• E-mail
• Print
• Comment
• Font Size
• Digg
• del.icio.us
• Discuss article

Download full size image

Climate Change And The Mystery Of The Shrinking Sheep

Posted on: Thursday, 2 July 2009, 13:45 CDT

It’s not often that one has an opportunity to find humor in the topic of global warming.  However, a peculiar new study pointing to climate change as the explanation behind a shrinking flock of wild sheep on a lonesome island in northern Scotland elicits a grin from even the most trenchant of climate change warriors.

For some twenty-five years now, the untamed Soay sheep of Hirta island, part of the remote St. Kilda archipelago, have been an odd source of interest and, more recently, befuddlement for evolutionary biologists.  In 2007, scientists released 24 years of data amassed on the wooly ruminants only to realize that the average weight of individuals in the flock had been consistently falling.  The data also indicated that lambs were developing, on average, at a slower rate than their predecessors and that smaller sheep were much more likely to survive to adulthood.

Given the sheep’s environmental milieu—particularly the long grueling winters and severely limited resources—the flock’s progressive shrinkage came as something of a shock to researchers.

According to basic Darwinian evolutionary theory, such conditions should produce increasingly larger sheep.  Those sheep able to grow quickly and pile on body mass stand a better chance of surviving the harsh winter and periods of curtailed food supply.  Eventually, as the smaller, weaker sheep died out—along with their suboptimal genetic make-up—the herd’s gene pool should have consisted of predominantly larger, beefier animals.

Instead, however, scientists have observed exactly the opposite trend: the average body mass of the sheep has decreased by 2.85 ounces (81 grams) per year since measurements began and young sheep are not growing as rapidly as they once did.

In a report issued on Thursday, British biologists shared their “ah-ha” moment with the science community, pointing to changing climate conditions as the culprit behind the mysterious growth patterns.

Tim Coulson, a professor at Imperial College London and lead author of the study believes that increasingly shorter and milder winters have put less pressure on lambs to gain weight quickly as more food is available for a longer period.

“In the past, only the big, healthy sheep and large lambs that had piled on weight in their first summer could survive the harsh winters on Hirta,” explained Coulson.

“But now, due to climate change, grass for food is available for more months of the year and survival conditions are not so challenging—even the slower-growing sheep have a chance of making it, and this means smaller individuals are becoming increasingly prevalent in the population."

Another factor likely contributing to the dwarfing herd is what researchers call the “young mum effect.” Ewes that tend to give birth earlier in their developmental cycle tend to have smaller lambs.  As more and more of these lambs are surviving their first winter and reaching reproductive age, their once disadvantageous genes are finding their way more and more frequently into the community gene pool.

“This study addresses one of the major goals of population biology, namely to untangle the ways in which evolutionary and environmental changes influence a species’ traits,” said Andrew Sugden, deputy and international managing editor for the journal Science, which published the report.

Biologists say it is difficult to tease-out evolutionary pressures from environmental factors to predict which species will profit and which will suffer from changing weather patterns.  Still, advances in research and high-tech computer modeling techniques have made possible a greater degree of accuracy in their predictions.

“Biologists have realized that ecological and evolutionary processes are intricately intertwined, and they now have a way of dissecting out the contribution of each,” said Coulson .

“Unfortunately, it is too early to tell whether a warming world will lead to pocket-sized sheep.”


----------

Image Credit: Wikipedia

------------

On The Net:

Imperial College London

Source: redOrbit Staff

More News in this Category