May 25, 2026
By Eurasia Review
Azara’s owl monkeys, a small primate species found in South America, are heavier today than those that lived a quarter-century ago, and evidence suggests that rising temperatures might have driven the weight gain, according to a Yale-led study of a wild population.
The study — the first to link climate change to weight changes in living primates — is based on 287 weight measurements of 180 owl monkeys collected between 1999 and 2023 in Formosa, Argentina. The researchers found that the monkeys were about 50 grams (1.8 ounces) heavier in 2023 than in 1999, an increase equivalent to 4% of the mean adult weight of 1,300 grams (2.87 pounds).
The weight gain coincided with a period when daily mean temperatures in the region increased by more than 1 degree Celsius. The researchers also found that that warmer temperatures in a monkey’s first year of life predict heavier weights when they’re older.
“We found that owl monkeys today weigh more, not less, than they did in 1999, even though average temperatures have increased since then,” said lead author Jonathan Pertile, a Ph.D. student in anthropology in the Yale Graduate School of Arts and Sciences. “That’s surprising because scientists have long thought that being lighter is an advantage in warmer temperatures because it helps the body shed excess heat.”
The finding that warmer temperatures in the animal’s first year of life predicts heavier weight later suggests that the amount of energy monkeys spend staying warm while young might limit their growth, he said.
The study was published in the journal Proceedings of the Royal Society B. Eduardo Fernandez-Duque, professor of anthropology in Yale’s Faculty of Arts and Sciences and director of the Owl Monkey Project in Argentina, is the study’s senior author.
The finding conflicts with a longstanding ecogeographical principle known as “Bergmann’s rule,” which states that individuals of a warm-blooded species inhabiting colder climates have larger mean body sizes than their counterparts in warmer climates. The theory is based on the notion that lighter bodyweights offer an advantage to species in warmer climates due to more efficient thermoregulation — the ability of an organism to maintain a stable body temperature, the researchers said.
Azara’s owl monkeys are omnivorous, pair-living, and monogamous primates that inhabit the Gran Chaco region of Argentina, Paraguay, and Bolivia. The new findings are based on data collected by the Owl Monkey Project over 24 years from 180 owl monkeys at a field site on a privately owned cattle ranch in Formosa, Argentina.
The mean daily temperatures in the region over the course of the study period increased from 22.2 degrees Celsius in 1999 to 23.8 degrees Celsius in 2023, according to the study.
For the study, researchers weighed monkeys at three life stages: as young monkeys still attached to their birth groups, as solitary young adults competing for access to breeding positions within groups, and as adults that have acquired reproductive status within a group. They also measured the animals’ body lengths, from the crown of their skulls to the base of their tails. (Some individuals were measured repeatedly over the years.)
The research team analyzed several variables that could possibly explain the weight gain, including reproduction, which benefits from enhanced energy reserves provided by higher bodyweights, and increased availability of food. But according to their analysis, warmer temperatures during a monkey’s first year of life was the factor that best predicted heavier weights later in life, they said.
The researchers posit that the warmer temperatures required the young monkeys to spend less energy on thermoregulation, which allowed them to use extra calories to grow heavier.
While the monkeys got heavier, their body lengths remained steady. The calorie surpluses caused by warmer temperatures early in life may not translate into increased body length if the monkeys’ minimum energetic and nutritional requirements for development are already met, the researchers explained. In humans, a similar trend is illustrated by the flattened rate of increase of mean height in many economically developed populations.
“Our study offers insight into how physical traits in a species can change when you don’t have underlying changes to its genetics,” Pertile said. “Temperatures will continue to rise as climate change unfolds, and it’s important to understand the dynamics of how changing environmental factors will affect animals’ bodies. This study provides a good start to that work.”
Eric Sargis, professor of anthropology in FAS, is a coauthor of the study.
Monkey business: artist chimps paint in their own style, study shows
Several studies conducted in captivity have demonstrated that chimpanzees, like other great apes, enjoy painting and drawing. But new resreach led by French and Japanese primatologists has shown that they each have their own drawing style – and that it can evolve and improve over time.
Issued on: 24/05/2026 - RFI
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Several studies conducted in captivity have demonstrated that chimpanzees, like other great apes, enjoy painting and drawing. But new resreach led by French and Japanese primatologists has shown that they each have their own drawing style – and that it can evolve and improve over time.
