A recent study led by Yale University has found that Azara's owl monkeys, a small primate species native to South America, are heavier now than they were 25 years ago. The research, published in the journal *Proceedings of the Royal Society B*, indicates that these monkeys have gained approximately 50 grams (1.8 ounces), or 4% of their mean adult weight, coinciding with a 1 degree Celsius increase in daily mean temperatures in their habitat in Formosa, Argentina. This finding challenges the long-held belief that lighter body weights are beneficial in warmer climates, as it suggests that warmer temperatures during a monkey's first year of life may lead to increased weight later on. The study analyzed 287 weight measurements from 180 monkeys over 24 years, revealing that while their weights increased, their body lengths remained stable. Researchers believe that the energy savings from reduced thermoregulation needs may allow young monkeys to grow heavier without increasing their body length. This research highlights the importance of understanding how climate change impacts animal physiology and could have broader implications for other mammal species as temperatures continue to rise.