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Primate Brains Have Evolved To Rapidly Detect Snakes

Our brains process countless visual signals every second, but some triggers spark instant recognition. New research shows that when it comes to spotting snakes, it's their scales that set off ancient alarm bells in primate brains – including our own.

Dr. Nobuyuki Kawai, a researcher at Nagoya University in Japan, has discovered that monkeys respond to snake scales as a key visual warning sign, suggesting this trait is deeply embedded in primate evolution.

Fear of snakes in primates

Snakes have posed a serious danger to primates since the earliest stages of our evolution.

For our ancestors, the ability to quickly detect and react to snakes was crucial for survival, as many species of snakes are venomous and can be deadly. What makes this fear particularly fascinating is that it is not learned through experience.

Both monkeys and human babies, even those who have never encountered a snake in real life, still show strong reactions to images of snakes.

This suggests that the fear of snakes is an innate survival mechanism, hardwired into the brain through millions of years of evolutionary pressure. This instinct ensures that primates, including humans, remain vigilant and can avoid potentially life-threatening encounters.

Monkeys reacting to snake images

To uncover the specific triggers in primates behind this instinctive fear of snakes, Dr. Nobuyuki Kawai's team from Nagoya University designed a straightforward test.

The researchers presented monkeys with two sets of images: one showing snakes and the other depicting salamanders.

Although both creatures share similar elongated, slithering bodies, the monkeys reacted immediately to the snake images but showed no such response to the salamanders. This highlighted that the reaction wasn't simply about the body shape.

This observation raised an intriguing question: what if a harmless salamander were altered to display the distinctive scales of a snake? Would the monkeys recognize it as non-threatening, or would the scaly appearance alone activate their fear response?

Kawai's curiosity set the stage for a deeper investigation into the visual cues that spark this innate fear of snakes in primates.

Triggered by snake skin

Kawai designed a clever experiment to find out. He set up a game where monkeys had to spot the odd image out among nine pictures to get a reward.

When he placed a single snake image among salamander pictures, the monkeys found it super quickly. But here's where it gets interesting – when he showed them doctored images of salamanders wearing snake skin, they spotted these just as fast, sometimes even faster than real snake images.

"Previously we demonstrated that humans and primates can recognize snakes quickly; however, the critical visual feature was unknown," Kawai said.

"The monkeys did not react faster to salamanders, a species that shares a similar elongated body and tail with snakes, until the images were changed to cover them with snake skin."

Primates' snake detection abilities

The discovery points to something remarkable about primate evolution.

"This may be because during evolution our primate ancestors evolved a visual system to identify scales, which are characteristic of snakes," Kawai explained.

"These insights into primate evolution will likely improve our understanding of vision and brain evolution in animals, including ourselves."

Primate brains evolved to spot danger

The study, published in Scientific Reports, extends beyond simply explaining why primates are quick to spot snakes.

It shows how our visual processing system developed specific tricks to keep us safe. Our brains didn't just evolve to see better – they evolved to see danger better.

This research opens up new questions about other visual cues that might be hardwired into our brains. What other ancient warning signs might we be unconsciously processing? How else has evolution shaped the way we see the world?

The next time you jump at the sight of something snake-like, remember – you're reacting like all primates have for generations. This instinct is the result of millions of years of evolution, designed to shield you from one of nature's oldest threats. And it all comes down to those scales.

The full study is recently published in the journal Scientific Reports.

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Why Are Women The Only Primates With Enlarged Breasts?

We love exploring the traits that set humans apart from other primates and animals. For example, our bipedal posture, hairless bodies, higher encephalization, rich cumulative culture, and language. Yet, we often overlook one of the most conspicuous differences: the development of prominent breasts in women at puberty, something that does not happen in other primates. The intriguing question is — why?

Desmond Morris tackled this question in his 1967 seminal book, The Naked Ape. His ideas became fixed in the collective imagination like a scab that was difficult to peel off, as they were innovative, striking, intuitive and easy to understand. But they were also pure speculation, and subsequent research has chipped away at their foundations.

Morris proposed that female breasts evolved alongside bipedalism, as they resembled buttocks. In quadruped primates, sexual signals are most visible on the backside. In contrast, in the upright position of humans, most interactions are face-to-face, and the buttocks are not as visible, so why not bring them to the chest? If men were attracted to buttocks, Morris argued, it was better to have two sets: one in front, and one behind.

Other researchers later suggested that breasts could be attractive as they serve as indicators of sexual maturity. Whether due to Morris's hypothesis or this one, the idea of breasts evolving through sexual selection has been the most widely discussed hypothesis. Consequently, much research has focused on male perspectives regarding breast attractiveness. And the results are very varied.