Issued on: 24/05/2026 - RFI
\A group of chimpanzees at the Beauval zoo in France. © AFP - Alain Jocard
"Zamba draws nothing but dots. Loi sketches curves and triangles. Misaki fills the page with large fan-shaped patterns. These aren’t children in an art class. They are chimpanzees, and they each have their own style," writes ethologist Cédric Sueur on Instagram, sharing images of each chimp's distinctive art.
Sueur is one of four primatologists, three from France and one from Japan, who collaborated on a study published this month in the scientific journal Primates.
The colleagues analysed nearly 500 drawings produced by six chimpanzees at the Great Ape Research Institute, a sanctuary in southern Japan that takes in chimpanzees and bonobos that were once used as laboratory test subjects.
The team provided the animals with paper, paint and brushes. They were not trained to use them and weren't offered a reward for doing so.
By analysing 494 drawings over eight years, each piece dated and attributed, the researchers discovered that every chimpanzee has its own unique graphic signature, which evolves over time.
"Three dimensions structure the artwork of all individuals – the way they occupy space, the diversity of shapes, the richness of colours – exactly the same way as in orangutans and human children," Sueur says.
"These styles evolve over time: the chimpanzees increasingly fill the frame, diversify their shapes and develop more complexity."
Changing styles, colours
Researchers observed that the colourful fans that Misaki systematically drew took up more and more space on the page, for example. Their shapes became more intricate, the colours changed with the seasons and, much like in children’s drawings, their style refined itself over the years.
These changes may also reflect chimps' mood and energy levels. In winter, chimpanzees go out less and see fewer bright colours around them, Sueur posits.
"Their drawings change with the seasons, being less elaborate in winter, as if their inner state were leaving its mark on the paper," he writes.
Although the chimps' drawings were always abstract, never figurative, they seem to be influenced by what they see. One chimpanzee named Molly began painting in blue and yellow after being visited by Japanese schoolchildren wearing uniforms in those colours.
Ethologists are continuing their research to better understand what motivates animal artists such as chimpanzees, orangutans and elephants, who have been known to paint with their trunks.
This article was adapted from the original in French by RFI's Caroline Lachowsky.
"Zamba draws nothing but dots. Loi sketches curves and triangles. Misaki fills the page with large fan-shaped patterns. These aren’t children in an art class. They are chimpanzees, and they each have their own style," writes ethologist Cédric Sueur on Instagram, sharing images of each chimp's distinctive art.
Sueur is one of four primatologists, three from France and one from Japan, who collaborated on a study published this month in the scientific journal Primates.
The colleagues analysed nearly 500 drawings produced by six chimpanzees at the Great Ape Research Institute, a sanctuary in southern Japan that takes in chimpanzees and bonobos that were once used as laboratory test subjects.
The team provided the animals with paper, paint and brushes. They were not trained to use them and weren't offered a reward for doing so.
By analysing 494 drawings over eight years, each piece dated and attributed, the researchers discovered that every chimpanzee has its own unique graphic signature, which evolves over time.
"Three dimensions structure the artwork of all individuals – the way they occupy space, the diversity of shapes, the richness of colours – exactly the same way as in orangutans and human children," Sueur says.
"These styles evolve over time: the chimpanzees increasingly fill the frame, diversify their shapes and develop more complexity."
Changing styles, colours
Researchers observed that the colourful fans that Misaki systematically drew took up more and more space on the page, for example. Their shapes became more intricate, the colours changed with the seasons and, much like in children’s drawings, their style refined itself over the years.
These changes may also reflect chimps' mood and energy levels. In winter, chimpanzees go out less and see fewer bright colours around them, Sueur posits.
"Their drawings change with the seasons, being less elaborate in winter, as if their inner state were leaving its mark on the paper," he writes.
Although the chimps' drawings were always abstract, never figurative, they seem to be influenced by what they see. One chimpanzee named Molly began painting in blue and yellow after being visited by Japanese schoolchildren wearing uniforms in those colours.
Ethologists are continuing their research to better understand what motivates animal artists such as chimpanzees, orangutans and elephants, who have been known to paint with their trunks.
This article was adapted from the original in French by RFI's Caroline Lachowsky.
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