Senescence and age-related reproduction of female Columbian ground squirrels

1. Three hypotheses have been proposed to explain age-structured patterns of reproductive investment and somatic investment: residual reproductive value, senescence and evolutionary restraint. We evaluated these hypotheses for female Columbian ground squirrels (Spermophilus columbianus) by examining age-related patterns of somatic and reproductive investment. Females were designated as successful (those that weaned litters) and unsuccessful (those that did not wean litters). 2. Somatic investment varied among both successful and unsuccessful females of different ages, with yearlings having the highest investment. Considering all females, reproductive investment varied among age classes with yearlings and the oldest (6-9-year-olds) having the lowest investments. However, when only successful females were considered, reproductive investment was lowest in the yearlings and not significantly different among older females. 3. The highest proportion of successful females occurred in the middle adult age classes, while yearlings and the oldest females displayed the lowest proportion of successful females. During the breeding season, somatic investments of successful and unsuccessful females differed significantly only in the yearling age class, with unsuccessful females having the highest investment. 4. Evolutionary restraint or constraint explained patterns of reproduction in the yearling age class, where both reproductive investment and proportion of reproductive females were low. There was evidence for senescence of reproduction by some of the oldest females.