Twinning in natural pinniped populations is often inferred from observations of suckling behavior, but this approach has been criticized because nonfilial nursing occurs at high frequencies in many seal species. Consequently, we used 9 highly polymorphic microsatellite markers to examine the parentage of 11 putative pairs of twins in Antarctic fur seals (Arctocephalus gazella) breeding at Bird Island, South Georgia. Only 3 pairs (27%) were found to be genuine twins, indicating that suckling observations are an unreliable means of identifying twins in this species. All of the twins were female; 1 pair was monozygotic and the other 2 were dizygotic. Using a strict exclusion approach, paternity was assigned to the monozygotic but not the dizygotic twins. However, likelihood tests revealed that, of the latter, 1 pair was significantly more likely to be full siblings against the null of half sibship suggesting shared paternity, whereas the other pair was more likely to be half siblings against the null of full sibship indicating probable multiple paternity. Our results provide novel insights into the reproductive ecology of fur seals and also support an earlier study showing that molecular genetic analysis can provide an effective means of validating field observations of pinniped twins.
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