Brown howler monkeys prefer to rest on long, hot days and during leaf feeding

Brown howler monkeys (Alouatta guariba) rest more during long, hot days and when consuming fibrous leaves, according to a study published on July 8, 2026, in the International Journal of Primatology. Conducted in Brazil, this research highlights how environmental conditions and interspecies interactions influence the monkeys' daily movements.

Impact of Environmental Factors on Howler Monkeys

The study found that warmer temperatures and longer daylight hours lead to increased rest periods for brown howler monkeys. When temperatures rise, these monkeys travel shorter distances, as the heat necessitates more time spent resting. This behavior is particularly pronounced when they are feeding on leaves, which require longer digestion times.

Erika Alejandra Chaves-Diaz, the study's first author, tracked a group of howler monkeys from August 2023 to January 2024 at Carlos Botelho State Park in São Paulo, Brazil. The research team monitored the monkeys over 63 days, focusing on their behaviors, feeding habits, and interactions with other primate species.

Social Interactions Affecting Travel Distance

Interactions with other primate species, such as southern muriquis (Brachyteles arachnoides) and capuchin monkeys (Sapajus cucullatus), significantly influence how far brown howler monkeys travel each day. These interactions often lead howler monkeys to seek new feeding or resting sites to avoid conflict.

Chaves-Diaz noted, "The muriqui is larger, and its mere presence is enough to scare off howler monkeys, which prefer to leave the area rather than confront them." In contrast, capuchin monkeys, which travel in larger groups, can aggressively drive howler monkeys away from their resting spots.

Significance of Vocalizations in Tracking Behavior

Vocalizations among brown howler monkeys also vary significantly between fragmented habitats and continuous forests. In smaller areas, these monkeys vocalize frequently to alert other groups to their presence, aiding researchers in tracking their movements.

Chaves-Diaz explained that in the expansive area of Carlos Botelho State Park, howler monkeys vocalize, on average, only once a day, making tracking challenging. This low frequency is likely due to the lower density of howler monkeys in this preserved habitat.

• Average distance traveled: 531 meters (1,742 feet)
• Shortest distance: 202 meters (663 feet)
• Longest distance: 866 meters (2,841 feet)
• Average daily activity time: 8 hours

The study utilized a statistical model that accounted for 70% of the variation in daily travel distance, a significant improvement over previous models that only accounted for 37% to 43%.