"We do not yet know the exact mechanisms (机制) involved, or why some organisms are getting smaller while others are unaffected," the researchers said. "Until we understand more, we could be risking negative consequences that we can't yet quantify. "

　　The change was big in cold-blooded animals. Only two decades of warmer

　　temperatures were enough to make creatures smaller.

　　An increase of only lC caused nearly a 10 percent increase in metabolism(新陈代谢).

　　Greater use of energy resulted in tiny tortoises and little lizards.

　　Fish are smaller now too.

　　Though overfishing has played a part in reducing numbers, experiments show that warmer temperatures also stop fish growing.

　　Warm-blooded animals weren't immune (兔疫) from the size change caused by climate change. Many birds are now smaller.

　　Red deer are weaker.

　　And polar bears are smaller, compared with historical records.

　　This isn't the first time that has happened in Earth's history.

　　55 million years ago, a warming event similar to the current climate change caused bees, spiders and ants to shrink by 50 t0 75 percent over several thousand years. That event happened over a longer time than the current climate change.

　　The speed of modern climate change could mean organisms may not respond or adapt quickly enough,

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