Owl Monkeys Have Gotten Heavier as Temps Have Risen

Climate Change May Be Causing Primates to Gain Weight—Defying Long-Standing Biological Theories

A new study of wild Azara’s owl monkeys in Argentina challenges a century-old ecological principle by showing that rising temperatures may be driving weight gain rather than the expected loss. Researchers found that these small primates have grown heavier over 24 years as regional temperatures increased, with warmer early-life conditions linked to lasting changes in body weight.

The findings, published in Proceedings of the Royal Society B, mark the first direct evidence that climate change can alter body size in living primates. The study, led by Jonathan Pertile of Yale University’s Department of Anthropology, tracked 180 owl monkeys from 1999 to 2023, recording 287 weight measurements across three life stages: infants, young adults, and reproductive adults. By 2023, the monkeys weighed an average of 50 grams (1.8 ounces) more than in 1999—a 4% increase relative to their mean adult weight of 1,300 grams (2.87 pounds).

This weight gain occurred despite a rise in mean daily temperatures from 22.2°C to 23.8°C over the study period, defying the widely held assumption that warmer climates favor lighter body weights for better heat dissipation. Instead, the researchers found that warmer temperatures in a monkey’s first year of life predicted heavier weights later in adulthood. Pertile suggests this may be because young monkeys expend less energy on thermoregulation in warmer conditions, allowing them to redirect calories toward growth.

“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.”

The research contradicts Bergmann’s rule, a long-standing ecological principle stating that warm-blooded species in colder climates tend to have larger body sizes than those in warmer regions. The owl monkeys’ weight gain, however, did not translate to increased body length, suggesting that while calorie surpluses may drive weight increases, they do not necessarily affect linear growth when basic nutritional needs are met.

The study was conducted by the Owl Monkey Project in Formosa, Argentina, under the direction of Eduardo Fernandez-Duque, professor of anthropology at Yale and senior author of the study. The team analyzed multiple variables—including reproduction, food availability, and temperature—to determine the primary driver of the weight gain. Warmer early-life temperatures emerged as the strongest predictor, reinforcing the idea that environmental conditions in early development can have lasting physiological effects.

While the findings are specific to owl monkeys, they may have broader implications for understanding how climate change influences wildlife. The study highlights the need for further research into how rising temperatures affect species across different ecosystems, particularly as global warming continues to reshape natural habitats.

The research was published in Proceedings of the Royal Society B, a peer-reviewed journal of the Royal Society, and was first reported by Futurity, a science news outlet affiliated with universities including Yale.