title: Understanding stable bi-female grouping in gibbons: feeding competition and reproductive success.
abstract: BACKGROUND
  Species of the order Primates are highly gregarious with most species living in permanent heterosexual social groups. According to theory in socioecology maximum social group size is limited by rates of intra-group feeding competition and associated increases in travel costs. Unlike other hylobatids, which are predominantly pair living, cao vit gibbons (Nomascus nasutus), and two other species of crested gibbon (Nomascus spp.) living in northern seasonal forest, regularly exhibit larger bi-female groups. To better understand the ability of northern gibbons to live in stable bi-female groups, we examined food distribution, feeding competition and reproductive success over a period of six years in a small cao vit gibbon population at Bangliang, Guangxi, China.
  
  
  RESULTS
  In general, we found weak evidences for within-group contest or scramble competition in our two study groups, which we attribute to high spatial and temporal heterogeneity in the distribution of their important food species. Nevertheless, the larger of the two groups studied increased feeding time and group spread during lean periods, factors that may limit cao vit gibbon group size to a maximum of two breeding females. Relative to tropical pair-living gibbons, there is no evidence that cao vit gibbons travel farther or spend more time feeding, but they did consume more leaves and buds and less fruit and figs. Despite their highly folivorous diet, the average inter-birth interval is comparable to tropical gibbon populations, and the survival rate of infants and juveniles in our study groups is high.
  
  
  CONCLUSIONS
  Cao vit gibbons do not suffer obvious costs in terms of feeding competition and reproductive success by living in bi-female groups, but within-group feeding competition may determine the upper the limit of cao vit gibbon group size to a maximum of two breeding females. These findings contribute to a growing body of evidence that bi-female grouping can be a stable grouping pattern of gibbons in certain habitats and further emphasize the flexibility of gibbon social organization.
authors.affiliation: Institute of Himalaya Biodiversity Research, Dali University, Dali, 671003 Yunnan People's Republic of China.
authors.forename: Peng-Fei
authors.initials: PF
authors.lastname: Fan
authors.affiliation: Department of Anthropology, The University of Texas at San Antonio, San Antonio, TX 78249 USA.
authors.forename: Thad Q
authors.initials: TQ
authors.lastname: Bartlett
authors.affiliation: Institute of Himalaya Biodiversity Research, Dali University, Dali, 671003 Yunnan People's Republic of China.
authors.forename: Han-Lan
authors.initials: HL
authors.lastname: Fei
authors.affiliation: Institute of Himalaya Biodiversity Research, Dali University, Dali, 671003 Yunnan People's Republic of China.
authors.forename: Chang-Yong
authors.initials: CY
authors.lastname: Ma
authors.affiliation: Institute of Himalaya Biodiversity Research, Dali University, Dali, 671003 Yunnan People's Republic of China.
authors.forename: Wen
authors.initials: W
authors.lastname: Zhang
keywords: Cao vit gibbon; Feeding competition; Heterogeneity; Pair living; Reproductive success; Social system; Socioecological model