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.

The field site is in the Citalahab area of the Gunung Halimun-Salak National Park (6°44′S, 106°31′E) in West Java, Indonesia. We collected data from three habituated wild Javan gibbon groups A, B, and S. All three groups consist of an adult female-male pair and their offspring (total N = 12). We followed the age classification (infant; 0-2 years, juvenile; 2-5 years, adolescent; 5-8 years, subadult; 8-dispersal). Groups A and B had two offspring each throughout the study. Group S had three offspring and we excluded the oldest offspring from the data analysis since the individual dispersed in April 2016, near the end of the study.

We carried out all-day field observations on the three habituated gibbon groups from one sleeping tree to the next over 22 months between November 2014 and July 2016. We observed them for 2,209 h over 306 days (group A: 776.25 h over 105 days; B: 720.5 h over 101 days; S: 712.25 h over 100 days). During the study period, we collected data from an adult female-male pair in each group, and a younger offspring (10-30 mo), and an older offspring (45-65 mo) from group A, and two younger offspring (5-25 mo) and two older offspring (41-61 mo) from group B and S. We recorded the occurrence of social grooming (hereafter ‘grooming’) and social playing (hereafter ‘playing’) bouts between all family members using all occurrence sampling. We defined a grooming bout as grooming without ceasing for longer than one minute in between. Particularly for the mother-offspring dyad, we recorded the time mothers breastfeed and carry infants using 15-min focal sampling (all occurrence sampling during 15-min) for every hour.