More nails in the monogamy coffin: Offspring in stable, monogamous gibbon families have an extra-group father in 40pct of cases; the breeding system of crested gibbons is more flexible than previously thought (understatement of the week)

Disassociation of social and sexual partner relationships in a gibbon population with stable one-male two-female groups. Xia Huang et al. American Journal of Primatology, May 25 2022. https://doi.org/10.1002/ajp.23394

Abstract: Adult males living in a one-male multi-female social group are expected to try to monopolize copulations with resident females to increase reproductive fitness. Gibbons have traditionally been described as living in monogamous groups, with the sole resident adult male assumed to sire all of the group's offspring. Here, we used microsatellite analyses and behavioral observations to examine rates of extra-group paternity (EGP) over 16 years in a population of crested gibbons (Nomascus concolor) that form stable and long-term one-male two-female social units. Forty percent of offspring (N = 14) were sired by extra-group males. To understand this high level of EGP, we tested whether inbreeding avoidance was related to EGP. Females who engaged in EGP did not show larger pairwise relatedness with their resident male compared to females who did not engage in EGP. Nevertheless, the standardized heterozygosity of EGP offspring was significantly higher than for offspring sired by the group's resident male. These results provide partial support for the inbreeding avoidance hypothesis. It appears that resident male crested gibbons are unable to monopolize resident females' matings. Our results indicate that long-term social partners are often distinct from sexual partners in this population. Clearly, the breeding system of crested gibbons is more flexible than previously thought, indicating a need for integrating long-term behavioral data and genetic research to re-evaluate gibbon social and sexual relationships derived from concepts of monogamy and pair-bonding.

Highlights

First study of reproductive strategies in a gibbon population with stable one-male two-female groups.

Confirmed unexpectedly high rate of extra-group paternity.

Females engaged in extra-group paternity to increase offspring heterozygosity.

High flexibility in gibbon breeding systems.