Some animals stand to gain from warming climates, say researchers who have looked at the effect of changing rainfall on mating and sexual selection in grey seals in Scotland.
Sean Twiss, at Durham University, UK, and his colleagues studied the grey seals that mate at the North Rona colony in Scotland.
They found that the reduction in freshwater pools in dry years forced females to wander away from their usual breeding spots, and the watchful eye of their dominant male.
This allowed a greater number of previously unsuccessful males to copulate with them, and decreased the dominant males’ access to females. The result is an increase in genetic diversity in these populations of grey seals.
Rain or shine
Every year, from September to mid-November, heavily pregnant females return to North Rona for 18 days. During this time, they give birth to a pup. Then, on about day 16, they mate, before setting off to sea again.
The females each have preferred spots, gathered around pools of rainwater which keep them cool and supplied in drinking water. This clustering makes it easy for dominant, polygamous males to keep an eye on their 10 to 15 females and mate with them when they are ready.
Twiss found that in drier years, females return to the same spot even if their pool is empty. Once they have given birth to their pup, they have to leave their spot to find the more scarce water pools.
“In the very dry seasons, you get lots of movement among the females,” says Twiss. As a result, while a dominant male is watching one female leave her pup and go off in search of water, he might lose sight of another female, who may succumb to the attentions of a subordinate male.
Between 1996 and 2004, Twiss and colleagues found that only 23 males copulated in the wettest year, but 37 mated during the driest. The researchers identified at least 101 males in the colony each year.
A normal mating season, says Twiss, would be wet and windy throughout the 8 to 9 weeks. But climatologists predict that global warming will make rainfall more irregular. Twiss and his colleagues have noticed that the beginning of the season is increasingly dry.
The females return to North Rona at different times, and the early breeders have increasingly been faced with dry conditions.
Twiss points out that although climate change seems to be increasing genetic diversity in North Rona’s grey seals, the link between climate change and sexual selection in other species could be reversed.
The researchers note that although many factors affect the degree of polygamy in animals, most are linked to the availability of natural resources – which are in turn often dependent on the weather.