Amphibians
Spadefoot Toad
Pelobatoidea · also called spadefoot
A spadefoot toad lives most of its life as a lump in the soil. It has a hard, sharp spade built into each hind heel, and it uses these to slice its way backward into the ground, where it sits out the dry season — sometimes a whole year — with almost everything switched off. Then a big storm comes, and the desert briefly fills with toads.
Overview
Spadefoot toads are the superfamily Pelobatoidea, around 20 species, in the dry country of North America, Europe, northwest Africa, and Asia. They are squat, short-legged burrowing amphibians of sandy soils, carnivores that feed on ground invertebrates. Despite the name they are not true toads (family Bufonidae) — they are an ancient separate branch of the frogs, called toads for their looks and habits.
The spade, and life underground
Each hind foot carries a hard, dark spade that the toad uses to shuffle backward into loose soil, up to a metre down. In its sealed chamber it slows its metabolism, tolerates high urea to hold water, and may wrap itself in shed skin — enough to wait out months or years of drought.
Breeding against the clock
Heavy rain triggers the whole population to emerge and rush to newly filled pools, where they breed within a night or two. The eggs hatch in a day, and the tadpoles can transform in one to two weeks — among the fastest of any amphibian. In shrinking, crowded pools, tadpoles of several species switch to a larger, big-jawed carnivorous form that eats shrimp and other tadpoles and develops faster still.
Status
Spadefoots as a group are Not Evaluated. Several are widespread, but many are declining as their shallow breeding pools are drained, ploughed, or paved and the light diggable soils they need are lost to farming and development. The European common spadefoot has vanished from large parts of its range.
Spadefoot Toad: common questions
Is a spadefoot toad a real toad?
Not in the strict sense. The true toads are the family Bufonidae, and spadefoots are not part of it. Spadefoots belong to their own group, the Pelobatoidea, an old branch of the frog tree that split off long before the true toads appeared. They are called toads because they look and act the part, being squat, short-legged, dry-country animals that hop rather than leap and have somewhat warty skin. This is the same reason names like fire-bellied toad and midwife toad are attached to other non-bufonid frogs. Biologically, calling something a toad is often just shorthand for a frog with a toad-like lifestyle.
How do spadefoot toads survive in the desert?
By spending most of their lives hidden underground and compressing everything they need to do into a few wet nights. Using the hard spade on each hind foot, a spadefoot digs itself backward into loose soil, sometimes a metre down, and settles into a sealed chamber. There it drops its metabolic rate, tolerates high concentrations of urea in its body fluids to hold on to water, and can secrete a cocoon of shed skin around itself. In this state it can wait out months or even years of drought. When heavy rain finally soaks the ground, the toad digs out, feeds heavily, breeds explosively, and returns underground, all within a week or two.
Why do spadefoot tadpoles develop so fast?
Because the pools they hatch in are temporary and can vanish within days. Natural selection strongly favours any tadpole that reaches metamorphosis before its pool dries, so spadefoots have evolved eggs that hatch in a day or two and larvae that can transform in one to two weeks under warm conditions, far faster than pond-breeding frogs that have all summer. The development rate is also flexible, and when a pool is shrinking the tadpoles speed up further and emerge smaller, trading body size for survival. In some species this includes switching to a carnivorous, cannibalistic form that grows faster on a diet of shrimp and other tadpoles.
Do spadefoot toads really only come out when it rains?
Almost. Adult spadefoots are strongly tied to rainfall and spend the great majority of the year underground, emerging mainly on warm, humid, or rainy nights, and above all after the heavy storms that fill breeding pools. In wetter parts of their range they may surface to feed on a fair number of nights through the active season, but in true deserts an individual might be above ground for only a handful of nights a year. This is why spadefoots are rarely seen even where they are common, and why local people are sometimes astonished by the sudden appearance of hundreds of calling toads after a big storm.