Families in space: relatedness in the Barents Sea population of polar bears (Ursus maritimus)


  • Box 1 Definitions of terms used

    Kin structure: spatial structure within populations where more related individuals are closer to each other in space than to unrelated individuals.

    Dispersal: process by which organisms constantly move from one place to another.

    Natal dispersal: dispersal from the natal area to the area where animals first reproduce. Herein estimated as the distance between adult mothers and their adult offspring.

    Secondary dispersal: dispersal of adult animals after natal dispersal.

    Breeding dispersal: secondary dispersal for breeding purposes.

    First-order relatives: parent–offspring pairs and sibling/half-sibling pairs.

    Second-order relatives: distant relatives such as cousin pairs, aunt/uncle–niece/nephew pairs, grandparent–grandoffspring.

    Reproductive pairs: female and male which fostered common offspring, i.e. parents.

    Family: individuals linked by a pedigree.

    Matriline: females linked by a pedigree through female ancestors.

    Patriline: males linked by a pedigree through male ancestors.

Eve Zeyl, Fax: +47 22 851837; E-mail: eve.zeyl@nhm.uio.no


The kin structure and dispersal pattern of polar bears (Ursus maritimus) of the Barents Sea was investigated during the spring mating season using two complementary approaches. First, individual genotypes based on the analyses of 27 microsatellite loci of 583 polar bears were related to field information gathered from 1146 bears in order to reconstruct the animals’ pedigrees and to infer geographical distances between adult bears of different relatedness categories. According to the data, the median natal dispersal distance of the male animals was 52 km while that of the females was 93 km. Second, the relatedness of pairs of adult bears was estimated and correlated to the geographical distance between them. The female dyads had a much stronger kin structure than the male dyads. The ‘pedigree approach’ revealed a male kin structure which could not be detected using the ‘relatedness approach’. This suggests that, on a broader scale, effective dispersal is slightly male biased. Despite fidelity to natal areas, male-mediated gene flow may nevertheless prevent genetic differentiation. Males might occasionally shift their home range which could therefore lead to a male-biased breeding dispersal. Our results showed that a nonterritorial species such as the polar bear that has a high dispersal potential, lives in a highly unstable environment and migrates seasonally is still able to exhibit a distinct kin structure during the mating season.