Seal Dark, Seal Light

Explanation: Here we are, on a late winter day with some uncommon sunshine, on the rocky shores of the central coast of Oregon, USA. Lounging on the exposed rock bench while the tide is out, are these sleepy Pacific harbor seals.

At first glance, one might think there are two species here. But ... no ... these are all the same species: harbor seal.

The species is known to have a range of colors, and mainly two color morphs of dark with light spots, and light with dark spots, as evident in this week's images.

I found that the local population consisted of approximately 10% dark morphs and the rest light morphs.

Although the occurrence of these color morphs, and other color variations, have been commonly reported, apparently there are no studies of any specific adaptive variation of the differences. It may be that neither morph conveys any greater adaptive advantage than the other, and that the two persist in what is called a balanced polymorphism, where different forms (colors, in this case) persist within a single population. Balanced polymorphisms can occur in a wide range of other animal groups and species. They can live in harmony with each other.

Harbor seals are protected under the Marine Mammal Protection Act.

Information:
     Kelly, B. 2011. Pelage polymorphism in Pacific harbor seals. Canadian Journal of Zoology 59(7):1212-1219.

EPOW - Ecology Picture of the Week