Seals Less Successful in Equatorial Oceans

An assessment of 34 species of seals, sea lions and walrus finds that certain natural environmental factors link with a species' vulnerability to extinction. Other factors associated with the animals' life histories do not distinguish between the species at risk and those with healthy populations.

The life history variables concern whether the marine mammals form harems, how extensively females nurse their young, and how fast the pups mature. Instead, living where weather varies little throughout the year seems to predispose a species to precarious population declines.

Among the 34 species, none of those endangered are the 13 that live at polar or subpolar latitudes. Six of the endangered species are from equatorial seas and another four inhabit temperate waters.

Seals and sea lions, collectively known as pinnipeds, living far from the equator experience sharp contrasts among the seasons. Seasonal variation affects how the amount of energy in the environment is distributed throughout the year.

Additionally the group most at risk, the ten equatorial species, live where ecosystem primary productivity is low. These include the extinct West Indian monk seal and critically endangered Mediterranean monk seal.

How well suited the seals are to their environment affects their resilience to human-posed risks such as harvesting, habitat loss and pollution. Pinniped ancestors originated in the north Pacific where they evolved for tens of millions of years, adapting to climates with large seasonal fluctuations. Much later, pinnipeds ranged south into temperate regions and consequently they may be less evolved for survival under equatorial conditions.