April 25, 2008

For The Frogs, It Is Still Winter

Here in the eastern Sierra Nevada, signs of spring are everywhere. After a winter during which the only common birds were Mountain Chickadees and Dark-eyed Juncos, a host of birds not seen for months are now in every bush and tree. The aspen and cottonwood trees are clothed in catkins, and the trunks of Jeffrey pines smell strongly of vanilla-scented sap. With all of these reminders that summer is on its way, I've been busy preparing for another field season studying mountain yellow-legged frogs up in the High Sierra. On a warm day down here at 7,000' it's easy to forget that for the mountain yellow-legged frogs in the high country it is still winter and will be for another month.

In the fall when the air turns cold and insect prey becomes scarce, mountain yellow-legged frogs retreat into the deep waters of lakes, coming to shore to sun only on the warmest of days. By late October, the lakes are skimmed with ice and by November or December the landscape is clothed in a thick blanket of snow. For the next seven months, the frogs live underwater in a world of near-freezing temperatures and complete darkness, breathing solely through their skin. Like many amphibians, mountain yellow-legged frogs capture prey primarily with their sticky tongues, but this method doesn't work underwater. So, for these seven months the frogs probably don't feed at all, living solely off of the fat reserves they accrued during the previous summer. Most mammals, including humans, die from starvation after a few weeks without feeding, and yet the mountain yellow-legged frog can survive without food for seven or more months! How do they do it?

This survival ability is a direct consequence of amphibians being ectothermic (often called "cold-blooded"), meaning that body temperature is controlled by factors outside of their bodies (e.g., air or water temperature). Body temperature of ectothermic animals controls metabolism, with body temperature and metabolism being positively correlated. During winter, the body temperature of mountain yellow-legged frogs is near the freezing point and their metabolism is therefore extremely low. As a consequence, their need for food is also greatly reduced. The mountain yellow-legged frog pushes this ability to survive without food to an extreme seen in few other amphibians. During winters with unusually heavy snowfall, lakes can thaw as late as August and then freeze over again in October. Under these conditions, mountain yellow-legged frogs can be without food for ten months and have only August-October to replenish their fat reserves for the next winter.

When spring finally comes to the high-elevation haunts of the mountain yellow-legged frog, lakes thaw and frogs crawl to shore in search of warmth, mates, and food. What has always amazed me when I've been at a lake during this time is that the frogs aren't particularly skinny. They haven't eaten anything for months and yet they are only slightly less plump than they were the previous fall.

I wish I could go months without eating. That ability would certainly make my backpack a lot lighter during the summer when I'm doing my frog research in the Sierra Nevada backcountry.

Back to The Mountain Yellow-legged Frog Site.

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