Chapter 1 FROZEN FROGS Animals in warm habitats have to protect themselves from heat Animals in cooler habitats have to protect themselves from cold Some animals that live in places with cold winters grow more fur But sometimes animals simply sleep through the winter and wake up when it is easier to survive These animals are hibernators Hibernators often sleep in dens or other hidden spots What is a hibernator 4 44
Wood frogs are hibernators They live in forests as far north as the Arctic Circle When the weather is warm they live under logs or in wetlands When it gets cold they burrow under fallen leaves on the forest floor Then they freeze solid for the whole winter Wood frogs are the only frogs that live north of the Arctic Circle 5
Frogsicles Most animals die if they freeze The sharp edges of ice crystals in frozen blood can harm their bodies But wood frogs are different Freezing actually allows them to survive in cold weather A frozen wood frog does not move It doesn’t breathe Its heart doesn’t beat Its blood doesn’t flow Ice surrounds its internal organs Ice crystals form between its skin and muscles About two-thirds of the water in its body freezes But none of this kills a wood frog When a wood frog freezes its eyes turn white because the lenses freeze 6
Compare It Arctic ground squirrels can also survive in extremely cold temperatures They hibernate in burrows for seven to eight months each winter During that time their body temperature drops below freezing Hibernating arctic ground squirrels have the lowest body temperature ever recorded for any mammal Sleeping ground squirrels warm themselves up every two to three weeks by shivering and shaking for several hours at a time Then they stop shaking and their bodies cool off again An arctic ground squirrel breathes only about three times a minute when it hibernates Its heart beats one or two times per minute 777777
