Estimating proportions of identical twins and twin survival rates in cetaceans using fetal data

Abstract

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.

Geographical distribution of close kin in southern right whales on feeding grounds

“…It should be emphasized that this is a probabilistic classification, which will not be error-free. For instance, twins are rare [ 44 , 45 ], and we do not have any evidence of monogamy in this species, so FS can also be expected to be rare. Hence, although some pairs are classified as full siblings using the maximum likelihood classification, the full sibling distribution (not shown in Fig 2 ) has a large overlap with both the half sibling and parent-offspring distribution, and the evidence that they are true full sibling pairs is weak.…”

Estimating reproductive costs in marine mammal bioenergetic models: a review of current knowledge and data availability

“…been reported in all taxonomic groups of marine mammals (Kawamura, 1969;Spotte, 1982;Jameson and Bodkin, 1986;Fay et al, 1991;Osborn et al, 2012;Davison et al, 2016;Gutiérrez et al, 2021;Drinkwater and Branch, 2022), but these reports are uncommon to rare. In his review on the incidence of twins in pinnipeds, Spotte (1982) noted that twins are typically underweight for their age, with no reports of both twins surviving to weaning in the wild.…”

Report of the Scientific Committee

“…The Committee welcomed this information, especially given the paucity of recent information on breeding off the west coast of southern Africa. Drinkwater and Branch (2022) investigated the rate of twin pregnancies in 16 cetacean species using whaling data held by the IWC. They found that overall 0.87% (2,197 out of 252,651) pregnancies included multiple foetuses, 0.81% in blue whales (244 out of 30,969).…”