Molecular evidence for twinning in Weddell seals (Leptonychotes weddellii)

We used microsatellite genotyping to determine the genetic relatedness of 7 apparent twin dyads of Weddell seals (Leptonychotes weddellii) born in McM urdo Sound, Antarctica. Genetic evidence for twinning in wild pinnipeds has not been reported previously. A review of 14 years of demographic data com bined with pathological exams, behavioral observations, and molecular genet ic evidence suggests that twinning is extremely rare in Weddell seals and t hat females do not wean both pups in the wild. The incidence of live twin b irths was about 0.1% (2/1,439 births), recorded over 3 seasons in Erebus Ba y, McMurdo Sound. Additionally, a single case of true twinning was document ed from 23 known pregnancies observed in an isolated population of Weddell seals located within McMurdo Sound. The 3 twin sets were dizygotic full sib lings, 1 nontwin dyad represented a case of adoption, and the 3 remaining p utative twin sets were identified as instances of foster nursing, These res ults indicated that observation of mother-offspring behavior was not a reli able method for identifying a twin birth in this species. Use of genetic te chniques to verify presence of twins in species with low or unknown twinnin g rates offers the opportunity for a refinement of estimates in studies of reproductive success, fostering behavior, and adoption.