Horn growth variation and hunting selection of the Alpine ibex

Selective hunting can affect demographic characteristics and phenotypic traits of the targeted species. Hunting systems often involve harvesting quotas based on sex, age and/or size categories to avoid selective pressure. However, it is difficult to assess whether such regulations deter hunters from targeting larger "trophy" animals with longer horns that may have evolutionary consequences. Here, we compile 44,088 annually resolved and absolutely dated measurements of Alpine ibex (Capra ibex) horn growth increments from 8,355 males, harvested between 1978 and 2013, in the eastern Swiss Canton of Grisons. We aim to determine whether male ibex with longer horns were preferentially targeted, causing animals with early rapid horn growth to have shorter lives, and whether such hunting selection translated into long-term trends in horn size over the past four decades. Results show that medium- to longer-horned adult males had a higher probability of being harvested than shorter-horned individuals of the same age and that regulations do affect the hunters' behaviour. Nevertheless, phenotypic traits such as horn length, as well as body size and weight, remained stable over the study period. Although selective trophy hunting still occurs, it did not cause a measurable evolutionary response in Grisons' Alpine ibex populations; managed and surveyed since 1978. Nevertheless, further research is needed to understand whether phenotypic trait development is coinfluenced by other, potentially compensatory factors that may possibly mask the effects of selective, long-term hunting pressure.