Many species of animals exhibit distinct differences in the sizes of the male and female of the species. Differences between males and females of a species, whether the differences are in size, coloration, or seemingly decorative enhancements, are termed sexual dimorphism, meaning different forms based on the sex of the individual in the species. Notable examples include the Indian peacock (Pavo cristatus), African lions (Panthera leo), humans (Homo sapiens), peacock spiders (Maratus spp.), and elephant seals (Mirounga spp.), among many others.
Found along the Pacific coast of North America from the Aleutian Islands to Baja California, the northern elephant seal (Mirounga angustirostris) exhibits distinct sexual dimorphism, with males weighing as much as 4,500 pounds (2,000 kg) and measuring up to 15 feet (4.6 m) in length, while females usually weigh no more than 1,500 pounds (600 kg) and measure no more than 10 feet (3 m) in length. Their southern hemisphere cousin, the southern elephant seal (Mirounga leonina) is even more dimorphic sexually, with males weighing as much as 8,150 pounds (3,700 kg) and measuring up to 19 ft (5.8 m) in length with females weighing 1,980 pounds (900 kg) and measuring 10 feet (3 m) in length.
Such physical differences between the sexes in a species seem to be driven by the differences of all the selective evolutionary pressures experienced by the males as compared to those experienced by the females. In the case of the northern elephant seal, males fight to control areas on the beach occupied by large groups of females, with the winners usually being the larger, more aggressive males. Larger size in males may also increase their reproductive success, by allowing them to stay in the breeding area longer by utilizing bigger energy reserves in their blubber.
For whatever the full set of selective pressures that resulted in the sexual dimorphism of elephant seals, the size differences are extreme. In today’s photo, a male northern elephant seal is clearly much larger than the female in front of him.
This photo was taken at the northern elephant seal rookery at Piedras Blancas on the California coast with a Canon EF100-400 mm f/4.5-5.6L IS USM lens zoomed to 285 mm on a Canon EOS 5D Mk III. The exposure was set to 1/500 sec at f/6.3 and ISO 800.