Summary1. 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.

Summary1. Factors leading to differences in age of primiparity and possible life-history implications arising from these differences were studied in a population of Columbian ground squirrels ( Spermophilus columbianus ; Ord, 1815). We used data collected from 1992 to 2000 on two neighbouring colonies of ground squirrels in south-eastern Alberta, Canada. 2. Earlier born females were more likely to wean successfully a litter at the age of 2. Among 2-year-old females, those who emerged earlier and heavier from hibernation were more likely to wean offspring than those females who emerged later and lighter from hibernation. The presence of the mother had a significant positive impact on reproductive output in 2-year-old females. 3. Although there was a positive correlation between lifetime reproductive success and age at death, there was no difference in lifetime reproductive success or in age at death, depending on age of primiparity. For individual females, litter size at weaning at 2 years old did not differ from that at 3 years old. 4. We conclude that body condition, presence or absence of the mother and environmental conditions are the main factors leading to differences in age of first successful reproduction in Columbian ground squirrels. Previous breeding experience does not seem to play a role in determining reproductive success. Finally, differences in age of first successful reproduction do not seem to have an effect on lifetime reproductive success or longevity.

To maximize fitness, organisms must optimally allocate resources to reproduction, daily metabolic maintenance, and survival. We examined multiple years of live-trapping and observational data from a known-aged population of female Columbian ground squirrels, Spermophilus columbianus (Ord, 1815), to determine the influences of stored resources and daily resource income on the reproductive investments of females. We predicted that because yearling females were not fully grown structurally while producing their first litter, they would rely exclusively on income for reproduction, while reproductive investment in older females (≥2 years of age) would be influenced by both stored resources (capital) and daily income. Results from path analysis indicated that both yearlings and older females were income breeders. However, initial capital indirectly influenced investment in reproduction of yearling and older females. Females with the greatest initial capital maintained high body masses while investing relatively more income in reproduction. By considering influences of both capital and income, important relationships can be revealed between these resources and their influence on life histories.

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2005. Somatic senescence: evidence from female Richardson's ground squirrels. Á/ Oikos 108: 591 Á/601.We studied reproductive and somatic investments in /700 female Richardson's ground squirrels (Spermophilus richardsonii ) of known age over a 14-year period to evaluate three hypotheses, restraint, senescence, and residual reproductive value, proposed to explain age-specific life history patterns in iteroparous vertebrates. We found that reproductive investment, measured as litter mass at first emergence from the natal nest, did not differ among age classes. Although yearling female Richardson's ground squirrels made a greater somatic investment during reproduction than older females, they produced similar numbers and mass of offspring as older females. Reproductive investment did not decline with age, though active season somatic investment was lowest in the oldest females. Somatic investment during reproduction was highest in yearlings. This combination of age-related changes in somatic investment unaccompanied by changes in reproductive investment was not well explained by any of the hypotheses examined, though the senescence hypothesis best explained the combination of declining somatic investment and declining survival of the oldest females. Our results supported the ideas that reproductive and somatic senescence evolve independently, and that somatic senescence may be more common in relatively smaller species.