Data from: Is bigger better? The relationship between size and reproduction in female Asian elephants

The limited availability of resources is predicted to impose trade-offs between growth, reproduction and self-maintenance in animals. However, although some studies have shown that early reproduction suppresses growth, reproduction positively correlates with size in others. We use detailed records from a large population of semi-captive elephants in Myanmar to assess the relationships between size (height and weight), reproduction and survival in female Asian elephants, a species characterized by slow, costly life history. Although female height gain during the growth period overlapped little with reproductive onset in the population, there was large variation in age at first reproduction and only 81% of final weight had been reached by peak age of reproduction at the population level (19 years). Those females beginning reproduction early tended to be taller and lighter later in life, although these trends were not significant. We found that taller females were more likely to have reproduced by a given age, but such effects diminished with age, suggesting there may be a size threshold to reproduction which is especially important in young females. Because size was not linked with female survival during reproductive ages, the diminishing effect of height on reproduction with age is unlikely to be due to biased survival of larger females. We conclude that although reproduction may not always impose significant costs on growth, height may be a limiting factor to reproduction in young female Asian elephants, which could have important implications considering their birth rates are low and peak reproduction is young – 19 years in this population.

据预测，动物体内的资源有限性会导致生长、繁殖与自我维持之间存在权衡取舍。然而，尽管部分研究表明早期繁殖会抑制生长，但另有研究显示繁殖与体型呈正相关。本研究利用缅甸一个大型半圈养亚洲象种群的详细记录，对体型（身高与体重）、繁殖与存活之间的关系展开评估，该物种以缓慢且高成本的生活史为典型特征。尽管该种群中雌性个体生长阶段的身高增长与繁殖起始几乎无重叠，但首次繁殖的年龄存在较大差异，且在种群水平的繁殖峰值年龄（19岁）时，个体仅达到最终体重的81%。早期开始繁殖的雌性个体在生命后期往往体型更高、体重更轻，但上述趋势未达到显著性水平。研究发现，在特定年龄时，身高更高的雌性个体繁殖概率更高，但该效应随年龄增长而减弱，这表明繁殖可能存在体型阈值，这一点对年轻雌性尤为重要。由于繁殖期雌性的体型与存活概率并无关联，因此身高对繁殖的影响随年龄减弱的现象，不太可能源于体型更大的雌性存活偏倚。本研究结论认为，尽管繁殖并非总会对生长造成显著成本，但身高可能是年轻雌性亚洲象繁殖的限制因素；考虑到该物种出生率较低且繁殖峰值年龄较早（本种群为19岁），这一结论具有重要的研究意义。