The biology and behaviour of these primates has fascinated scientists over the years. Charles Darwin suggested that, within the vertebrates, the loudest male would attract the most females by advertising his strength (2). However, there seems little evidence of this in howler monkeys and it is now thought that their calls announce a troop's right to food trees in the forest. Unlike other howlers, this species does not give a dawn chorus and seems to reserve most of its howling for inter-group encounters (5). These loud calls therefore play a role in avoiding conflict between groups and thus in saving energy which can be better used for foraging and digestion (2) Brown howling monkeys feed mainly on leaves, flowers and fruit, though the composition of their diet varies according to the season and their location in the forest (6). During autumn and winter, individuals must also spend more time feeding due to the inferior quality of the food and the higher demands on energy in these colder months (6). Generally howlers spend over half of their waking hours resting to conserve energy for feeding. Though leaves, flowers and fruit are abundant in tropical forests, they are low in nutrients and high in cellulose and so do not provide much energy (2). Mammals lack the enzymes to digest cellulose, and while primates of the subfamily Colobinae
have evolved specialised stomachs containing bacteria to digest the leaves, howlers have a simple, acid stomach similar to that of humans. However, howlers also have two enlarged sections where fermentative bacteria exist that are able to break down the material efficiently (2). In the process of fermentation, energy-rich gases (known as volatile fatty acids) are produced. Howlers absorb and use these gases as an energy source for their daily activities (2). Home ranges are small, up to around 31 hectares for a group of 15-20 individuals, as brown howling monkeys limit the distances they travel to feed (2). Males defend the home ranges by intimidation and fighting, which protects the group and allows females to invest more energy in reproduction and care of the young (2). Females usually have a single offspring, which are weaned before they are one year old (6).