A.N. Severtsov Institute of Ecology and Evolution, RAS
119071 Moscow, Leninsky Pr., 33
Reproduction is a key and energetically costly aspect of animal life-history, which requires accurate choice among alternative mating tactics and reproductive decision-making in the context of species-specific life-history trade-offs to maximize individual lifetime reproductive success and fitness. In this paper we summarize and discuss the results of our long-term study of life-history and reproduction in yellow ground squirrel (Spermophilus fulvus). Despite their relatively big size and extremely long hibernation, yellow ground squirrels exhibit fast-living life-history pattern — they grow fast, maturate early and do not live long (2—3 years only) as compared with similar-sized or even smaller sciurid species. We show that, unlike many other ground squirrels, in S. fulvus female annual reproductive effort and success is independent from age, body condition or previous reproduction. The only factor found to influence female reproduction is the date of emergence from hibernation: the later a female emerge, the lower is the offspring mass, and fewer offspring survive. At the same time, female annual reproductive investment in a single litter, as measured by litter mass at weaning, is very large and close to mean lifetime reproductive investment estimated for female mammal. We suggest that females do not try to spread reproductive effort in many breeding attempts over the life span but maximize reproductive effort in current reproduction, regardless of physical condition and age (i.e., do not exhibit bet-hedging reproductive strategy) because of high costs of missed reproductive opportunity in this short-living species combined with low costs of breeding. However, this does not explain why, annually, the substantial portion of females (about 30%) do not produce litters. We suggest that female reproductive failures should be caused by a shortage of males. Adult sex ratio within the colony during the mating season is female-biased due to higher mortality and later maturation in males as compared with females. Moreover, males in poor physical condition demonstrate a trade-off between somatic needs and reproduction - they use "passive" tactic of residing near their burrow, do not exhibit mate-searching tactic and encounter fewer females as compared with adult males in good condition. Finally, in the mid-mating season, when a lot of emerging females come to estrus, operational sex ratio (OSR, the ratio between females and males ready to mate) is skewed toward females. All this combined with solitary living at low population density and low mobility of females lead to positive relationship between male availability and female breeding success. We found out that a female's probability to produce a litter increased with the local density of males and at more male-biased OSR at time of female's receptivity. Thus, for the first time we showed that male limitation can significantly contributes to female mating failure in a wild intact mammal population. This pattern is rarely recorded in animals with conventional sex roles, and in yellow ground squirrel it can be attributed to a combination of species socio-demographic and life history traits related to fast living.