Illegal tusk harvest and the decline of tusk size in the African elephant - PubMed (original) (raw)

. 2015 Oct 22;5(22):5216-5229.

doi: 10.1002/ece3.1769. eCollection 2015 Nov.

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

Patrick I Chiyo et al. Ecol Evol. 2015.

Abstract

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.

Keywords: Anthropogenic impacts; evolution of morphology; hunting; inheritance of incisors; ivory; selection; tusk evolution; tusklessness.

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Figures

Figure 1

Figure 1

The location of elephant populations sampled in 1966–1968 (Tsavo East and Mkomazi National Parks) and in 2005–2013 (Shimba hills National reserve, Narok and Ol Pejeta wildlife conservancies and Tsavo East and Tsavo West National Parks).

Figure 2

Figure 2

Predicting total tusk length from exposed tusk length in contemporary females (A, total tusk length = 23.73 + 1.159 × exposed tusk length_, R_ 2 adj = 0.956), and males (B, total tusk length = 20.05 + 1.294 × exposed tusk length, R 2 adj = 0.974).

Figure 3

Figure 3

Variation in tusk length as a function of shoulder height for the elephant samples collected in 1966 – 1968 (black) and 2005–2013 (red). Contemporary males (A) and females (B) born after 1995 and onwards and males (C), and females (D) born by 1970 (survivors) compared with male and female elephants of similar age sampled in 1966–1968. Lines of model fit are for visualization only.

Figure 4

Figure 4

Variation in tusk length as a function of tusk circumference at base in (A) male and (B) female African elephants from Tsavo National Park in 1966–1968 and 2005–2013. The black circles show data for the 1966–1968 tusk samples, and the red or gray circles show the 2005–2013 tusk samples.

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