Toastier temps make for smaller sheep

STANFORD (US)—Researchers may have solved the case of the incredible shrinking sheep on the Scottish island of Hirta. Blame the heat.

The average size of Soay sheep on the island has declined about 5 percent in both body weight and stature since researchers began taking measurements of the herd in 1985. The finding is the exact opposite of how sheep were expected to respond to the consistent warming trend that global warming has brought to the island.

“Since the trend has been for milder winters, that should actually make things much happier for everybody, because they don’t have to cope with that severe winter,” explains Shripad Tuljapurkar, a professor of biology at Stanford University and one of the coauthors of a paper published online July 2 by the journal Science.

Hirta lies in the Scottish archipelago of St. Kilda, renowned for harsh, stormy winters. But, with less punishing winters and a longer growing season, the sheep have more grass available for grazing and more time in which to shovel it in before winter hits. More eating, bigger sheep, right? Findings suggest otherwise.

“Survival rates have been lifted for everyone, including the smaller sheep, so sheep that might simply have not made it 20 or 30 years ago are definitely making it now,” says Tuljapurkar. With more runts surviving, the size of the average sheep in the herd has declined.

The trend is exacerbated by the fact that, with more sheep surviving, there is more competition for a limited commodity: food. The island of Hirta is just under a square mile and there is only so much grass.

“When you start looking at the population effect, it’s very clear that what is driving the decline in growth rates is simply that there are more sheep jostling for the resources that are at hand,” Tuljapurkar says.

The researchers gathered annual data by making measurements each August on the female members of the flock. Natural selection generally works to favor animals growing larger over time, as bigger animals are better equipped to survive hard times and are thus more likely to reproduce successfully.

While studying the data, the researchers found another surprising trend.

“We know that body weight is heritable,” Tuljapurkar explains, so it would be expected that mothers would give birth to daughters of about the same weight at birth, or slightly larger, as the mothers had been when they were born.

“It turns out that is not true for young mothers,” he says. Unlike more mature mothers, the young mothers have offspring that typically weigh less than the mothers did and also stay smaller as they grow. And every year, a significant percentage of the new lambs are born to young mothers.

Tuljapurkar says that this “young mother” effect, as the researchers have dubbed it, pushes down the distribution of birth weight in the population, counteracting the increase in birth weight that would be expected to result from natural selection favoring bigger animals through survival and reproduction.

Tuljapurkar says people sometimes ask why he would spend 25 years “studying sheep in the middle of nowhere.”

His response: “There really is a tremendous amount that you can learn from studying the same thing in the same place for long enough so that you can see what happens across changes of climate regime. This is one of those payoffs for long-term research.”

Researchers from Imperial College of London and from Leeds, Cambridge, and Edinburgh universities contributed to the study, which was funded by the U.S. National Institute on Aging.

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