Many vertebrates follow the pattern of growing larger in colder climates known as Bergmann's Rule, giving them a smaller surface-area-to-volume ratio that helps minimize loss of body heat. But experts say there are other upsides to being big in cold weather, and factors such as sexual selection, fat storage and food availability can drive body size. And for some species, the pattern is the opposite: Certain ectothermic animals are smaller in cooler climes and larger in warmer environments.

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