Is It a Boy or a Girl? Testing Hypotheses to Explain Variable Sex Ratios in Columbian Ground Squirrels

International audienceThree main hypotheses have been proposed for explaining departures from even sex ratios in animal populations, based on variation in maternal and social characteristics. The Trivers–Willard (1973) model proposes that mothers in good condition will produce more males, based on their competitive ability, and superior future reproduction. The local resource competition model proposes that philopatric females will produce more sons in large matrilines, to reduce future competition with daughters. A final model invokes advantages of maintaining matrilines, despite increased competition among females. We evaluated these hypotheses in the Columbian ground squirrel (Urocitellus columbianus), a semi-social species characterized by strong female philopatry