• North Atlantic right whale;
  • DNA sex identification;
  • sex ratio


To test hypotheses involving reproduction and demographics, the sex of individuals must be established, but many species of Cetacea are not obviously dimorphic. In the North Atlantic right whale, Eubalaena glacialis, population, the sex of 61 males and 55 females had been determined previously by observation of the urogenital region, and the sex of 43 more females had been inferred from repeated sightings with a calf. To confirm the sex of some of these animals and to identify the sex of mote animals, genomic DNA was isolated from skin samples of 95 individual right whales (54 from among those described above and 41 additional recognizable individuals). The DNA was surveyed using the human Y-chromosome probe pDP1007. With EcoRI-digested DNA, a clear, sex-discriminating banding pattern was apparent. This method verified the sex of all 54 animals whose sex was previously known or inferred and identified the sex of an additional 41 recognizable individuals. A total of 89 male and 111 female right whales was identified in the population. The most unbiased estimate of sex ratio available is the 36 male and 34 female calves identified by genital morphology and DNA techniques. The sex ratio of this sample does not differ significantly from unity (P = 0.811). Only 38% (58/152) of the females in the North Atlantic population are known to have been reproductively successful compared with 54% in the population of right whales in the western South Atlantic. The population growth rate reported for the North Atlantic population is only 33% of that reported for right whales in the South Atlantic. Thirteen adult North Atlantic females have been identified that have not been known to calve during the past 11 yr. These data suggest that the absence of measurable recovery may be due to a combination of fewer actively reproducing females and lower reproductive rates of some females.