The tadpoles of some toads and frogs, for instance, can morph from placid plant eaters to fierce cannibals if their pools threaten to run dry. By becoming meat eaters that snack on other tadpoles, the cannibals speed up their development, hoping to transform into toads before the water runs out. Spadefoot toads, for example, live in dry areas. Adults wait to lay their eggs until temporary pools are formed by infrequent rainstorms. Though the tadpoles take only two weeks to develop into toads, sometimes the pools don't last that long.
Spadefoot toads live in dry areas.
The tadpoles get a tip that the pools are shrinking when they start bumping into each other and fairy shrimp, insect-like animals that also thrive in puddles. If a tadpole eats one of the shrimp, it can spark a transformation into a full-fledged cannibal. Even the cannibals can be somewhat careful about their diets, however, as researcher David Pfennig of North Carolina State University has shown. In some species, the cannibals generally avoid eating their brothers and sisters, preferring to eat unrelated tadpoles instead. Exactly how they tell the difference between friend and foe isn't known.