CONSTRAINTS ON FIRST REPRODUCTION IN NORTH-AMERICAN RED SQUIRRELS

Age at first reproduction influences lifetime reproductive success of individuals and growth rates of populations, and is thus of general in terest to ecologists. In red squirrel (Tamiasciurus hudsonicus) popula tions, yearlings and older nulliparous females (age two and above) are less likely to have a litter than multiparous females (i.e. those tha t have bred before). To explain these life history traits we tested se veral hypotheses for why particular females fail to breed in a given y ear. In a five-year study of red squirrels in a jack pine (Pinus banks iana) forest in central Alberta, Canada, the probability of primiparit y (i.e. producing a litter for the first time) was correlated with con e crop size, but doubling the caches of cones available to females fai led to increase the probability of first time reproduction. Cones were never completely depleted on any territory, so first reproduction rar ely appeared to be restricted by absolute food availability on a terri tory. We hypothesize, that first reproduction is constrained by the sq uirrel's ability to obtain seed energy enclosed in serotinous cones th at vary in distribution on a territory. We found that the probability of primiparity is lower and more variable between years than that of m ultiparity, because foraging and cone-handling efficiencies vary more in younger than in older squirrels. Nulliparous females given sunflowe r seeds to reduce handling time were two to six times more likely to p roduce a first litter than controls extracting seed from jack pine con es. 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 cons traint in one year, as did absolute cone crop size. Multiparous female s strip the bracts off cones in the autumn, reducing handling costs in winter and spring, while young nulliparous females lacked such behavi oural compensation. In jack pine, reproductive success of red squirrel s depends upon behaviour adapted to a seed supply that is abundant but costly to extract from serotinous cones.