Trevor A. Branch

Multiplet pregnancies are rare in cetaceans, and live‐born multiple births even rarer. We used whaling data collected from pregnant females and held by the International Whaling Commission to examine multiplets in 16 cetacean species, finding that 0.87% (2,197 out of 252,651) of pregnancies included multiple fetuses, including 12 instances of five or more fetuses in five different species, all rorquals. For six species, we estimate the proportion of twins that are identical using Weinberg's differential rule. Identical twins are less common than fraternal twins in all species (4%–34%) except humpback whales (57%). To infer survival rates of twins to term, we fitted models to the proportion of twins by fetal length, finding that the probability of surviving twins declined exponentially to near zero with increasing fetal length for sei, fin, and blue whales; but not for sperm, humpback, and Antarctic minke whales. Because some fetuses may be missed at smaller lengths, we repeated this analysis excluding fetuses shorter than 2 ft (61 cm), finding that twin survival declined with increasing fetal length for sei, fin, blue, and Antarctic minke whales. We conclude that few multiple pregnancies (<1 in 500) end in live birth, and even these pregnancies experience high mortality after birth.