Reproductive Decisions in a “Fast-Living” Sciurid: A Case Study of the Yellow Ground Squirrel (Spermophilus fulvus)

Abstract

The problem of the optimal distribution of time and energy between reproduction and the other vital functions and the choice of optimal reproductive tactics concerns all animals. The sequence of decisions largely determines overall individual reproductive success and fitness. The goal of the study was to summarize the factors determining the reproductive decision of the yellow ground squirrel (Spermophilus fulvus). Despite its relatively large size and extremely long hibernation, the yellow ground squirrels is characterized by rapid growth and maturation, in combination with a short life-span; the life cycle of the yellow ground squirrel proceeds according to the “fast” scenario. Analysis of the relationship between breeding indices of females and their individual characteristics has shown that the reproductive investment and success of females do not depend on the age, body condition, or previous reproductive history of the female. The only factor found to influence female reproduction is the date of emergence from hibernation: the later a female emerges, the lower the offspring mass is and the fewer offspring survive. At the same time, female annual investment in reproduction is very large and close to the mean lifetime reproductive investment estimated for female mammals. With a short life-span (approximately 2–3 years), females do not have the opportunity to distribute the reproductive effort across many breeding attempts, they seem to be able to recover quickly after reproduction. Nevertheless, a substantial portion of mature females (about 30%) annually do not produce litters for unknown reasons. The sex ratio among reproductively active individuals during the mating period is strongly biased toward females. This is due to the lower survival of males, the late maturation of young males in comparison with females, and the fact that, even in adult males, individuals in poor physical condition are not very active and hardly search for sexual partners, i.e., males demonstrate a trade-off in the distribution of costs for somatic processes and reproduction. When there is a surplus of females emerging from hibernation, there are more receptive females in the population than reproductively active males, and operational sex ratio (OSR, the ratio between females and males ready to mate) is skewed toward females. Since females live sparsely and are sedentary during the mating period, the probability of breeding for them depends on the availability of sexual partners: the females’ breeding chances increase with an increase in the local density of males and the proportion of males in the population at the time of the female’s estrus; the only explanation for the large proportion of non-breeding females is the shortage of sexual partners. Thus, the negative effect of the shortage of males on the probability of female reproduction is shown for the first time for a natural population of mammals; this opens a new perspective on the theory of sexual conflict. This phenomenon, which is not so typical of species in which the parental investment of the female far exceeds the male’s investment, is explained by the combination of sociodemographic conditions and the life-history patterns of the yellow ground squirrel.