Effects of spatio-temporal variation in food supply on red squirrel (Sciurus vulgaris) body size and body mass and its consequences for some fitness components

Food availability is likely to influence body condition and, in turn, fitness. The intensity of this response may
vary between populations of the same species on a small spatial and temporal scale. We used 5 yr of data from 6
Eurasian red squirrel Sciurus vulgaris populations from the southern Alps to explore differences in body size and
body mass among neighbouring populations, in relation to habitat type and variation in food supply. We also
investigated sexual dimorphism in these traits and whether phenotypic variation affects local survival and female
reproductive success. Mean hind foot length, a measure of body size, did not differ between sexes but differed
between areas. Seasonal variation in body mass was small with no evidence for fattening in autumn. Females
were slightly heavier than males, but this difference was largely explained by mass gain of females during
reproduction. The size of conifer seed crops, the major food supply, varied strongly over years and between
habitats, but this variation corresponded only weakly with autumn body mass. Differences in size and mass
between populations were partially explained by habitat-related differences in body size and variability of seedcrops,
suggesting differential selection for smaller squirrels in spruce-larch forests against selection for larger and
heavier animals in mixed broadleaves and conifer forests and in Scots pine forests with more stable seed
production. The probability of reproduction by females increased with body mass, but varied strongly between
habitats and years, with more females reproducing in years with rich seed-crops. In both sexes, body mass
positively affected probability of settlement and length of residency. Our results suggest that in temporally
variable environments that differ in overall amount of food resources, individual variation in body mass is
related to habitat type, and that having a relatively high body mass, within each population, positively affects
male and female settlement success and local survival, and female reproductive success.