Disturbed Habitats Leading to Increased Stress Levels in Mexican Howler Monkeys

Habitat disturbance by humans is causing increased stress levels in monkeys and is also resulting in poor diets for the animals, reveals a new study.

Researchers from University of Cambridge, U.K., studied the endangered Mexican howler monkeys in the tropical rainforests of the Mexican state of Veracruz, an area which is deforested by humans.

The research team analyzed the feces of the monkeys for glucocorticoid stress hormones, in order to evaluate their stress levels. They found that the increase in travel time to search for food is causing high levels of stress in them.

Monkeys prefer to eat more fruits than what was previously thought. With their habitats shrinking due to deforestation, it has become difficult for the primates to find fruits. These monkeys have resorted to consume more leaves, which is their second choice when fruits are scarce.

"As forests are fragmented, the howlers become cut off, isolated on forest 'islands' that increasingly lack the fruit which provide an important component of their natural diet. This has led to the monkeys expending ever more time and effort foraging for food, often increasing leaf consumption when their search is, quite literally, fruitless," lead researcher Jacob Dunn, from Cambridge's Department of Biological Anthropology, said in a statement.

Plants such as lianas have increased in the Mexican howlers' diet. Although leaves are abundantly available in rainforests, they are hard to digest and could also have toxins - a natural defense mechanism used by trees and plants.

The monkeys are forced to search for leaves that are less toxic and could be digested easily. They visit different trees to select leaves for consumption. This has led to an increase in their travel time and subsequently, it has led to a surge in glucocorticoid stress hormone levels. These hormones are also related to lower reproductive success and survival rates of the monkeys.

The study highlights the need to preserve fruit trees, in particular different species of fig trees that could provide fruit during periods of scarcity.

Dunn and his research team believe the study could serve as a model for behavioral change and the resulting health implications in primates living in habitats disturbed by humans. They also insist on further studies to understand the importance of increases in stress hormone levels in howler monkeys living in such disturbed habitats.

The findings of the study are published in the International Journal of Primatology.