Darker sheep are bigger, but now we learn why they're not better.
Soay sheep living wild on the Scottish island of Hirta come in two colors: light brown and dark brown. Researchers have known that the dark sheep are bigger than the light ones. Extra heft translates into an advantage in winter survival and reproduction. "We would have expected [the dark sheep] to have increased in frequency over a 20-year time series," says Jon Slate of the University of Sheffield in England.
Not so, Slate and his collaborators say in the Jan. 18 Science. Biologists have kept records on the Hirta population since 1985, and big dark sheep have not been taking over the