Why do Columbian ground squirrels copulate underground?

Abstract

Columbian ground squirrels (Spermophilus columbianus) usually copulate underground in a burrow. Underground copulations might be associated with 2 nonexclusive benefits: reducing probability of predation during copulation and reducing interference by conspecific males. We tested whether either of these benefits was involved in determining if the copulation site was underground or aboveground. In 2006 and 2007, we obtained detailed information on the copulatory behavior and social interactions of free-ranging individuals in southwestern Alberta, Canada. During the 3-week annual breeding period, we also recorded the activity of predators of Columbian ground squirrels such as ravens, foxes, and hawks. Squirrels that lived on the periphery of the population were more susceptible to predation than squirrels in the center. Despite this risk, aboveground copulations usually occurred on peripheral territories. In addition, aboveground copulations were not further removed in time from predator attacks or sightings than underground copulations. Copulations that occurred aboveground were sometimes disrupted by previous mates of the estrous female. Probability that copulation would occur aboveground increased when the density of reproductive males around an estrous female was low. Our results suggest that although underground copulations protect individuals from predation, male-male competition for females and interference with copulations have been more important than predation in determining copulatory sites for Columbian ground squirrels in our study population. © 2008 American Society of Mammalogists.