Sheep Dip

The latest reality for the global-warming deniers and minimizers to ignore or explain away is the shrinkage of Scottish sheep.

Anglo-American researchers have determined that on Hirta island, located in the wild and windswept Atlantic some 100 miles west of mainland Scotland, Soay sheep are shrinking at a rate of roughly 3 ounces per year. The cumulative effect has been a 5% reduction in total body size over the past 24 years.

While evolution had previously favored larger sheep, as better able to survive Hirta’s harsh winters, “researchers have concluded that warming temperatures have made it easier for scrawnier sheep to survive, thus reducing the average size of animals in the herd.” 

As the Los Angeles Times puts it, this study “offers unusual proof that large animals are already evolving to adapt to changes wrought by climate change.” 

The forebears of today’s sheep were transported to Hirta by a Scottish marquess in 1932, two years after the island was abandoned by its human inhabitants. In the 1980s scientists began dropping by from time to time to study that flock’s descendants. Because there are no permanent human occupants of the island, “it’s like an outside laboratory,” says Arpat Ozgul of Imperial College London, lead author of the Soay-sheep weight-loss report.

Soay sheep are a “genetic archive” of the Neolithic origins of today’s domesticated sheep. They take their name from Soay island, where they have lived probably since the Bronze Age. “Soay” is Old Norse for “island of sheep.” These beasts are both smaller and hardier than domesticated sheep, and are extremely agile, which they need to be, in order to get around on the isle of Soay, and on Soay’s neighbor, Hirta.

Below is an image of something from today’s Hirta. I have no idea what it is, but it passes the time to speculate.

For their study:

Ozgul and his colleagues analyzed a plethora of data, including the number of lambs born each year since 1985, the age of ewes giving birth and the survival rates of sheep at different ages. The team included body-weight measurements and the length of the hind leg to see whether the sheep were just thinner or actually smaller overall.

To measure the effect of climate, the researchers also incorporated the North Atlantic Oscillation index, which affects the strength of westerly winds in Europe and determines whether winters will be wet and mild or cold and dry. All of these terms were plugged into a mathematical formula that allowed them to measure the individual components that contributed to the change in sheep body size.

As expected, the researchers found that evolutionary pressure pushed the sheep to grow bigger. But that was offset by another, unexpected factor: Lambs born to yearling ewes instead of fully grown sheep weighed less at birth than their mothers did and remained smaller throughout their lifetimes.

Still, the “young mum effect” wasn’t enough to account for the decrease in sheep size.

When the researchers added in the effect of changing environmental conditions, “you get just about the exact rate of decline that we’ve seen,” said study coauthor Shripad Tuljapurkar, a biology professor at Stanford.

The Hirta study is one of a number of inquiries that is demonstrating the reverse of the German biologist Christian Bergmann’s 1847 observation that as members of a species migrate to elevated regions with colder temperatures, their body size tends to increase. Bergmann opined that larger bodies helped these animals conserve heat by reducing their surface area relative to their volume.

The Hirta people operated on the theory that the same mechanism might prompt body sizes to shrink, when animals stayed put, but the climate around them grew warmer.

And they were right.

On average, a one-year-old Hirta sheep today weighs 3.3 ounces less than did a one-year-old Hirta sheep in 1985. Today’s shorter, milder winters allow grass to grow later into the year; thus, sheep can successfully soldier on through even the coldest months with fewer fat reserves; thus, more lambs born to young mothers can survive in spite of their smaller size. The milder winters permit the overall herd to grow larger, even as the average size of the animal decreases.

As Tim Coulson of Imperial College (any chance they could lose that name?) observes:

“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.

“But now, due to climate change, grass 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.”

It is this that the deniers and minimizers will no doubt seize upon, if they choose to acknowledge this study at all:

Global Warming Good For Sheep! They’re Not As Fat, And There’s More Of ‘Em!
So Get On Down and Buy That Hummer! Yee-Haw . . . .