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 on May 20. 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.