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Does global warming cause animals to shrink?

A study of copepods found that the growth rates of the tiny marine crustaceans is highly sensitive to temperature, with implications for the entire food chain.

By Wynne ParryLiveScience Senior Writer / October 4, 2011

A study of tiny marine crustaceans called copepods suggests that rising global temperatures can result in smaller animals, at least cold-blooded ones. .

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Melting ice, outbreaks of disease, more intense storms and more forest fires are just some of the effects scientists say will accompany human-caused climate change. Scientists are now exploring another, perhaps more surprising, potential effect: Shrinking animals.

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A new study has examined how warmer temperatures can result in smaller individuals within a species.

This relationship between size and temperature change only holds for cold-blooded animals, which rely on external sources, such as sunlight, to warm themselves. Scientists don't understand why this relationship exists. But it's important because size influences an individual's reproductive success, as smaller animals tend to have fewer offspring, and its role in a food chain, among other things. [Cold-Blooded Creatures: Album of Lizards & Frogs]

To warm-blooded creatures like humans, this might not sound like a big deal. But we make up only a tiny percentage of Earth's animals, and we rely upon cold-blooded creatures for food, to pollinate crops and for many other crucial, but perhaps not obvious, reasons. So, climate-influenced changes could have cascading effects.

Scientists have already established the "temperature size rule," which says that individual animals reared at colder temperatures will become larger adults. Likewise, animals reared in warmer temperatures produce smaller adults. However, it's unclear just how this happens, according to Jack Forster, a doctoral student at Queen Mary, University of London, and the lead researcher.

Forster and his colleagues looked at data on copepods — tiny, water-dwelling crustaceans — to study what was happening for a range of species. Using data already collected for 34 marine species of copepod, they looked at how non-extreme temperatures influenced growth rate (how quickly an animal puts on weight) and development (how quickly they passed through life stages). For copepods, there is plenty of development to track, since they go through 13 life stages, from egg to adult.

The researchers' analysis revealed that development rate is more sensitive to temperature than growth rate.

"If you warm up, you put on mass more quickly, but the rate at which you pass through life stages is even faster, and when you reach an adult size, you end up being smaller at warmer temperatures," Forster said.

It's not clear why this is the case, he said.

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