria University of Wellington tested the birds in a natural setting, giving them no training and no rewards, and showed that they knew perfectly well when a scientist had showed them two mealworms in a box, but then delivered only one. The researchers reported the work this fall in the journal Behavioural Processes. The experiment is intriguing to watch, partly because it looks like a child’s magic trick. The apparatus used is a wooden box that has a sliding drawer. After clearly showing a robin that she was dropping two mealworms in a circular well in the box, Dr. Garland would slide in the drawer. It covered the two worms with an identical-looking circular well containing only one worm. When the researcher moved away and the robin flew down and lifted off a cover, it would find only one worm. The robins pecked intensely at the box, behavior they didn’t show if they found the two worms they were expecting. Earlier experiments had also shown the birds to be good at counting, and Dr. Garland said that one reason might be that they are inveterate thieves. Mates, in particular, steal from one another’s food caches, where they hide perishable prey like worms or insects. “If you’ve got a mate that steals 50 or more percent of your food,” she said, you’d better learn how to keep track of how many mealworms you’ve got. © 2014 The New York Times Company
Carl Zimmer In the early 1970s, Sarah Blaffer Hrdy, then a graduate student at Harvard, traveled to India to study Hanuman langurs, monkeys that live in troops, each made up of several females and a male. From time to time, Dr. Hrdy observed a male invade a troop, driving off the patriarch. And sometimes the new male performed a particularly disturbing act of violence. He attacked the troop’s infants. There had been earlier reports of infanticide by adult male mammals, but scientists mostly dismissed the behavior as an unimportant pathology. But in 1974, Dr. Hrdy made a provocative counter proposal: infanticide, she said, is the product of mammalian evolution. By killing off babies of other fathers, a male improves his chances of having more of his own offspring. Dr. Hrdy went on to become a professor at the University of California, Davis, and over the years she broadened her analysis, arguing that infanticide might well be a common feature of mammalian life. She spurred generations of scientists to document the behavior in hundreds of species. “She’s the goddess of all this stuff,” said Kit Opie, a primatologist at University College London. Forty years after Dr. Hrdy’s initial proposal, two evolutionary biologists at the University of Cambridge have surveyed the evolution of infanticide across all mammals. In a paper published Thursday in Science, the scientists concluded that only certain conditions favor the evolution of infanticide — the conditions that Dr. Hrdy had originally proposed. “My main comment is, ‘Well done,'” said Dr. Hrdy. She said the study was particularly noteworthy for its scope, ranging from opossum to lions. The authors of the new study, Dieter Lukas and Elise Huchard, started by plowing through the scientific literature, looking for evidence of infanticide in a variety of mammalian species. The researchers ended up with data on 260 species, and in 119 of them — over 45 percent — males had been observed killing unrelated young animals. © 2014 The New York Times Company