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Spadefoot Desert Chorale

Broadcast on June 23, 2011

A plains spadefoot looks for termites to eat. (Photo by Mary Anne Dingus, courtesy of the Sibley Nature Center)

By Megan Wilde

The concert starts with a mighty summer storm. Thunder stirs the singers from their subterranean slumber. They convene at an impromptu stage—maybe a rain-filled arroyo, maybe a waterlogged ditch. And just as the curtain of evening falls, the chorus begins. Who are these vocalists? And what’s the occasion for their performance?

Summer’s rains give all Chihuahuan Desert dwellers something to sing about, but especially frogs and toads. As amphibians, most of these animals live on land as adults but need water to reproduce, which in our arid region presents a challenge.

So, when rain falls, many desert frog and toad species have learned to seize the day. Summer storms send these so-called explosive breeders into a raucous reproductive frenzy. Each species’ vocalizations play an important part in their hasty courtship.

Spadefoots are masters of this fast-and-furious way of life. Sometimes mistakenly called toads, these chunky little frogs spend most of their lives burrowed underground in a hibernation-like dormancy. They may remain in this state for years during prolonged droughts. But as soon as a storm approaches, they explode into action.

The vibrations of thunder or raindrops hitting the ground are thought to cue spadefoots and other explosive breeders to surface. Whatever stirs them, upon emerging, males rush en masse to wherever rain has pooled nearby—an arroyo, a livestock tank, or even a roadside ditch.

In these teeming congregations, the males face a problem familiar to any single guy at a noisy, crowded party: attracting a female’s attention. So they advertise their eligibility with clamorous come-hither calls. Each frog and toad species has its own distinctive advertisement call, which in spadefoots can summon females a half-mile away.

Then the race to breed begins. Males scramble around their watery bachelor pad mounting any frog they can grasp. They occasionally seize another male or even another species of frog or toad, who may respond with a squeak, known as a release call, that essentially says, “Back off, you’re not my type.” Whenever a male finally grabs a female, he may not have her all to himself, as other males pile on to create a writhing knot.

Amidst this boisterous chaos, advertisement calls help females find the most attractive males of their species. But it’s not uncommon for a female’s eggs to be fertilized by multiple males, or for hybrids of similar species to be produced.

Not all males are up to the task of calling, which requires a huge investment of energy. In some species, calling raises a frog’s metabolism ten to twenty times its normal rate. Males without the needed energy have to be sly to reproduce. These so-called satellite males hang out quietly near a more-robust male. When his calls attract a female, they try to intercept. They’re not alone in taking advantage of others’ vocal virtuosity. Research has shown that in choruses, smaller, less-fit spadefoots tend to associate with large males whose calls are more enticing to the opposite sex.

Should you stumble upon a spadefoot chorus, savor it. They’re as fleeting as the storms that inspire them, lasting just one or two nights. Their offspring must mature quickly, before their birthplace evaporates. Eggs hatch within thirty-six hours, and tadpoles transform into froglets in less than fifteen days—among the fastest developmental rates in North American amphibians. As the rains pass and the desert dries, spadefoots—young and old—burrow into the soil, where they’ll wait patiently for the next chance to perform.

Thanks to Lang Elliott, Carlos Davidson and Carl Gerhardt for use of recordings from their CD and book, The Frogs and Toads of North America.

Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page.

References & Resources for Educators

• Frogs and Toads of North America: by Lang Elliott, Carl Gerhardt, and Carlos Davidson
• Couch’s Spadefood: Arizona-Sonora Desert Museum
• “Evolution of male mating behavior: male spadefoot toads preferentially associate with conspecific males” by K. S. Pfennig, Katrina Rapa and Regan McNatt. Behavioral Ecology and Sociobiology (Volume 48, Number 1, 69-74)

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