A Pain in the Butt: The Association Between Endo-Parasite Diversity and Horn Growth in Rocky Mountain Bighorn Sheep

A counterintuitive link between parasite diversity and horn growth in male bighorn sheep challenges established theories of sexual selection. Researchers found that young Rocky Mountain bighorn sheep (Ovis canadensis canadensis) with higher gastrointestinal and lungworm parasite species richness exhibited faster annual horn growth, contrary to expectations. The study, conducted in Sheep River Provincial Park, Alberta, analyzed fecal samples from individually marked males over two years, measuring parasite egg and oocyst counts alongside horn growth metrics. While adult and older males showed no significant relationship between parasite diversity and horn growth, young males appeared to prioritize resource allocation toward horn development, potentially at the expense of immune function or parasite resistance. This finding contradicts the traditional 'parasite-mediated sexual selection' hypothesis, which suggests that elaborate traits like horns signal robust health and parasite resistance.

Henry, T. C., Rijal, S., Alves, J., Neuhaus, P., Kutz, S., & Ruckstuhl, K. E. (2025). A Pain in the Butt: The Association Between Endo-Parasite Diversity and Horn Growth in Rocky Mountain Bighorn Sheep. Pathogens, 14(7), 623. https://doi.org/10.3390/pathogens14070623