Mammals and birds are endotherms, which means they generate their own body heat through relatively high metabolic rates. That high metabolism requires energy, which these animals garner from food. We typically think of endotherms as warm-blooded; however, some of them are not warm all of the time.

Most active birds and mammals maintain relatively high and stable body temperatures — often around 100°. But they also lose heat to the surrounding environment, especially during the cold winter months. The heat loss is greater for small endotherms because they have a large surface area compared to their size, and although fur and feathers certainly help retain heat, there is a limit to how much insulation a small animal can carry. So how do they manage to make it through the winter?

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The Outside Story is assigned and edited by Northern Woodlands magazine ( and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation (

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