Department of Animal and Plant Sciences, University of Sheffield, Sheffield, United Kingdom.
PLoS One. 2012;7(3):e32335. doi: 10.1371/journal.pone.0032335. Epub 2012 Mar 1.
Juvenile mortality is a key factor influencing population growth rate in density-independent, predation-free, well-managed captive populations. Currently at least a quarter of all Asian elephants live in captivity, but both the wild and captive populations are unsustainable with the present fertility and calf mortality rates. Despite the need for detailed data on calf mortality to manage effectively populations and to minimize the need for capture from the wild, very little is known of the causes and correlates of calf mortality in Asian elephants. Here we use the world's largest multigenerational demographic dataset on a semi-captive population of Asian elephants compiled from timber camps in Myanmar to investigate the survival of calves (n = 1020) to age five born to captive-born mothers (n = 391) between 1960 and 1999. Mortality risk varied significantly across different ages and was higher for males at any age. Maternal reproductive history was associated with large differences in both stillbirth and liveborn mortality risk: first-time mothers had a higher risk of calf loss as did mothers producing another calf soon (<3.7 years) after a previous birth, and when giving birth at older age. Stillbirth (4%) and pre-weaning mortality (25.6%) were considerably lower than those reported for zoo elephants and used in published population viability analyses. A large proportion of deaths were caused by accidents and lack of maternal milk/calf weakness which both might be partly preventable by supplementary feeding of mothers and calves and work reduction of high-risk mothers. Our results on Myanmar timber elephants with an extensive keeping system provide an important comparison to compromised survivorship reported in zoo elephants. They have implications for improving captive working elephant management systems in range countries and for refining population viability analyses with realistic parameter values in order to predict future population size of the Asian elephant.
幼仔死亡率是影响密度独立、无捕食、管理良好的圈养种群增长率的关键因素。目前，至少有四分之一的亚洲象生活在圈养环境中，但无论是野生种群还是圈养种群，按照目前的生育率和幼仔死亡率，都无法维持下去。尽管需要详细了解幼仔死亡率的数据，以便有效地管理种群，并尽量减少从野外捕获的需求，但人们对亚洲象幼仔死亡率的原因和相关因素知之甚少。在这里，我们利用世界上最大的、关于亚洲象半圈养种群的多世代人口统计数据集，该数据集由缅甸木材营中收集而来，调查了 1960 年至 1999 年期间，圈养出生的母象（n=391）所生的幼仔（n=1020）在 5 岁之前的生存情况。死亡率风险在不同年龄段有显著差异，任何年龄段的雄性死亡率都更高。母象的繁殖史与死产和活产死亡率风险的巨大差异有关：首次生育的母象和在之前分娩后不到 3.7 年（<3.7 年）再次生育的母象以及年龄较大时生育的母象，其幼仔的损失风险更高。死产（4%）和断奶前死亡率（25.6%）明显低于动物园大象的报告数据，也低于发表的种群生存力分析中使用的数据。很大一部分死亡是由事故和缺乏母乳/幼仔虚弱引起的，这两者都可以通过对母象和幼象进行补充喂养以及减少高危母象的工作量来部分预防。我们对缅甸木材象的研究结果提供了一个重要的比较，与动物园大象报告的生存情况不佳形成了对比。这些结果对于改善亚洲象分布国家的圈养工作象管理系统，以及对于完善种群生存力分析，使用更现实的参数值来预测亚洲象未来的种群规模，都具有重要意义。
