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.
