The blood of the North American wood frog helps it survive freezing temperatures for as long as seven months via a natural antifreeze.
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"The North American wood frog (Rana sylvatica), for instance, can survive freezing temperatures for as long as seven months, relying on a natural antifreeze in its blood to protect its organs." (Morell 2001)
Journal article The fragile world of frogsVirginia Morell