Some studies suggest men prefer medium and large breasts — though not excessively large, as they may lack firmness. Others claim men favor smaller breasts. Men like some breasts more than others, depending on cultural factors. Poor men prefer larger breasts than rich men. Men do not care about the size of the breasts. Men in Mali believe that being sexually attracted to breasts is perverted. The sheer variability of findings could fill an entire article.

In 2021, Bogusław Pawłowski and Agnieszka Zelazniewicz from the University of Wroclaw in Poland published a literature review criticizing the theory that breasts evolved through sexual selection. Their review argued that it's unlikely enlarged breasts were initially deemed attractive, as in primates they are typically more pronounced during periods of lower fertility: lactation and pregnancy.

A woman breastfeeds her baby during a breastfeeding protest in London, England.Peter Summers (Getty Images)

The researchers also find it strange that such an arbitrary trait evolved simply to attract men. In nature, male selection is often based on characteristics that represent a direct benefit to the individual, such as willingness to mate, health or reproductive potential. In contrast, there is evidence that breast size does not correlate with any of these characteristics. In fact, women begin to develop breasts before they are even fertile.

Pawłowski and Zelazniewicz acknowledge the obvious role of breasts in human sexuality, but believe this was not their original evolutionary purpose. Often in evolution, traits that arise for one function are co-opted for another, a process known as exaptation. They suggest that the female breast may have initially been a byproduct of increased subcutaneous fat in humans.

Most studies on the development of primate breasts have been carried out on macaques, because of their usefulness as models for studying breast cancer. In these animals, the mammary glands also develop during puberty, but their volume barely increases. Therefore, women's breasts are different not because they develop earlier, but because they accumulate a greater amount of fat. In fact, while there is considerable variability, a positive correlation exists between breast size and body fat index.

Molecular and archaeological data suggest that, approximately 2 million years ago, Homo ergaster began to increase its level of subcutaneous fat. This helped newborns sustain energy for enhanced brain development and allowed adults to better adapt to colder climates. In women, the surge in estrogen during puberty further increased their fat levels, preparing them for pregnancy.

But why does fat accumulate more in breasts than elsewhere on the body? Fat distribution is related to the number of estrogen receptors. According to Pawłowski and Zelazniewicz, female chimpanzees, for instance, have higher concentrations of these receptors around the genitals, and in the uterus and breasts. It is therefore likely that this receptor distribution — already present in our common ancestor — predisposed humans to increased breast development.

But why is there a higher level of fat in the breasts than in other parts of the body? Fat distribution is related to the number of estrogen receptors. According to Pawłowski and Zelazniewicz, female chimpanzees have a higher density of these receptors around the genitals, in the uterus and on the breasts. It is therefore likely that our common ancestor already exhibited this distribution, which facilitated a greater increase in mammary size.

The accumulation of fat in the breasts and hips became a distinctive feature between men and women, which, according to Pawłowski and Zelazniewicz, may have subsequently driven their attractiveness. Sexual selection, they argue, would then have acted on breast traits, explaining why some women with low body fat still have large breasts, and vice versa. Indeed, genetic studies have even identified two loci (location of a gene on the chromosome) associated with breast size, independent of overall subcutaneous fat, though they account for only about 1% of size variation.

While more research is needed to validate Pawłowski and Zelazniewicz's hypothesis, their explanation is grounded in evidence and represents progress over speculative theories made in the last century. Still, it is striking that discussions about the evolutionary origin of breasts barely mention their critical role in women's sexual experiences.

The function of enlarged breasts extends beyond attraction, as proper stimulation during sex is pleasurable for most women. One study found that 80% experience arousal from breast stimulation, and some can achieve orgasm through this alone due to the neural connection between the nipples and uterus. There are also quite a few studies on how to regain nipple sensitivity after a mastectomy, given its importance in female sexuality.

Additionally, nipple stimulation — whether from a sexual partner or breastfeeding — triggers the release of oxytocin. Psychiatrist Larry Young posits that romantic love may have evolved as an adaptation of neural circuits originally designed for maternal bonding. Therefore, female breasts, through oxytocin release during sexual intercourse, may activate this bonding mechanism.

The origin of breasts' allure may not stem so much from their size, shape, color or supposed resemblance to buttocks. Perhaps, it all began when a curious Homo ergaster with fuller breasts discovered with her partner that they were a wonderful way to achieve pleasure, desire and love.

