Endocrinology of howler monkeys: Review and directions for future research

Abstract

Endocrine studies that investigate the interplay between hormones, behavior, and social and ecological environment are critical for our understanding of proximate, physiological mechanisms underlying the biology and sociality of a species. Nonetheless, only recently have endocrine studies been incorporated into research on howler monkeys (Alouatta spp.), and only few aspects of endocrinology in 6 out of 12 species have been addressed. These include androgen and estrogen profiles of juvenile A. palliata, and progestin and estrogen profiles of the ovarian cycle of A. arctoidea, A. caraya, and A. pigra. In addition, socioendocrine studies in A. pigra and A. palliata have investigated how male androgen levels and male and female glucocorticoid levels are influenced by intra- and extragroup male-male competition, whereas ecologically oriented endocrine studies have revealed that in A. pigra, A. palliata, A. belzebul, and A. seniculus male and female glucocorticoid levels are influenced by a scarcity of high-quality food resources, habitat fragmentation, human disturbance, and translocation. Endocrine studies have shed light on howler monkey biology and sociality that were not anticipated based on behavioral data alone. This includes a nonaggressive form of intragroup male-male competition over access to females, a more prominent reliance on high-quality food resources such as fruits for these primarily folivorous primates, and an apparent higher sensitivity to social and ecological stress in females than in males. Additional endocrine studies across howler monkey species are needed to further elucidate relationships among diet, mating competition, and social interactions.