Under pressure: changes in maximum tusk size in hunted savanna elephant (Loxodonta africana) populations

Abstract

Conservation concerns are not limited to ensuring the survival of species, populations and individuals, but also about maintaining the variation within and between populations, including genetic and phenotypic variation. For African savannah elephants (Loxodonta africana), large intact tusks are a phenotypic trait targeted by human hunters. This represents a potential selective pressure, with the capacity to impact the average tusk size of a given population. In this study, we used trophy record data to investigate trends in larger tusk size (considering weight and length) in elephants at a continent-wide level across a 100-year period. We accounted for baseline differences between populations by incorporating phylogenetic data into generalised mixed effects linear models. We found no significant change in tusk weight over time and a significant decrease in tusk length (1.28cm/year). One explanation for this pattern, given that elephant tusks grow continuously, is that elephants with longer tusks could be hunted at a younger age before their tusks reach their full length, and that those that survive to an older age have shorter tusks with a stable weight. Because elephants have a generation time of 17 years and a lifespan of up to 65 years, the finding of a significant decrease in tusk length in the space of a century is of concern. Policy implications for this study include reviewing illegal hunting measures and adapting local legal hunting laws to minimise phenotypic loss. This would contribute to the conservation of phenotypic diversity in savannah elephant populations across the African continent.