Hormonal and behavioral patterns of reproduction and parental care in the Hylobatidae

Parental involvement is crucial for the normal development of physical, psychological, and behavioral characteristics in all mammalian species. The form and degree of parental care, however, varies considerably within the mammalian order. While extensive maternal care is generally the exclusive pattern of parental care in polygamous primates, in some monogamous primates males contribute varying degrees of paternal care to ensure the successful rearing of offspring (Trivers 1972; Clutton-Brock 1991). In this dissertation research, basic reproductive biology and variation in behavioral and hormonal patterns of parental care are explored in the Hylobatidae.;North American zoos maintain populations of three major hylobatid species, including Nomascus leucogenys, Hylobates lar, and Symphalagus syndactylus. The hylobatids are the least studied group of apes, and very limited information is available related to basic reproductive physiology and variation in parental behavior. The hylobatids are also evolutionarily interesting because they are the only monogamous ape species and because only one species of hylobatid, the siamang (Symphalangus syndactylus), demonstrates direct paternal care in the form of infant-carrying. First, basic reproductive traits and patterns of reproductive behavior were characterized in zoo-housed hylobatid females. The length of the hylobatid estrous cycle was determined and found to be similar across individuals and species, gestation length was estimated for Nomascus leucogenys, and reproductive behavior was found to occur throughout the estrous cycle. Then, maternal behavior was characterized and variation in behavior and associated hormonal patterns was examined in zoo-housed hylobatid females. Variation in maternal behavior was found, but there were no species differences in maternal behavior. Two distinct types of inadequate mothers were identified, the first of which rejects an infant at birth, and the other that exhibits appropriate maternal behavior until around 2 months post-partum followed by inappropriate care. A unique pattern in the E:P ratio and in 1 month post-partum fecal glucocorticoid concentrations was associated with each type of inadequate mother. Finally, hormonal correlates of paternal care were examined in male hylobatids. Father-infant proximity was found to be the best measure of paternal care, and this increased over the post-partum period in siamangs but not in gibbons. Additionally, fecal androgen metabolite concentrations decreased over the post-partum period in siamang fathers but not in gibbon fathers. Finally, fecal glucocorticoid metabolite concentrations during 1 month postpartum showed the greatest increase from individual baseline in the siamang males than all other gibbon males. Taken together, these results suggest a relationship between specific hormonal patterns and species differences in paternal care. In the concluding chapter, a comprehensive model was developed to determine which life-history and environmental factors could predict inadequate maternal care. However, none of the factors were able to significantly predict whether a mother would be adequate or inadequate, highlighting the need for the development of a better predictive model.;The hylobatids are an especially understudied group of primates, which is surprising given their unique phylogenetic position and behavioral characteristics. Zoo-housed populations of hylobatids provided an ideal model system to study individual and species variation in parental care. By combining behavioral and hormonal data, this dissertation research project was able elucidate proximate factors that contribute to variation in maternal care in the zoo-housed population, as well as proximate mechanisms related to the evolution of species differences in paternal care. This research has added to the current understanding of hylobatid biology, and it is also relevant to the continued management and successful maintenance of hylobatids in zoos.