Data from: Illegal tusk harvest and the decline of tusk size in the African elephant

Harvesting of wild populations can cause the evolution of morphological, behavioral, and life history traits that may compromise natural or sexual selection. Despite the vulnerability of large mammals to rapid population decline from harvesting, the evolutionary effects of harvesting on mega-fauna have received limited attention. In elephants, illegal ivory harvesting disproportionately affects older age classes and males because they carry large tusks, but its' effects on tusk size for age or tusk size for stature are less understood. We tested whether severe historical elephant harvests eliminated large tuskers among survivors and whether elephants born thereafter had smaller tusks. Adjusting for the influence of shoulder height – a metric strongly correlated with body size and age and often used as a proxy for age – we compared tusk size for elephants sampled in 1966–1968, prior to severe ivory harvesting in the late 1970s and early 1980s, with tusk size of survivors and elephants born during population recovery in the mid-1990s. In a regional population, tusk length declined by ˜21% in male and by ˜27% in female elephants born during population recovery, while tusk length declined by 22% in males and 37% in females among survivors. Tusk circumference at lip declined by 5% in males but not in females born during population recovery, whereas tusk circumference reduced by 8% in male and by 11% in female survivors. In a single subpopulation, mean tusk length at mean basal tusk circumference declined by 12.4% in males and 21% in females. Tusk size varied between elephant social groups. Tusk homogeneity within social groups and the often high genetic similarity within social groups suggest that tusk size may be heritable. Our findings support a hypothesis of selection of large tuskers by poachers as a driver of the decline in tusk size for age proxy and contemporary tusk evolution in African elephants.

野生种群的捕获活动可引发形态、行为及生活史特征的演化，进而可能干扰自然选择与性选择的正常进程。尽管大型哺乳动物极易因捕获活动出现种群快速衰退，但捕获行为对巨型动物群（mega-fauna）的演化影响却鲜有研究关注。在大象种群中，非法象牙捕获因目标个体拥有较大象牙，更多针对高龄个体与雄性个体，但其对按年龄划分的象牙尺寸或按体型划分的象牙尺寸的影响仍有待阐明。本研究旨在验证：历史上严苛的大象捕获活动是否已将存活种群中的大型象牙个体清除殆尽，以及后续出生的大象是否拥有更小的象牙。本研究以肩高作为校正变量——该指标与体型大小及年龄高度相关，常被用作年龄的替代指标——将1966-1968年（即20世纪70年代末至80年代初大规模象牙捕获活动发生前）采样的大象象牙尺寸，与1990年代中期种群恢复阶段的存活个体及新生个体的象牙尺寸进行对比。在区域种群中，种群恢复阶段出生的雄性与雌性大象的象牙长度分别下降了约21%与约27%；而存活个体的象牙长度则分别下降了22%（雄性）与37%（雌性）。唇部处的象牙围度在种群恢复阶段出生的雄性个体中下降了5%，但雌性个体无明显变化；而存活个体的象牙围度则分别下降了8%（雄性）与11%（雌性）。在单个亚种群中，以平均基部象牙围度为基准的平均象牙长度，雄性下降了12.4%，雌性下降了21%。不同大象社群间的象牙尺寸存在差异。社群内部象牙尺寸的同质性，以及社群内普遍较高的遗传相似性，提示象牙尺寸可能具有可遗传性。本研究结果支持以下假说：偷猎者对大型象牙个体的选择性捕杀，是导致以年龄替代指标衡量的象牙尺寸下降，以及非洲大象当代象牙尺寸演化的驱动因素。