Fan Peng-Fei, Bartlett Thad Q, Fei Han-Lan, Ma Chang-Yong, Zhang Wen
Institute of Himalaya Biodiversity Research, Dali University, Dali, 671003 Yunnan People's Republic of China.
Department of Anthropology, The University of Texas at San Antonio, San Antonio, TX 78249 USA.
Front Zool. 2015 Mar 5;12:5. doi: 10.1186/s12983-015-0098-9. eCollection 2015.
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.
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.
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.
灵长目动物具有高度群居性，大多数物种生活在永久性的异性社会群体中。根据社会生态学理论，最大社会群体规模受到群体内觅食竞争率以及随之而来的旅行成本增加的限制。与其他主要成对生活的长臂猿不同，高棉白眉长臂猿（Nomascus nasutus）以及生活在北部季节性森林中的另外两种冠长臂猿（Nomascus spp.）经常形成较大的双雌性群体。为了更好地了解北部长臂猿生活在稳定双雌性群体中的能力，我们在中国广西邦亮的一个小高棉白眉长臂猿种群中，对食物分布、觅食竞争和繁殖成功率进行了为期六年的研究。
总体而言，我们在两个研究群体中发现了群体内竞争或争夺竞争的证据较弱，我们将其归因于其重要食物物种分布的高度时空异质性。然而，在食物匮乏时期，两个研究群体中较大的那个群体增加了觅食时间和群体扩散范围，这些因素可能将高棉白眉长臂猿群体规模限制在最多两只繁殖雌性。相对于热带成对生活的长臂猿，没有证据表明高棉白眉长臂猿的活动范围更远或觅食时间更长，但它们确实消耗了更多的树叶和嫩芽，而果实和无花果较少。尽管它们的食物以树叶为主，但平均产仔间隔与热带长臂猿种群相当，并且我们研究群体中的婴儿和幼猿存活率很高。
高棉白眉长臂猿生活在双雌性群体中，在觅食竞争和繁殖成功率方面并未遭受明显代价，但群体内觅食竞争可能决定了高棉白眉长臂猿群体规模上限为最多两只繁殖雌性。这些发现为越来越多的证据提供了补充，即双雌性群体可以是长臂猿在某些栖息地的一种稳定群体模式，并进一步强调了长臂猿社会组织的灵活性。
