Bigger had always been better for sheep living on the remote, windswept Scottish island of Hirta.

So why have the animals been getting smaller lately? One possible explanation, say scientists studying the sheep, is climate change.

An international team of researchers tracked the population of Hirta’s Soay sheep over 24 years and found that average body size mysteriously shrank 5 percent, despite the apparent evolutionarily benefits of being larger. They report their findings today in the journal Science.

“As climate changes, the way selection operates changes as well,” says Tim Coulson, a professor of population biology at Imperial College London, and principal investigator on the study. The island’s longer springs and warmer winters mean less competition for food because there’s more of it. Living conditions are also better for weak animals. “Sheep that are a little bit smaller are no longer as disadvantaged as they were when winters were longer and harsher,” he says.
Why then, if being larger had been so advantageous, had sheep stayed about the same size until the recent warm up? The answer, Coulson and his colleagues suggest, is the “young mum” effect. When a sheep first starts reproducing, she is physically unable to deliver large lambs, explains Coulson. In fact, her first-borns are almost always smaller than she was at birth.

Over the past several years, scientists have identified several other species whose physical traits have also changed quickly—often against the grain of prior trends. Fish, for example, have become smaller due to factors such as fisheries selectively catching larger fish.

So, what does all of this say for the future of Soay sheep?  “I’m not going to speculate that we’ll end up with pocket size sheep,” says Coulson, adding that modeling of future trends for sheep and other animals is on his team’s to-do list. “I would hope over coming years other researchers will apply these methods, and we can move to a more general view across populations and species.”

Images of Hirta's Soay sheep © Science/AAAS by A. Ozgul