The ontogeny of feeding behavior of Nicaraguan mantled howler monkeys (Alouatta palliata)

This thesis is an investigation of the relationship between diet and life history in Primates. The goal of this research is to evaluate the hypothesis that accelerated growth and maturation schedules of folivorous primates enable these species to rapidly attain the ability to masticate and digest mechanically demanding leaf resources. In order to determine how dietary patterns underlie the evolution of primate life history strategies, I conducted a year-long study on the acquisition of foraging proficiency in juvenile mantled howler monkeys (Alouatta palliata) inhabiting La Isla de Ometepe, Nicaragua (11??40' N, 85??50' W). I predicted that juveniles are less efficient foragers than adults, that juvenile and adult diets differ, that juveniles rely on their mothers??? milk during periods of seasonal food stress, and that juvenile mortality increases when tough resources are an important dietary component. To evaluate these predictions, I followed two groups of mantled howler monkeys???collecting behavioral data on juveniles, adult females, and adult males???and I quantified the toughness of the leaves they consumed. Results indicate that juveniles were less efficient foragers than adults: juveniles spent more time foraging relative to the time they spent feeding, and juveniles devoted a greater proportion of their day to foraging than did adults. Juvenile and adult diets were similar, in terms of gross dietary category and plant species consumed; however, juveniles consumed a diet that was lower in overall toughness than was adults???. Milk served as a ???fallback??? food for juveniles during the dry season when food availability was limited and howlers increasingly relied on tough resources. Nonetheless, juvenile mortality rate increased: half of all juveniles died or disappeared during this time. These results support the initial hypothesis that the accelerated growth rate of folivorous primates is related to dietary toughness. Although juveniles adopted an adult-like diet early in their ontogeny, their inefficient foraging, lower toughness profile, extended reliance on maternal milk, and increased mortality rate during periods of dietary stress indicates that they struggled with the transition to dietary independence. Growing quickly throughout the juvenile period may be a life history strategy that howlers evolved to minimize mortality risks prior to reproduction. Moreover, howlers begin consuming solid food early in their ontogeny while still continuing to nurse, particularly during times of dietary stress. This early intake of solid food reduces mothers??? energetic costs to current offspring and enables them to rapidly shift their energy investment to future offspring