Amphibians
Couch's Spadefoot Toad
Scaphiopus couchii · also called Couch's spadefoot, spadefoot toad
Couch’s spadefoot is a desert amphibian that has solved the problem of being a frog in a place with almost no water: it spends most of its life asleep underground, surfaces for a few frantic nights a year when the monsoon breaks, and rushes its tadpoles through metamorphosis before the puddle dries.
Overview
Couch’s spadefoot lives in the deserts, arid grasslands, and thornscrub of the southwestern US and Mexico. For 8 to 10 months a year it is buried — up to a metre deep, in a burrow it digs backwards into the soil, wrapped in a cocoon of shed skin that holds in moisture. Above ground it is a plump, greenish toad with vertical pupils and a hard black “spade” on each hind foot.
Waiting for rain
The toad stays underground until the first heavy summer storm, then surfaces within hours — cued, it seems, by the low-frequency vibration and sound of rain striking the ground. What follows is a compressed burst of activity: males gather at newly formed pools and call with a loud, bleating groan, mating happens over one or a few nights, and the adults feed heavily — often on termites — sometimes taking in half a year’s energy in a single night before conditions dry and they dig back down.
The fastest tadpoles
A desert rain pool may last only a week or two, so the larvae are built for speed. Eggs hatch in about a day and a half, and tadpoles can become toadlets in 7 to 8 days — among the fastest development of any amphibian. In fast-drying pools some tadpoles turn cannibalistic, growing bigger heads and eating other tadpoles and fairy shrimp to finish even sooner.
Surviving the dry
Underground, the toad drops its metabolism, seals itself in its skin cocoon, and lets urea build up in its body, raising its internal concentration enough to draw water out of dryish soil instead of losing it. This lets it ride out not just the dry season but a failed rainy season if it comes.
Status
Couch’s spadefoot is Least Concern and common, well suited to arid conditions and tolerant of some land disturbance. Its main vulnerabilities are the loss of the temporary breeding pools it depends on and, potentially, shifts in monsoon timing under climate change.
Couch's Spadefoot Toad: common questions
Is a spadefoot a true toad?
No. Spadefoots look and behave like toads — squat, warty-ish, terrestrial — but they belong to their own family, Scaphiopodidae, separate from the true toads (Bufonidae). Tell-tale differences are the vertical, cat-like pupils (true toads have horizontal pupils), the relatively smooth, moist skin, the lack of large parotoid glands behind the eyes, and the single hard, sharp-edged "spade" on each hind foot, which true toads do not have. The spade is a keratinised digging tool.
How does it survive nearly a year without water?
When the surface dries out, Couch's spadefoot digs itself backwards into the soil using the spades on its hind feet, corkscrewing down as deep as a metre. There it enters a state of very low metabolic activity and sheds layers of skin that build up into a semi-permeable cocoon around its body, sealing in moisture and leaving only the nostrils clear. Its body also accumulates urea, which raises its internal solute concentration so that it can actually pull water out of the surrounding dryish soil rather than lose it. In this condition it can wait out not just a dry season but, if necessary, a failed rainy season.
How can the tadpoles develop so fast?
They have to. A desert rain pool may last only a week or two before it evaporates, so any tadpole that develops on a normal timescale dies. Couch's spadefoot tadpoles hatch within about a day and a half and can be fully formed toadlets in seven to eight days in the warmest water — one of the shortest larval periods of any frog or toad. In pools that are drying especially fast, some tadpoles switch to a carnivorous form with a bigger head and jaw muscles and eat other tadpoles and fairy shrimp to grow even quicker. It is a race against the sun.
What does the call sound like?
On a wet monsoon night, a chorus of male Couch's spadefoots produces a loud, low, drawn-out bleating or groaning sound, often compared to the bleat of a sheep or goat, or to a distressed cow. Because the toads are only above ground for a handful of nights a year, hearing this call is a distinctive and short-lived event tied tightly to the arrival of the summer rains in the Southwest.