Frogs at 12,000 feet?

In the back of our minds, I think most of us have a picture of what frog habitat looks like, perhaps a warm pond filled with lily pads and logs. Given this preconception, it comes as quite a surprise to see mountain yellow-legged frogs basking by the thousands at lakes in the Sierra Nevada that lie well above timberline, some at elevations as high as 12,000' (3,660 m). How do they survive in these cold, stark conditions?

The fact that frogs are ectothermic, as described in my 4/25/08 post, plays an important role in allowing mountain yellow-legged frogs to flourish at high elevations. Given the cold water and air temperatures even in mid-summer, food requirements are relatively modest, allowing frogs to add the necessary fat reserves to get them through the long winter despite low densities of prey.

Despite the general paucity of food in this harsh environment, food is actually plentiful during short periods and mountain yellow-legged frogs take full advantage of this ephemeral abundance. In early summer (a few weeks after ice-out), mayflies and chironomids ("midges") transform from aquatic larvae into terrestrial winged adults and provide a brief dietary bounty for the frogs. The mayfly, Ameletus edmundsi, thrives in cold, high elevation lakes, and its emergence style makes it particularly vulnerable to frogs. Unlike many mayfly species that emerge well offshore, A. edmundsi crawls up onto a shoreline rock and emerges there. In lakes harboring mountain yellow-legged frog populations, I've watched frogs eyeing A. edmundsi larvae from the second they crawl out of the water and then engulfing the winged adult just as it completes metamorphosis. Midges provide a similar feeding opportunity for frogs. On windy days, chironomid adults aggregate in mating swarms on the lee side of boulders. Within minutes of these aggregations forming, mountain yellow-legged frogs will arrive and spend hours picking off chironomid adults when they land on the rock to rest.

During late summer, most aquatic insect emergences are over and mountain yellow-legged frogs depend more heavily on terrestrial prey. Anything crawling along the lake shore that fits in their mouths is fair game. I've watched grasshoppers get blown into the water, swim back to shore, and be eaten by a frog as soon as they crawl up onto a shoreline rock. Although mountain yellow-legged frogs generally spend their time right at the lake shore, some take advantage of another food source away from water: ants. Frogs will position themselves along ant trails and pick off the hapless ants as they pass by. The metamorphosis of Pacific treefrogs (Pseudacris regilla) from tadpole to froglet in late summer provides another food source. The froglets don't have the well-developed hopping skills of adults and make easy prey for the much larger mountain yellow-legged frog.

Despite the various tricks that mountain yellow-legged frogs employ to fill their bellies, food is scarce in this high elevation environment, especially when there are thousands of frogs at a lake all competing for the same limited food resource. And yet, these high elevation lakes offer the frogs an important advantage over lakes at low elevations: a lack of snake predators. More about that next week....

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