Breastfeeding, carrying, grooming, and playing behaviours in wild Javan gibbons (Hylobates moloch)

In pair-living species, female and male pairs may maintain stable social
bonds by adjusting spatial and social associations. Nevertheless, each sex
invests differently to maintain the pair bond, and the investment can
depend on the presence of paternal care or ‘male services.’ While most
species live in pairs, the sex responsible for pair bond maintenance in
gibbons is still controversial. We investigated pair bond maintenance and
parental care in three pairs of wild Javan gibbons in Gunung Halimun-Salak
National Park, Indonesia, for over 21 months. We found that Javan gibbon
fathers groomed their offspring more than adult females, especially as
offspring get older. While both parents increased playing time with
offspring when offspring became older and more independent, fathers played
with offspring 20 times more than mothers on average. Grooming within
Javan gibbon pairs was male-biased, suggesting that pair bond maintenance
was heavily the job of males. However, offspring age as a proxy for
paternal care did not affect the pair bond maintenance. Our study
highlights that adult male Javan gibbons may have an important role in
pair bond maintenance and the care of juveniles.