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.