Red howler monkey birth data I: Seasonal variation.

A large set of birth data from a population of red howler monkeys (Alouatta seniculus) in two habitats found a consistent birth "dip," with births occurring less frequently in early wet season (May-July) than during the remaining months. In one of two habitats, births were negatively correlated with rainfall, and viable conceptions were positively correlated with rainfall. The observed degree of within-troop birth synchrony appeared to reflect populational synchrony with environmental factors. Correlations between births and the phenology of plant foods were not striking, but there was weak support for the hypothesis that births are timed to maximize the availability of weaning foods. Reduced births in early wet season also minimize the number of Infants who would experience the transition to independent locomotion and feeding during the peak of the dry season when probable low moisture content of foods and high daytime temperatures might be disadvantageous. However, there was no relationship between season of birth and infant mortality.