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

Abstract

(1) Background: Parasites can significantly impact growth, reproductive success, and the development of secondary sexual characteristics in various species. Our study investigated the effect of gastrointestinal and lungworm parasite infections on the total annual horn growth of male bighorn sheep (Ovis canadensis canadensis) in Sheep River Provincial Park, Alberta, Canada. (2) Methods: We collected fecal samples of individually marked male bighorns over two years to investigate parasite egg and oocyst numbers and parasite diversity in feces, and how these could potentially affect their annual horn growth. (3) Results: We found that parasite species richness, year, age class, and the interaction between age class and species richness were significant predictors of horn growth, while individual parasite species did not have a significant effect. Notably, parasite species richness positively affected horn growth in young males, whereas it did not predict horn growth in adult and old males. (4) Conclusions: One possible explanation is that young males might prioritize resources for body and horn growth, potentially at the expense of immunity or parasite resistance. Our finding contradicts the idea of parasite-mediated sexual selection, where traits like bright plumage signal good health and parasite resistance.

Keywords: Eimeria; age; bighorn sheep; horn growth; lungworms; nematodes; parasite diversity.

Figure 1

Counts of the different parasites found in bighorn sheep feces belonging to different age classes of males (young = aged 2–4, adult = aged 5 to 8, and old = aged 9 to 14 years old); these counts are numbers of eggs/larvae/oocysts per 4 g of wet fresh feces. Shown are median counts with interquartile (IQR) ranges (boxes), lines extending from the smallest and largest values within 1.5 times the IQR from the lower and upper quartiles, and dots representing potential outliers.

Figure 1

Counts of the different parasites found in bighorn sheep feces belonging to different age classes of males (young = aged 2–4, adult = aged 5 to 8, and old = aged 9 to 14 years old); these counts are numbers of eggs/larvae/oocysts per 4 g of wet fresh feces. Shown are median counts with interquartile (IQR) ranges (boxes), lines extending from the smallest and largest values within 1.5 times the IQR from the lower and upper quartiles, and dots representing potential outliers.

Figure 2

Predicted effects of parasite species richness on bighorn sheep horns growth across age classes, based on the linear mixed-effects model. Lines represent marginal predictions with 95% confidence intervals for the interaction between age class and standardized parasite species richness.