Mating Tactics and Reproductive Success in Male Columbian Ground Squirrels (Urocitellus columbianus)

Abstract

The widespread discovery of multiple mating tactics, particularly among males, highlights the diverse ways sexual selection may operate within a population. In polygynandrous populations, multiple paternity may arise from females who can produce multiple offspring at once. Alternate mating tactics can arise if there is an increase in competition among males or in the presence of sperm competition. Questions have been raised on how fitness is maximized in mating systems in which there is multiple paternity in a single litter. To understand sexual selection in polygynandrous mating systems, I used Columbian ground squirrels (Urocitellus columbianus) as my study species. Male Columbian ground squirrels have two main tactics, territorial and non-territorial. Futhermore, their reproductive success is influenced by the male’s mating order in the female’s queue. If the first male mate sires the majority of the offspring, he is said to have the “first male advantage.” Variation in the number of female mates that the male is able to obtain in a season may also influence his reproductive success (Bateman’s 3rd principle). The purpose of my first study was to examine the relative importance of number of mates (Bateman’s 3rd Principle) and average rank for male Columbian ground squirrels. Using a standardized partial regression, I quantified the relative contributions the number of female mates and average rank to the reproductive success for Columbian ground squirrel males. I found that the number of mates influenced male reproductive success three times more than average rank. I also examined if territoriality affected reproductive success and found that territorial males sired more offspring, had more mates, had lower average rank, and were older than non-territorial males. In my second study, I examined whether being a subordinate male was an equivalent alternative mating tactic to being a territorial male or whether subordinate males are simply “making the best of a bad job.” I found that territorial males received a higher proportion of matings, had a higher proportion offspring, and had higher odds at receiving a mating for the first four matings. Territorial males were not more efficient than non-territorial males at inseminating females at any copulation rank.