Tadpoles can become cannibalistic.
If you are a tadpole, sometimes it pays to watch out for headhunters. 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.