No place like home: The benefits of slow maturation in hylobatids (#589)
Maturation is multifaceted and includes physical, reproductive and social aspects that may develop asynchronously. The small apes (family Hylobatidae) are arboreal primates that live in small, territorial family groups. Hylobatids have an equally long juvenile period between weaning and first reproduction as do the other nonhuman apes, despite their smaller body size. Part of this slow maturation is explained by slow physical growth, but dispersal and first reproduction in the wild can occur up to 2 – 6 years after cessation of growth, during which offspring may remain in the natal group. This study explores the benefits of delayed maturation in hylobatids by 1) reviewing markers of maturity in the literature and 2) quantifying changes in activity levels and social behaviour across development in captive immatures of 3 species (Hylobates moloch, Nomascus leucogenys and Symphalangus syndactylus). Behaviours of both captive and wild individuals indicate that adolescence begins ~ age 5 in the natal group, but there is inter-individual variability in the timing of physiological, sexual and social maturity. Developmental timing differs according to the social context, with some markers occurring later in the natal group than they do outside the natal group. Older offspring in the natal group display prosocial behaviour including spending time in close proximity to parents, grooming, and playing with younger siblings, despite showing evidence of adolescent behaviours (e.g. lower activity levels, spatial distancing from parents). Slower development and conflict-avoidance behaviour of older offspring in the natal group may reduce conflict in the family and facilitate delayed dispersal. In addition, participation in social play with younger siblings increases the inclusive fitness of delayed dispersers. These benefits of delayed dispersal and flexibility in developmental timing can thus explain the evolution of the late and variable age at first reproduction in hylobatids.