Conservation of the cao vit gibbon 781 Bi-female groups prevail amongst cao vit gibbons
Although most gibbons live in monogamous groups, the majority of cao vit gibbon groups are polygynous and usu- ally include two breeding females (Fan et al., 2010, 2015; Guan et al., 2018; this study). The two adult females are generally nonaggressive towards each other and are com- monly observed grooming (Fan et al., 2010). In addition, both females may breed at the same time (Fan et al., 2015). As a result, the mean group size of these bi-female groups was 6.3–6.4 individuals, which is comparable to bi-female groups of thewestern black crested gibbon Nomascus conco- lor (Fan et al., 2006) and Hainan gibbon (Zhou et al., 2005), and larger than monogamous gibbon groups (normally with four individuals, Bartlett, 2011). During the 2016 survey we observed one group with three
females, which is rare in any gibbon population. We ob- served two groups with three females during the long-term behavioural study on the Chinese side of the border. In the first group one female stayed in her natal group and replaced an old female that was not her mother, so temporarily there were three females in this group before the older female dis- persed. The second group contained an old female who had not given birth since 2007 and lived in the group with a second adult female. In August 2016, we identified a third female, a young adult, in this group. We are not certain whether this young female was born in the group or immi- grated into it. However, we have never observed a group with three adult females that all produced offspring. We recorded a mean of 0.84–0.92 infants per group
(0.39–0.52 infants per female) during our surveys. However, the long-term monitoring of the gibbon population on the Chinese side of the border indicates that females are breeding at a rate comparable to that reported in other gibbon species (0.77 infants per female; Fan et al., 2015).Apossible reason for this discrepancy is that researchers may have misidentified older infants as small juveniles. As infants mature they spend more time travelling independently from their mothers (P.F. Fan &
C.Y.Ma, unpubl. data), although they continue to nurse. Thus, during a short observation period, an older infant may be observed to travel independently and therefore be misidentified as a juvenile. Groups containing more than one adult male have been
recorded in lar gibbons Hylobates lar (Reichard et al., 2011) and siamangs Symphalangus syndactylus (Lappan, 2007), but are rare in Nomascus gibbons. During the 2007 and 2016 surveys we recorded few groups with more than one fully grown adult male. Because we did not know the genetic relationships and life history of males, subadult males that are fully grown but remained in their natal groups may have been wrongly recorded as adults. Given that there were two breeding females in most groups, one or both may have produced male offspring that could have been subadult during our relatively short survey period.
In rare situations, there could be three or even four adult- sized black individuals living in the same group. However, based on behavioural observations, only one male mates with adult females in cao vit gibbon groups. This contrasts with bi-male lar gibbon and siamang groups, in which both adult males copulate with resident females (Lappan, 2007).
Transboundary conservation
Since the rediscovery of the cao vit gibbon in 2002 in Viet Nam and 2006 in China, no hunting of gibbons has been recorded in either country. The main threat to the species is habitat degradation caused by selective logging, charcoal making, fuel wood collection and farming (Fan et al., 2011). Since the establishment of the Trung Khanh Cao Vit Gibbon Species and Habitat Conservation Area in Viet Nam and the Bangliang Gibbon National Nature Reserve in China, the gibbons and their habitat are legally protected. Logging, charcoal making, fuel wood collection and farming have been eliminated in the protected areas. Livestock grazing is completely prohibited in the protected area in Viet Nam, but still occurs inside the protected area in China. Since the rediscovery of the cao vit gibbon population, conservation awareness education has been conducted annually with local communities in both Viet Nam and China. In addition, programmes that improved the liveli- hoods of the local people were implemented by local gov- ernment and international NGOs, such as growing grass for livestock, planting trees for firewood, expanding the use of biogas, and the Community Seed Fund pilot programme (FFI, 2016). The governments of Viet Nam and China permit inter-
national NGOs to conduct conservation projects in this area and facilitate transboundary conservation of the cao vit gib- bon. A memorandum of understanding between China and Viet Nam for cooperative conservation of the cao vit gibbon was signed by the Department of Forestry of Guangxi Zhuang Autonomous Region and the Department of Agriculture and Rural Development of Cao Bang Province in 2011. The two sides reached agreement on me- chanisms of bilateral exchanges, collaborative approaches to counter wildlife hunting, and emergency measures such as forest fire protection. Both countries take turns to host a joint meeting every 2 years, to exchange experience on mon- itoring, programme management, public education, law en- forcement and other aspects of daily administration. In addition, the two protected areas have held informal meet- ings on the border twice per year since 2014. Participants from both contries did not need to apply for visas to travel to the other country, which makes transboundary conser- vation flexible and cost-effective. A transboundary species conservation action plan 2016–2020 was developed for the cao vit gibbon. This includes plans to expand the
Oryx, 2020, 54(6), 776–783 © 2019 Fauna & Flora International doi:10.1017/S0030605318001576
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