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Data on the body length (cm) of polar bear (Ursus maritimus), collected as part of a long‐term ecological study in the Hudson Bay region of Canada, were examined to test the hypothesis that growth early in life influences adult body size. Data for 42 females and 30 males that were captured as two‐year‐olds (2.6‐2.9 years), after the period of maternal care, and later recaptured as adults were analysed. On average, females increased in length by 11.2% and males by 23.1 % between captures. Relative adult body length was significantly correlated with relative length as a two‐year‐old in females but not in males. Thus, body length attained by two‐year‐olds is a weaker determinant of adult body length in males than in females. We suggest that, in comparison with females, prolonged growth beyond the period of maternal care may predispose the growth and eventual adult body size of male offspring to a greater degree of environmentally mediated variation. Furthermore, while data on maternal investment strategies in polar bears are lacking, variation in the allocation of maternal resources to cubs, within and among litters, may have a limited impact on the eventual adult body size of any surviving male offspring.

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ABSTRACT. During a study of the reproductive ecology of polar bears (Ursus maritimus) in western Hudson Bay (Canada), we documented a case of litter adoption. In an eight-month period, a ten-year-old adult female lost a litter of two cubs-ofthe-year and adopted three other cubs-of-the-year. This is the first reported case of natural offspring adoption in polar bears, and its significance as a reproductive strategy is unknown. Nevertheless, the observation raises questions regarding the social circumstances under which adoption may occur and the benefits or costs to maternal fitness in a solitary mammal such as the polar bear.Key words: polar bear, Ursus maritimus, litter adoption, maternal fitness RÉSUMÉ. Au cours d'une étude portant sur l'écologie de la reproduction des ours polaires (Ursus maritimus) dans la partie ouest de la baie d'Hudson (Canada), on a documenté un cas d'adoption de litière. Au cours d'une période de huit mois, une femelle adulte de dix ans a perdu sa litière de deux petits âgés de moins d'un an et adopté trois autres petits de l'année. Cet événement est le premier cas d'adoption naturelle que l'on ait rapporté chez l'ours polaire, et on n'en a pas encore mesuré l'importance en tant que stratégie de reproduction. L'observation soulève cependant des questions concernant les conditions sociales dans lesquelles peut se produire l'adoption et les avantages ou les coûts pour la santé maternelle chez un mammifère solitaire tel que l'ours polaire. The polar bear (Ursus maritimus) is a large, solitary carnivore that inhabits arctic and subarctic sea ice and adjacent terrestrial regions (DeMaster and Stirling, 1981). This sexually dimorphic mammal appears to have a polygynous mating system, in which parental care is provided exclusively by females (Ramsay and Stirling, 1986). Relatively little is known, however, about the reproductive strategies of female polar bears (Ramsay and Stirling, 1988). Here we report an observation of offspring adoption in this species.We routinely sampled free-ranging polar bears at different seasons as part of a long-term study of polar bears in the Cape Churchill region of western Hudson Bay, Canada. A Bell 206B helicopter was used to search for and capture the polar bears. Animals older than one year were anaesthetized by remote injection of Telazol © (A.H. Robins Co., Richmond, VA, U.S.A.) using standard methods (Stirling et al., 1989). Cubs-of-the-year were anaesthetized by hand or jab-pole injection of Telazol © . Unmarked bears were assigned a unique identifying number, which was applied as a tattoo on both left and right buccal surfaces of the upper lip and also as two Delrin® ear tags. Adult females were fitted with very high frequency (VHF) radio transmitters to allow relocation. A vestigial first premolar tooth was extracted from adults and subadults for age determination (Grue and Jensen, 1979).After research activities were complete, we left the bears, which were allowed to recover normally.On 5 March 1994, a ten-year-old adult female polar bear was captured ...

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