Senescence and age‐related reproduction of female Columbian ground squirrels

Abstract

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.

Age-related patterns of reproductive success among female mountain gorillas

“…The pattern qualitatively resembles natural fertility populations of humans (i.e., those without contraception Wood, 1994), with the notable exception that the gorillas lack an extended PRLS. Now that the age-related patterns have been identified, the underlying causes should be discerned in more detail by measuring factors such as the weight of the mother and offspring at parturition and at weaning (Broussard et al, 2003), nursing behavior (Cameron et al, 2000), and the length of each phase within an IBI (Setchell and Wickings, 2004). Meanwhile, we present some preliminary conclusions below, based upon the particular reproductive measures in which the age-related patterns were most pronounced.…”

“…Offspring mortality was higher for primiparous mountain gorillas below age nine. Lower reproductive success has been reported for primiparous and/or younger females in many species (Anderson, 1986;Borries et al, 1991;Bercovitch et al, 1998;Broussard et al, 2003). Although differences between primiparous and multiparous females could reflect maternal experience, age-related patterns among primiparous females seem more likely to arise from differences in physical condition and/or maternal investment.…”

“…A key goal of life history theory is to explain how age and parity affect the reproduction of iteroparous organisms (Charlesworth, 1980;Stearns, 1992). Age-related patterns have been observed in pregnancy rates (Marsh and Kasuya, 1986), birth rates (Packer et al, 1998;Nishida et al, 2003), interbirth intervals (IBI) (Paul et al, 1993), the size and number of offspring (Mysterud et al, 2002;Broussard et al, 2003), maternal behaviors (Timmermans and Vossen, 1996;Cameron et al, 2000), and offspring survival frequencies (Ericsson et al, 2001). Agerelated patterns may reflect changes in the level of maternal investment, maternal experience, and the physical condition of the mother.…”

“…The evolutionary restraint hypothesis predicts that young mothers should have relatively low investment in their current offspring, and relatively high investment in their own maintenance and continued growth (Curio, 1983;Broussard et al, 2003). The terminal investment hypothesis states that aging mothers should increase their reproductive effort in current offspring, as their potential for having future offspring (reproductive value) declines (Fisher, 1930;Williams, 1966).…”

Senescence in Animals: Why Evolutionary Theories Matter

“…In fact, in recent years, several studies have demonstrated demographic senescence (Austad, 1993;Bronikowski et al, 2002;Ericsson et al, 2001;Orell & Belda, 2002) and reproductive senescence in mammals and birds in their habitats (Austad, 1993;Broussard et al, 2003;Ericsson et al, 2001;Reid et al, 2003;Saino et al, 2003).…”