Within- and among-male variation in roaring by black and white colobus monkeys (Colobus guereza): What does it reveal about function?

Abstract

To test hypotheses about loud call function, researchers have typically used playback experiments and/or analyzed the contexts in which calls are produced. Analyzing the variation within and among individuals' loud calls may also provide insights about their function, including advertisement of callers' fighting abilities. This study tests whether some male black and white colobus monkeys (Colobus guereza, 'guerezas') indicate their willingness to defend access to important feeding areas and/or mates by increasing the number of roars they produce/chorus, their roar lengths, and/or overall roaring effort. It also tests whether mean values for these roaring variables indicate male fighting ability. My assistants and I followed six guereza groups for three months each at Kanyawara, Kibale National Park, Uganda. I recorded males' morning chorus roars and used data on roars, intergroup aggression, feeding, and proximity of females to ovulation to test hypotheses about roaring variation. I show that males were the main participants in intergroup aggression, the six study groups can be ranked in a linear dominance hierarchy, and group rank (effectively male rank) predicts and varies positively with how long males roar, as well as overall roaring effort. I provide evidence that some males roar longer when their groups' females are in close proximity to presumptive ovulation. There was relatively little evidence that males roar more when near important feeding areas. Males in higher and lower ranking groups may have different roaring strategies in the presence of important resources, but further studies are needed to fully test this idea. While this study may not provide strong evidence for either a direct mate or food defense function of roars, it shows that roars may advertise a male's ability to defend mates and/or food.