How to be a good partner and father? The role of adult males in pair bond maintenance and parental care in Javan gibbons

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 got 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.