The varied path to adulthood: Plasticity in developmental timing in hylobatids.

Maturation includes physical, reproductive, and social aspects that develop asynchronously. Intra- and inter-individual variation in the relative timing of each aspect reflects context-dependent plasticity. Plasticity in developmental timing may be important for the family-living small apes (family Hylobatidae), where reproductive opportunities are monopolized by territorial breeding pairs. Data on development in hylobatids are so far limited due to their elusive arboreal niche and long lifespans. Herein, we provide the first quantitative analysis of behavioral development in 14 captive individuals of three species (Hylobates moloch, Nomascus leucogenys, and Symphalangus syndactylus), with some individuals being observed over 8 years. To establish a baseline for behavioral development, we compare the behavior of infants (0-2 years), juveniles (2-4 years), adolescents (4-6 years), and subadults (6-9 years) in their natal groups from data obtained over 250 focal observation hours. We then relate changes in behavior to measures of somatic and physiological development, and to wild populations, to identify the degree of intra- and inter-individual variation in developmental timing. We found greater concordance between behavioral and physiological markers of weaning than between different markers of later development, and the timing of later milestones varied more among individuals. Behavioral estimates from both captive and wild individuals in their natal groups indicate that puberty begins close to age 5, but physiological maturation can occur 0.5-3 years earlier in captive individuals than in wild individuals. Ages at onset of puberty, sexual, and social maturation vary in both captive and wild populations. The available evidence indicates that the social environment during development (e.g., presence of breeding female and appropriate mating partner), as well as nutrition, play a role in the timing of life history transitions in hylobatids. Am. J. Primatol. 78:610-625, 2016.