In hibernating species, the sperm storage timing coincides with hibernation. Examples include hibernating bears and torpor in bats. In hibernating species, males are known to mate with females in torpor. Groundhogs are excellent burrowers, using burrows for sleeping, rearing young, and hibernating. In winter, they may prey on hibernating bats. Brown adipose tissue is especially abundant in new born mammals and hibernating mammals. Brown adipose tissue is especially abundant in newborns and in hibernating mammals. Similar endocrine phenotypes are observed in fetal life and in hibernating mammals. They spend the winters in deep dens hibernating. Bears were hunted during the winter months when they were hibernating. This is particularly important in brown fat thermogenesis of newborn and hibernating mammals. For hibernating species, weight gain is important as it provides nourishment during winter dormancy. Queens may survive the winter by hibernating. Frog and Toad are hibernating again in their respective beds. A number of hibernating chrysalids fall prey to various enemies.