Subordinate seals get sex lives thanks to climate change

Research published recently in the Royal Society’s Biology Letters by Dr Sean Twiss, a lecturer in the school of Biological and Biomedical Sciences at Durham University and colleagues at the University of St Andrews, has shown that mating patterns within the grey seal colony on the remote Scottish Island of North Rona have altered dramatically as a result of higher temperatures and lower rainfall, leading to a greater genetic diversity within the population.

Warmer and drier autumns have led to a reduction in pools of rainwater which the female seals require to maintain their body temperature and provide them with a source of drinking water. As a result the females, who remain loyal to one breeding site, must travel further to access the vital resource for themselves and their pups, which removes them from the watchful eye of the dominant males and allows subordinate males to mate with them.

Dr Twiss said: “Grey seals are typically polygamous, with the more dominant males mating with approximately ten to fifteen females which they guard from other males within their territory. These males’ ability to dominate is easy when rainwater pools are abundant and females cluster in a small geographical area, but during dry seasons the area in which the females are located becomes too big and they can no longer successfully keep an eye on them all!

“The females must travel further to access water before returning to nurse their pups, which remain at the original pupping location. The increased movement amongst the females allows the weaker males to mate and results in more males contributing genetically to the next generation.”

During the nine year study, Twiss and his colleagues recorded a 61% increase in the number of males contributing to the genetic pool – from 23 males in the wettest years to 37 in the driest, from a total number of around 100 males present each year.

Grey seals are annual breeders with a discrete and predictable breeding season which in the UK sees them gather on remote island sites for 18 days during October and November. During this time each female gives birth to a pup before mating again on approximately day sixteen after which they return to the sea.

This autumnal breeding season would normally be wet and windy but between 1996 and 2004 Twiss and his colleagues recorded unusually dry starts to some seasons. This is in-keeping with predictions made by climatologists who state that global warming will make rainfall more irregular.

Dr Twiss continued: “Much current research is focusing on the geographical movement of animals as a result of climate change. What we are interested in finding out is what impact climate change has on the behaviour of animals – how it effects their social systems including mating patterns.

“The effect of climatic variation on temperature and rainfall has wide-spread implications for many species as there are very few animal populations whose mating patterns and levels of polygamy are not intimately linked to resource distribution. These findings show that climate change, whilst endangering many species could also help to increase the genetic diversity of some species, giving a leg up (or over!) to males who normally wouldn’t be so successful.”