Reproductive Parameters of Captive Female Northern White-Cheeked ( Nomascus leucogenys ) and Yellow-Cheeked ( Nomascus gabriellae ) Gibbons

Reproductive and life-history characteristics, such as age at first reproduction, interbirth interval (IBI), sex ratio of newborns, and infant mortality, are crucial for assessing the population dynamics and survival of threatened species. There are no longitudinal data on the life histories of the threatened northern white-cheeked gibbons ( Nomascus leucogenys ) and yellow-cheeked gibbons ( Nomascus gabriellae ). Here, we examine the reproductive parameters of these species in 19 Chinese zoos. Some of these captive individuals may be hybrids of N. leucogenys × N. gabriellae , but it is not possible to distinguish these hybrids from the parent species based on morphological characteristics. Thus, in this study we identified individuals to species level from the zoos’ archives. Based on breeding records, 46 infants were born to 14 female northern white-cheeked gibbons from 2000 to 2019 and 89 infants to 29 female yellow-cheeked gibbons from 1995 to 2018. The mean age at first reproduction was 10.1 ± SD 2.4 yr for northern white-cheeked gibbons and 10.0 ± SD 3.1 yr for yellow-cheeked gibbons. The mean IBI was 32.8 ± SD 11.7 mo for northern white-cheeked gibbons and 27.2 ± SD 14.8 mo for yellow-cheeked gibbons. There was no significant effect of parity on IBI in northern white-cheeked gibbons, but primiparous mothers had a significantly longer IBI than multiparous mothers in yellow-cheeked gibbons. There was no significant difference in infant mortality between primiparous and multiparous mothers for either species. The sex ratio of newborns did not deviate significantly from 1:1 for either species. Infant mortality was 13% for northern white-cheeked gibbons and 12% for yellow-cheeked gibbons. Infant death reduced the IBI by 19 mo in northern white-cheeked gibbons and 10 mo in yellow-cheeked gibbons. Our data suggest that the intrinsic reproduction rate is very low in these gibbons and that wild populations may continue to decline without suitable conservation intervention.