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Studying Primates Past And Present: The Founding And Growth Of The Duke Lemur Center

In honor of Duke's Centennial, The Chronicle is highlighting pivotal figures and events throughout the University's history. Here, we take a look at the founding of the Duke Lemur Center:

The Duke Lemur Center is no secret on campus — in 2020, one in 12 undergraduate admissions applications made mention of the DLC.

Yet, few people know the story of its founding and how it grew from a small behavioral research station into the international hub for lemur research it is today.

The DLC finds its roots in the Civil Rights Movement in the 1960s, through which the two co-founders, Professor Emeritus of Biology Peter Klopfer and John Buettner-Janusch, met.

Klopfer — who arrived at Duke in 1958 — conducted behavioral research in zoology. Taking part in the Civil Rights Movement, he was arrested in 1964 for protesting a segregated restaurant in Chapel Hill, which resulted in mounting legal fees.

"My friends and colleagues set up a defense fund, and they contacted my former teachers and friends at Yale for contributions," Klopfer, who obtained his doctorate from Yale University, said in a 2020 interview with the DLC.

After an unconventional ruling left his case undecided — but capable of being reinstated — Klopfer made an appeal to the North Carolina Supreme Court, citing his Sixth Amendment right to a speedy trial. While the court denied his request, the U.S. Supreme Court ultimately took on Klopfer's case and ruled unanimously in his favor.

One contributor to the legal defense fund was Buettner-Janusch, then a Yale professor studying lemur genetics. Although Buettner-Janusch did not know Klopfer at the time, he shared a passion for civil rights and — unbeknown to Klopfer — was being considered for an appointment at Duke.

"When he came down here to give a lecture … I went to the lecture and introduced myself to him to say 'thank you' personally," Klopfer said.

After meeting, Buettner-Janusch expressed interest in Klopfer's work. The two visited the Duke Zoology Behavior Station together, where Klopfer studied various animals, from deer to turkeys. Buettner-Janusch noted that he was "very impressed" by the 40-acre enclosed research facility and admitted that he was seeking an appointment where he would have space to keep his collection of 80 lemurs.

John Buettner-Janusch, co-founder of the Duke University Primate Center, holds a collared lemur.

Shortly thereafter, Buettner-Janusch proposed founding a joint facility with Klopfer.

"Would I consider letting him bring the lemurs out to the behavior station if he in turn allowed me to use the lemurs for my behavioral work?" Klopfer recounted.

The two agreed that if Buettner-Janusch accepted a position at Duke, they would "form a partnership." After obtaining funding from the National Institutes of Health and a $406,000 National Science Foundation research grant in 1966, along with additional land from the University, the DLC — then called the Duke University Primate Center — was born.

The Duke University Primate Center takes off

The center's inaugural year saw the first lemur birth and plans to build a new Animal Behavior Station in 1967. By 1970, Klopfer had published the first research paper that used DUPC-collected data. One year later, the first doctoral dissertation using DUPC research was published.

Buettner-Janusch resigned as director of the center in 1972, and following the years saw several changes in leadership. Jan Bergeron stepped in as DUPC interim director, until in 1977, Elwyn Simons, then-James B. Duke professor of biological anthropology and anatomy, assumed the role. A June 13, 1977, article in The Durham Sun that year described Simons as "the world's top authority on the history of primates."

In 1983, the DUPC made national headlines following the birth of a brown lemur named Chiclette. Chiclette was the smallest brown lemur in captivity, with an article in The Virginian-Pilot describing her as "one-quarter the weight of a normal lemur infant."

"She was so tiny at first we were afraid she wouldn't make it," Simons said in a 1983 interview with CNN. "… This type of lemur, the collared lemur, is only in captivity here at Duke, and it is very rare in the wild."

In 1985, Chiclette gave birth to her son, E.T., a nod to comparisons made between the film character and the infant Chiclette. A report from the Duke News Service explained that both lemurs were "products" of a "long-range conservation program."

In an interview with the DNS, Simons described the primate center as the "second line of defense against extinction in the wild."

In 1989, a University press release described plans to establish a center in Ivoloina, Madagascar, as part of larger DUPC conservation efforts. Five years later, the first golden-crowned sifaka to be conceived and birthed in captivity, named "Gaius Julius," was born at the center.

The DUPC welcomed famous primatologist Jane Goodall in 1996 to speak at its 30th anniversary celebration. She and actress June Lockhart each gave speeches as part of the festivities.

Louise Martin conducts ring-tailed lemur research in the 1990s. The center faces challenges

In its first decade, the DUPC also faced its share of challenges.

