Constraints on First Reproduction in North American Red Squirrels

Abstract

Age at first reproduction influences lifetime reproductive success of individuals and growth rates of populations, and is thus of general interest to ecologists. In red squirrel (Tamiasciurus hudsonicus) populations, yearlings and older nulliparous females (age two and above) are less likely to have a litter than multiparous females (i.e. those that have bred before). To explain these life history traits we tested several hypotheses for why particular females fail to breed in a given year. In a five-year study of red squirrels in a jack pine (Pinus banksiana) forest in central Alberta, Canada, the probability of primiparity (i.e. producing a litter for the first time) was correlated with cone crop size, but doubling the caches of cones available to females failed to increase the probability of first time reproduction. Cones were never completely depleted on any territory, so first reproduction rarely appeared to be restricted by absolute food availability on a territory. We hypothesize, that first reproduction is constrained by the squirrel's ability to obtain seed energy enclosed in serotinous cones that vary in distribution on a territory. We found that the probability of primiparity is lower and more variable between years than that of multiparity, because foraging and cone-handling efficiencies vary more in younger than in older squirrels. Nulliparous females given sunflower seeds to reduce handling time were two to six times more likely to produce a first litter than controls extracting seed from jack pine cones. Small body size, inexperience with seed extraction from serotinous cones, and a lack of strategies for gathering and using cones from a new territory are supported as mechanisms that constrain primiparity, especially in yearlings. Cold spring temperatures added a further constraint in one year, as did absolute cone crop size. Multiparous females strip the bracts off cones in the autumn, reducing handling costs in winter and spring, while young nulliparous females lacked such behavioural compensation. In jack pine, reproductive success of red squirrels depends upon behaviour adapted to a seed supply that is abundant but costly to extract from serotinous cones.