Age-related gestation length adjustment in a large iteroparous mammal at northern latitude

1. There is considerable interest in patterns of age-dependent reproductive effort and reproductive timing of large iteroparous mammals living in strongly seasonal environments. Due to lack of data on both timing of mating and birth, there is generally little insight into whether variation in gestation length play a role for life-history patterns observed for large mammals at northern latitudes. 2. Based on data on both timing of mating and birth of 88 female reindeer (and paternity confirmed with DNA fingerprinting), we explore the view that adjustment of gestation length plays a role in the reproductive tactic. 3. Observed gestation lengths of reindeer varied between 211 and 229 days (mean of 221 days). Consistent with a dynamic view of gestation length, variation could be predicted from life-history traits. Gestation length was longer for male than female offspring, which is expected in polygynous species where males benefit more from extra allocation of maternal resources. Gestation length increased with maternal age both due to direct effects and indirect effects linked to earlier mating of older, heavier females. Early mating females increased gestation length. A relatively small effect of female age on birth mass operated through variation in gestation length. 4. Our analysis supports the view that adjustment of gestation length is a part of the reproductive tactic of large mammals in northern environments.