476 L. J. Hale et al.
(Lande, 1993). Fromthe data collectedwe determined there to be at least eight adult females, although their ages and reproductive status cannot be determined using our meth- ods. There were seven young animals, at least four of which (two infants and two calves) were assumed to be dependents. Asian elephants are known to experience senescence, with reproductive success declining beyond the age of 18 years (Hayward et al., 2014). The age of first reproduction for females is 6–9 years and the average inter- birth interval is 2.5–4 years (Sukumar, 2003). Without fur- ther details of female ages it is not possible to predict future demographic trends. However, the detection of only one adult male in Nangunhe suggests a reduced effective population size, exacerbating the risks of inbreeding and reducing the long-term sustainability of the population (Frankham, 2005; Allendorf et al., 2008). The observed adult sex ratio of the population was female-biased (1:8). It is possible that the number of males was underestimated, particularly if they range more widely than females (Sukumar, 1989). We also acknowledge the low detection rate indicated by capture models, which could have resulted in individuals not be- ing detected. Anecdotally, the reserve manager reported knowledge of only two adult males in the Nangunhe popu- lation over the last 5 years (Li, pers. comm.). This suggests a strong female-biased adult sex ratio, seldom seen in undis- turbed populations, which tend to exhibit adult sex ratios of 1 adult male:2 adult females (Gupta et al., 2016). The underlying reasons for the skewed sex ratio in Nangunhe are unclear. There are recorded incidences of poaching of adult male elephants in Nangunhe, although not in the last 14 years. Of eight animal deaths reported during 1987–2003, one adult male was killed in retaliation for eating crops in 1996 and another was poached for ivory in 2003 (Liu et al., 2016). The sex of other animals killed was not recorded. Assessments of sex ratios at birth, or examination of dif-
ferential survival and mortality rates in younger animals, are thwarted by our inability to distinguish the sex of calves or infant elephants. Theories exist to explain sex ratio biases at birth and the effect of maternal (Trivers &Willard, 1973; Rosenfeld & Roberts, 2004) or paternal (Malo et al., 2019) conditions that may have relevance given the largely suboptimal habitat of Nangunhe and potential levels of inbreeding. An important consideration is that elephants are highly
complex social animals, and it is likely their breeding bio- logy is similarly complex. For example, Asian elephants do not breed well in captivity (Taylor & Poole, 1998; Rees 2003;Wiese&Willis, 2004), where groups are structured ar- tificially. In response to severely reduced population size, the elephant population in Cat Tien National Park, Viet Nam, coalesced into a single group comprising many matrilines (Vidya et al., 2007). The impact of historical hunting,
which is often highly selective, may affect population de- mography by removing key individuals such as experienced females or reproductively successful males and altering so- cial relationships (Archie & Chiyo, 2012) and, in African elephants, can result in a bias towards adult females (Jones et al., 2018). Prior to the recorded poaching in Nangunhe, the population will have been subject to the same pressures that have caused the decline of elephants across China more widely (Elvin, 2006). As a consequence, the structure of the Nangunhe population, probably like many other small populations, is an artefact of human activity rather than natural processes and therefore, in common with captive groups, the requisite social processes required to facilitate breeding in this complex species may be lacking. The identified presence of lone female elephants in Nangunhe corresponds with similar findings reported by Fernando & Lande (2000) who identified female Asian elephants in Ruhuna National Park, Sri Lanka, which spent considerable time away from their natal herds to maximize foraging opportunities. The low male to female ratio amongst adult elephants could also influence the dis- persal of females. In elephants males typically seek mates, but in African elephants a lack of mating opportunities has also been found to increase female dispersal rates (Archie et al., 2007). The picture developing for elephants in Nangunhe, from
our study and others, suggests a remnant population that is at risk of being lost because of social, genetic, ecological and human factors resulting from its isolation. The spatial and temporal scales that are relevant for elephant conserva- tion create further problems. The long generation length (20–25 years) of Asian elephants (Choudhury et al., 2008) means that any detrimental effects of inbreeding may take a substantial period of time to manifest in the population (Ling et al., 2016), but is likely to present a long-term prob- lem for the elephants of Nangunhe unless gene flow is restored between unrelated populations. The addition of only one breeding immigrant could substantially reduce inbreeding depression in an inbred population (Vilà et al., 2003). However, there are no current natural migratory routes between Nangunhe and the six other elephant popu- lations in China. Corridors for elephants have been success- fully created elsewhere (Green et al., 2018), but can require substantial land-use changes and agreement from stake- holders in the interstitial areas between reserves. Efforts to develop transboundary corridors linking Nangunhe to potentially large areas of suitable habitat (Leimgruber et al., 2003) and elephant populations in Myanmar would probably be complicated. Remaining options include trans- locations between elephant populations within China (Ishida et al., 2018), or assisted reproductive technologies to restore gene flow (Hermes et al., 2013), each requiring significant investment of effort and resources.
Oryx, 2021, 55(3), 473–478 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319000504
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