Amphibians
Plains Spadefoot
Spea bombifrons · also called plains spadefoot toad
For most of the year there is nothing to see. The plains spadefoot is a metre down in the prairie soil, sealed in a chamber, barely alive. Then a summer storm rolls across the Great Plains, the rain hammers the ground, and within hours the surface is alive with squat, cat-eyed amphibians digging their way up to breed in puddles that will be gone in a fortnight.
Overview
Spea bombifrons is a spadefoot, not a true toad. It is plump and grey-green, about 4 to 6 cm long, with a vertical pupil, a bony lump between the eyes, and a hard, sharp “spade” on each hind foot for digging. It lives across the North American prairies and into the desert south-west, wherever the soil is loose enough to burrow.
A life mostly spent buried
The plains spadefoot spends the great majority of its life underground, dormant in a self-dug chamber. In a dry year it may not surface at all. It seems to sense the vibration of heavy rain on the ground above and digs to the surface within hours, so a whole population can emerge on the night of the first big storm.
The race against the pool
Breeding happens in temporary rain pools. Everything is fast: eggs hatch in a day or two, and tadpoles can transform in two to three weeks, among the quickest of any amphibian. In crowded, shrinking pools, some tadpoles become cannibals, growing bigger heads and jaws and eating other tadpoles to speed their own development and beat the drying water.
Status
The plains spadefoot is Least Concern and generally secure, though it is patchy and easy to miss. Local declines follow the ploughing of native prairie and the loss of the shallow temporary pools it must have to breed.
Plains Spadefoot: common questions
Is a plains spadefoot a toad?
Not a true toad. It looks like one, being plump, dry-skinned, and short-legged, and it is often called the plains spadefoot toad, but spadefoots belong to their own family, the Scaphiopodidae, separate from the true toads. The clearest differences are the eyes, which have a vertical, cat-like pupil rather than the horizontal pupil of a true toad, the smoother skin with smaller warts, the absence of the large parotoid poison glands behind the eyes, and the hard digging spade on each hind foot. A raised bony lump between the eyes, called a boss, is another spadefoot feature.
How does the plains spadefoot survive years without rain?
It waits underground and shuts down. Using the spades on its hind feet, the spadefoot digs itself backward into loose soil, often to a depth of half a metre to a metre, and forms a smooth chamber. There it lowers its metabolic rate, and it can form a cocoon of shed skin around its body that reduces water loss. It draws on stored fat and reabsorbs water from its own bladder to stay hydrated. In this state it can remain dormant through a full dry season, and in a prolonged drought it may stay buried for a year or more, only surfacing when enough rain finally soaks down to reach it.
Why do plains spadefoot tadpoles eat each other?
It is a response to bad conditions in the breeding pool. Spadefoots breed in temporary rain pools that can dry up within weeks, and in a crowded pool with limited food, some tadpoles develop into a distinct carnivorous form, with a wider head, larger jaw muscles, and a shorter gut suited to eating animal prey. These carnivore tadpoles feed on fairy shrimp and on other, smaller tadpoles, including their own kind. A tadpole that eats meat grows much faster, and in a race against evaporation, the ones that transform first and leave the pool are the ones that survive, so cannibalism, though costly, can be the difference between a generation living or dying.
Where and when might you see a plains spadefoot?
You are far more likely to hear one than see one. Plains spadefoots live across the Great Plains and into the desert south-west, and they come to the surface only on warm, humid nights during the rainy season, mainly after heavy summer thunderstorms. On such a night they emerge in numbers, and males gather at flooded fields, ditches, playa lakes, and roadside pools to give a low, nasal, snoring call that has been likened to the bleat of a young goat. Within a week or two the adults have bred and gone back underground, so the window to find them is short and unpredictable.