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Gibbon families grow larger with bi-female groups.

By Dave Armstrong - 14 Apr 2015 8:6:5 GMT
Gibbon families grow larger with bi-female groups.

These two mature females are the ksy to success in the critically-endangered cao vit gibbons. Their bi-female groups enable this and several other crested gibbon species to enlarge the typical small gibbon family by up to 9 individuals. Nomascus nasutus image; Credit: Chao Zhao

The sociable apes are full of species with permanent groups made up of heterosexual mixes. Those groups tend to gather and fetch, with competition between the members and the need to avoid longer travel to food areas. It almost sounds like the human urge to reduce our carbon footprint. Gibbon family members normally seem to be made up of a pair, while the endangered crested gibbons of the more northern Nomascus. genus differ slightly in having bi-female groups with more animals in the group. Other gibbon species can feature bi-male groups, but theoretically, the whole gibbon family seem quite restricted by their niches, retaining the typical small family groups.

Peng-Fei Fan and his 4 colleagues from Dali University, China, and the University of Texas at San Antonio, US, studied Understanding stable bi-female grouping in gibbons: feeding competition and reproductive success over 6 years for their paper in Frontiers in Zoology. One population of Nomascus nasutus (cao vit gibbons) living at Bangliang Gibbon Nature Reserve in the Chinese Province of Guangxi were the subjects.

Foraging efficiency, predator protection, cooperative resource defence and protection from conspecifics are the recognised advantages provided within a typical primate group. In gibbon foraging, competition within the group (WGC) and within-group scramble (WGS) involve aggression and contest between group members where resources can disappear because group members have eaten them. Low ranking animals can be forced to forage further from home because of these limitations, but observations of this species indicated a preference for eating buds and leaves when fruit supplies became scarce. In the cao vit gibbon, the bi-female group appears much more stable than in some other species. Juveniles remain in-group for 10 years, meaning there is a significant increase in the size of a bi-female group. The key could be that the females are normally very sociable (ie. they maintain the social group rather than simply tolerate the other woman), with the feeding competition and travel costs for the group reduced in several ways.

The inter-birth intervals and early mortality of the gibbon groups studied were part of the research. These features of the research indicated the extent to which cao vit gibbons differ from other gibbon species. There was no significant difference, despite the northern location and extreme seasonal differences in food availability. This means the capacity to utilise leaves and buds during lean periods in the fruit and fig seasons could have great significance, along with the large body size (or even some storage of body fat) in this cao vit, Nomascus nasutus, species at least. Interestingly, females in bi-female groups produce similar-aged juveniles. Such female offspring could then more simply disperse together and form their own bi-female groups.

The stability of these cao vit groups seemed to hold with a number of up to 9 individuals. Beyond that number, juveniles dispersed, sometimes together to form new alliances. The fruit, leaf and bud diet meant that these animals could easily avoid depletion of resources in most circumstances, so they could reproduce at least as successfully as other gibbon species or populations. Here is another angle on Hylobates lar. Like all gibbons, their beautiful songs are renowned as a great way to have entertainment for your breakfast-time, but this paper discovers that they use adjectival phrases in a defined mode of speech!