The University threatened to close the center in 1975 amid budget tightening, as DUPC operating expenses cost around $100,000 per year. According to the Roanoke Rapids Daily Herald, the closure would have forced the "nearly 200 lemurs" to move to other research facilities or "major zoos."

Students expressed dissatisfaction with the decision. A March 1 report in the Winston-Salem Journal described President Terry Sanford meeting with student protesters and promising "careful consideration" of their demands. In addition to protesting the proposed closure of the DUPC, they also presented a petition with roughly 5,000 signatures calling for increased student and faculty input on a proposal to close the School of Forestry and Environmental Studies.

The center was saved later that year by a $300,000 donation from the May Charitable Trust of Pittsburgh. According to one Chapel Hill newspaper, the May Trust provided the grant "on condition the University seek long-term sources of support to keep the facility open."

Fifteen years later in the fall of 1990, the DUPC made headlines again after a U.S. Department of Agriculture veterinary inspector cited the center for nine violations ranging from "unsanitary and unsafe cages" to "inadequate staffing levels," according to the Durham Morning Herald.

"Just because at present time they have deficiencies doesn't mean that the facility is not trying to comply with the standards," USDA Official Rich Overton told the Associated Press at the time. "… The purpose of the inspection process is to identify problem areas and get them resolved."

Simons defended the center, noting that lemurs "sometimes roll about in their urine and feces." He maintained that removing those familiar smells would traumatize the animals, according to the AP.

Shortly thereafter, Simons was relieved of his duties as DUPC director and transitioned to work as the center's scientific director. Yet, John Burness, then-senior vice president of public affairs, maintained there was not a "direct relationship" between the USDA inspection and Simons' removal. Rather, the health and safety violations were only one factor in a holistic administrative review of Simons.

A May 1991 article by The Chronicle reported that Kenneth Glander, then-associate professor of biological anthropology and anatomy, had assumed the position of director and would "tackle problems of meeting federal regulations" in the role.

In spite of efforts to meet USDA standards, the DUPC again appeared in the news in 1996 after three primates died from cold weather within 48 hours. Another 27 animals faced injuries of varying degrees that season, ranging from heating lamp burns to frostbite. The investigation that followed charged the center with seven federal Animal Welfare Act violations and a $2,200 fine.

Following the deaths of the two lemurs and one slender loris, community members and animal rights advocates expressed mixed reactions. Some Durham community members supported the DUPC, while other observers called for the center to be shut down.

"Any death is sad and unfortunate, and any that can be prevented should be," wrote Ellen Kerley, Durham resident and DUPC volunteer, in an email to The Durham Herald-Sun. "But these deaths were not by any neglect or lack of care."

Mary Beth Sweetland, director of research investigation and rescues at People for the Ethical Treatment of Animals (PETA), claimed that the deaths were a sign of the DUPC's underpreparedness.

"All this suffering and death points up that Duke shouldn't have [the lemurs]," she told The Herald-Sun. "I would encourage them to give up on the project and get these animals to a sanctuary … or, if they can be, return them to the wild."

In July 1996, Glander announced the launch of a "winterization" project at the center to better protect the animals from "the most bitter winter weather." The plan included "new weatherproofed structures" to surround the enclosures and "new heated shelter boxes." At the time, the DUPC housed over 470 animals from 22 different species.

Prosimians in the present

In 1997, "Zoboomafoo" co-hosts Martin Kratt, Trinity '89, and Chris Kratt auditioned several family groups at the center to star on the popular children's show. Jovian — a Coquerel's sifaka at the DUPC — caught the attention of the Kratts, and footage of the lemur appeared in many episodes of the show, spanning from 1999 to 2001.

After a reevaluation of the center's mission, the DUPC changed its name to the Duke Lemur Center in 2006. The center celebrated 50 years of research in 2016, and it has now cared for about 4,000 animals.

Today, the DLC is led by Executive Director Greg Dye, and is home to around 13 species and more than 200 animals. Its most recent addition, a member of Zoboomafoo's family, was born Oct. 16. The center welcomes over 35,000 visitors each year, and DLC volunteers logged almost 11,000 service hours in 2023.

In 2021, DLC became the fifth-ever facility to earn a perfect score in an inspection by the Association of Zoos and Aquariums, which evaluates standards of animal welfare, health and nutrition.

Today, the DLC conducts non-invasive prosimian research and conservation. The lemur hub — among few of its kind — serves as a unique extension of the Duke community that attracts visitors from around the world.

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Ryan Kilgallen

Ryan Kilgallen is a Trinity sophomore and an associate news editor for the news department.

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