The young mum effect explains why Soay sheep have not been getting bigger, as we expected them to," Coulson said. "But it is not enough to explain why they're shrinking. We believe that this is down to climate change. These two factors are combining to override what we would expect through natural selection."

"Our findings have solved a paradox that has tormented biologists for years — why predictions did not match observation," he added. "Biologists have realized that ecological and evolutionary processes are intricately intertwined, and they now have a way of dissecting out the contribution of each. Unfortunately it is too early to tell whether a warming world will lead to pocket-sized sheep."

The traditional El Nino involves a periodic warming of the water in the eastern part of the tropical Pacific. Indeed, it was first noticed by Peruvian fishermen, who named it after the baby Jesus because it tended to first appear around Christmastime.

In El Nino Modoki, on the other hand, the warming occurs farther to the west, in the central Pacific.

It's not clear why this new form is occurring, said Peter Webster, a professor at Georgia Tech's School of Earth and Atmospheric Sciences and a co-author of a report on the finding in Friday's edition of the journal Science.

(editors note: exerts above taken from US News 7/6/2009)