Demographic changes over thirty years in a red howler population in Venezuela

During a 30-year span (1969 - 1999) the annual growth rate of a Venezuelan red howler (Alouatta seniculus) population fluctuated irregularly, but its size increased, remained stable for a short while, and finally declined sharply. The increase took place in three stages, and began as an increase in the size of established groups. The next two stages of population increase were due to the formation of new groups and their subsequent increases in size. These two stages likely occurred because of habitat regeneration, which increased the areas where newly formed groups could establish home ranges. The population decline of 74% was most likely due to disease. However, new groups died out more rapidly than established groups, indicating that food shortages, especially in recently regenerated areas, may also have contributed to the population crash. The food shortages could have been caused by unpredictable periods of drought, which may explain the irregular size fluctuations of the study population. Since many howler species show irregular size fluctuations and sharp declines, their demographic features may reflect adaptations to unpredictable events like droughts and disease epidemics. On this premise we explain the preponderance of unimale groups and female-biased birth sex ratios at low densities and the dispersal of both sexes as adaptations for increasing a population rapidly after a decline. Within the population, mortality of small juvenile females was higher in multimale than in unimale groups, though medium juvenile and older immature females were better represented in multimale than in unimale groups. These results may be explained in terms of group composition and the mating systems in red howlers.