Timing is everything: expanding the cost of sexual attraction hypothesis

Anthropogenic disturbances present challenges to animals. Behavioural plasticity is one way that animals adjust to degraded habitats. In the present study, we examined how ecological conditions impact reproduction of female red colobus monkeys, Procolobus rufomitratus, in Kibale National Park, Uganda. Wrangham (2002, Behavioural diversity in chimpanzees and bonobos, pp. 204-215) proposed the 'cost of sexual attraction' hypothesis to explain the relationship between ecology and female reproduction. Here for the first time we test and expand on this hypothesis in a folivorous species, the red colobus monkey. We compared four groups of red colobus, two in previously logged areas and two in old-growth areas, to examine differences in female reproductive behaviours and physiologies. We predicted that, because of differences in food availability, females living in logged areas would (1) have a shorter duration of genital tumescence, (2) mate less frequently and (3) constrain mating behaviours more to periods of maximal genital tumescence compared to females in old-growth areas. As predicted, females in logged areas were fully tumescent for a significantly shorter period, copulated significantly less frequently and showed mating behaviours when fully inflated significantly more than females in old-growth areas. This behavioural plasticity contributes to the maintenance of female reproductive function in the face of environmental constraints associated with anthropogenic disturbance that influences food resources.