MATERNAL INVESTMENT AND FACTORS AFFECTING OFFSPRING SIZE IN POLAR BEARS (URSUS-MARITIMUS)

We examined data on live-captured polar bears (Ursus maritimus) from w estern Hudson Bay relative to differential maternal investment and fac ultative sex ratio theory. In spring, shortly after den emergence, we found no differences in mass or body length of cubs in litters of one or two. In triplet litters captured in spring, male cubs were larger t han females in both mass and body length, possibly due to sibling comp etition for limited maternal resources. In cubs captured during autumn , sex-based differences in mass and body length were found only in lit ters of one with males larger than females. Yearling males captured in autumn were heavier and longer than females in litters of one and two . In twin litters of one male and one female cub in spring and autumn, we found no differences in mass or body length. Except singleton fema le cubs in spring and female and male triplets in spring, cub and year ling mass were correlated with maternal mass. Mothers with singletons in spring were more likely to have male cubs but, in general, there wa s little evidence to suggest that mass or age of mothers influenced of fspring sex. Litter size, maternal mass, and maternal age all influenc ed cub and yearling size, but offspring sex was less important. Result s suggest that female polar bears do not normally invest differentiall y between offspring of different sexes except in triplet litters in sp ring.