Twins or not? Genetic analysis of putative twins in Antarctic fur seals, Arctocephalus gazella, on the South Shetland Islands

Genetic analyses can reliably determine the relationships among putative cases of twins in pinniped species. These studies demonstrate that field observations of nursing twins may often be cases of adoption or foster nursing of unrelated pups. A recent study of Antarctic fur seals ( Arctocephalus gazella ) on South Georgia Island found that only 3 of 11 putative twin cases were truly twins. Here we report results of genetic testing of eight putative cases of twinning (twin siblings and mother) observed at Cape Shirreff (62°27′30″S, 60°47′17″W), Livingston Island, Antarctica. Parentage and relatedness analyses using 18 microsatellite markers confirmed six out of the eight cases as twins and two cases of adoption/foster nursing of unrelated pups. All twins analyzed were dizygotic and in five out of six cases, the twins were likely full siblings (relatedness coefficient, or r xy μ = 0.46, σ 2 = 0.004). In one case, the twins were likely half-siblings ( r xy = 0.17), supporting a previous finding of heteropaternity in Antarctic fur seals. This result suggests that mate infidelity during estrus maybe common in Antarctic fur seals, which has implications for our understanding of this species's mating system. The twinning rate estimated at Cape Shirreff (0.12% or 6 twins per 4,965 births) is consistent with the scarcity of twin births observed in pinnipeds, which is associated with the high cost of nursing multiple pups in these animals.