Study suggests link between female bighorn sheep horn size and reproductive traits

A new link between horn size and reproductive fitness in female bighorn sheep highlights a potential conservation issue, according to a University of Alberta study.

The study found that females with small horns are less reproductively fit and are slow reproducers, says Samuel Deakin, first author and PhD candidate in the department of Biological Sciences.

Researchers examined 45 years of data on bighorn sheep in Ram Mountain, a remote area about 30 kilometres east of the Rocky Mountains near Nordegg, Alta. Data examined showed not just horn size, but also how many offspring each female gave birth to, and how the offspring fared — making it possible to measure reproductive fitness in this population.

While Deakin says the horns themselves aren’t directly affecting female reproductive fitness, there’s potentially some gene associated with horn size that is having an influence — and that link between horn size and reproductive fitness means trophy hunters targeting male bighorn sheep with larger horns could be a factor.

“Perhaps by taking away the strong, fit males, you’re actually affecting female fitness, which could in turn eventually slow population growth,” says Deakin.

The link between female horn size and reproductive fitness is new because those two factors aren’t commonly studies. Secondary sexual traits such as horns or antlers have a clear purpose for males, as they help them compete for mates, Deakin explains, but the link isn’t as clear for females of many species.

Trophy hunting is no longer permitted at the site where these findings are from. However, if the sheep Deakin studies are typical of the species, the same link between horn size and female reproductive fitness could be happening in other bighorn sheep populations, he says.