Illegal tusk harvest and the decline of tusk size in the African elephant

Abstract

SummaryHarvesting 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.

When does selective hunting select, how can we tell, and what should we do about it?

“…Finally, species in which both sexes grow horns, tusks or antlers that are the target of selective hunting may show faster evolutionary change than species where females are not selectively hunted. That may be the case, for example, for African elephants Loxodonta africana (Chiyo et al 2015).…”

Size‐selective harvesting fosters adaptations in mating behaviour and reproductive allocation, affecting sexual selection in fish

“…Indeed, selective harvesting can directionally change sexually selected traits (e.g. Chiyo, Obanda, & Korir, 2015, Pigeon, Festa-Bianchet, Coltman, & Pelletier, 2016, Sordalen et al, 2018, but the role of sexual selection in FIE remains largely unexplored (but see Sordalen et al, 2018).…”

Adaptation, speciation and extinction in the Anthropocene

“…(a) Selection in built environments: feathers left on a window illustrate the high death toll of birds colliding with buildings and automobiles [12], selecting against migratory behaviour [13] and for manoeuvrability [14]. (b) Selection to avoid hunting or harvesting: humans target individuals with preferred traits, selecting against traits such as long ivory tusks [15]. (c) Selection in novel communities: both abiotic and biotic selection pressures are reshaped when humans bring together species in new assemblages, as found in cardinals nesting in introduced honeysuckles [16].…”

“…Hunting pressures have long been studied for the selective pressures induced, including an early analysis by Haldane [31] documenting the decline in foxes of the silver coat variant, which was highly prized by trappers. From reductions in tusk size among elephants [15] to horn size among bighorn sheep [32], hunting selects for those traits that keep animals out of the cross-hairs [30].…”