Barbary macaques from the Atlas Mountains of Morocco live in one of the most extreme environments for primates: Temperatures during the summer hit 40ºC (104ºF), and wintertime temperatures can drop to -5ºC (23ºF). According to a new Biology Letters study, these monkeys can adjust their metabolism to better adapt to temperature changes and the energetic demands of mating season. 

Having flexible metabolic strategies has helped primates respond to various demands of their environment, ranging from changes in rainfall to competition over mates. "Understanding the rules and mechanisms that govern key decisions such as energy allocation in existing primates is important in gaining insight into how our ancestors were able to thrive outside tropical Africa," University of Cambridge’s Jurgi Cristóbal-Azkarate said in a statement. 

The basal metabolic rate (BMR) is a measure of cell activity when an animal is at rest. Individuals with higher BMRs have higher energy requirements, and when food is limited, for example, this could have negative impacts on growth and survival. Thyroid hormones, such as T3 – which help regulate metabolism – act as an index of BMR.

Using fecal samples, Cristóbal-Azkarate and colleagues measured the thyroid hormone levels of two groups of adult male Barbary macaques (Macaca sylvanus) in the Atlas Mountains: a wild-feeding group and one that’s provisioned by tourists. In addition to temperature extremes, these monkeys also endure seasonal scarcity of food and water and strong breeding seasonality. The team also recorded their dominance rank, intensity of mating activity, and general activity, which included resting, traveling, and foraging. 

After extracting T3 from a total of 395 fecal samples, the team found that average macaque T3 levels varied a lot throughout the nine-month study period depending on temperature, food availability, and mating activity. Throughout the year, T3 levels increased as both foraging time and the minimum temperature decreased and as intensity of mating activity increased. 

Overall, T3 levels were higher in the provisioned group than in the wild-feeding one. After all, monkeys that weren’t fed by tourists had to conserve their energy while foraging for food. In both groups, T3 levels began to increase one month before the start of the mating season, peaking four to six weeks into this period. 

While both groups showed remarkable metabolic flexibility in their response to environmental demands and social challenges, this flexibility is affected by nutritional status and food availability. 

^{Image in the text: A Barbary macaque in the Atlas Mountains. NHK photo by Michael J. Sanderson/Ateles Films}