The incidence of twins in pinnipeds is reviewed. Correspondence and a literature search revealed 42 documented cases involving 11 species (six phocids, five otariids). The greatest incidence was found in the grey seal and northern fur seal (7 and 11 cases, respectively). Two verified sets of triplets are known, both delivered by a single, captive grey seal. The odds of both pups of a viable twin birth surviving are slim. Both adoptive suckling and the suckling of two pups occur in some phocids, and the chances that twins will be reared are perhaps greater in these species. Neither behavior has been observed in otariids in the wild; however, a California sea lion that gave birth to twins in captivity suckled both pups until one was removed on day 15. Only two sets of twins (one set each of grey seals and harbor seals) are known to have survived to weaning, and both were captive born. The reproductive biology of pinnipeds does not favor twins, and their occurrence is rare, but probably not so rare as currently believed, because many observations are not published.
