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C. Study Finds Evolution Doesn't Always Favor Bigger Animals

Narrator: This is Science Today. Biologists have long believed in a maxim known as Cope's Rule, which states that evolution favors larger animals since the tendency is for lineages to get larger over time. But biologist Kaustuv Roy of the University of California , San Diego says their research suggests Cope's Rule may be only partly true.

Roy : What we noticed is there's another pattern that goes hand-in-hand with Cope's Rule that's called Bergman's Rule. And that one has to do with patterns in space, not time. What it says is, if you go from warm to cold climates, on average in many groups, things tend to get bigger as you go north. And one of the things we got curious about is with, is there some causal relationship between Cope's Rule and Bergman's Rule.

Narrator: Roy found there was by studying the fossil record of crustaceans called deep-sea ostracodes. Their body size increased only when the global ocean temperature cooled.

Roy : So, what we find was if you look over time, there's a very, very tight correlation between body size and temperature. When climate remained flat, it didn't change much, size didn't change much either.

Narrator: For Science Today, I'm Larissa Branin.