Photograph By Joel Sartore
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This toad was photographed at UC Berkeley’s Museum of Vertebrate Zoology.
Photograph By Joel Sartore

This toad’s sex life hinges on finding the perfect pool

The Yosemite toad will mate and lay eggs only in certain sorts of pools. As climate change alters the pools, this endangered species is even more at risk.

This story appears in the May 2019 issue of National Geographic magazine.

The male Yosemite toad (Anaxyrus canorus) can mate like mad every year—for about two weeks. Only in late spring. And only in wet meadows, at elevations above 4,800 feet, in California’s Sierra Nevada. The male will wait in a pool, trilling. A female drawn by his mating call (canorus means “melodious”) will enter the water and submit to his advances—if the pool suits. Toads have precise specifications for where they’ll breed and leave their eggs, says U.S. Forest Service ecologist Christina Liang.

For six years Liang and colleagues observed 143 pools across 19 meadows in the toads’ range. Toads seek pools that will support life from the springtime when eggs are laid through late summer when toadlets emerge. By tracking which pools were and were not occupied, researchers found differences in conditions were at times quite small. Toads chose wider pools with more surface area; pools with warmer water (a mean temperature of 76.7°F versus 71.2°F); and pools that were deeper—by only about the diameter of a standard pencil.

To mate, Liang says, the male “clamps onto” the female’s back—but “she has the final say” on where to release eggs and may move around with him attached until she chooses a spot. Once she releases her eggs and he fertilizes them, she’ll leave; he’ll resume calling.

The Yosemite toad is considered endangered, and its numbers are falling. Scientists say the amphibian chytrid fungus is one reason, but climate change also may contribute to some pools drying up before tadpoles mature. The species “is on that knife’s edge,” Liang says, “where these really small changes in environmental conditions can have potentially large effects.”