CANTERBURY, U.K.--Tadpoles are masters at the art of adjusting their growth to their world. If a pond is likely to dry out, they speed up their development into toads. But the tadpoles of the Mallorcan midwife toad have set a new record for "developmental plasticity", according to a study reported here on 15 July at the annual meeting of the Society of Conservation Biology.
Scientists long believed that the Mallorcan midwife toad had gone extinct. The likely culprits, they thought, were viperine snakes, which had been introduced to the island of Mallorca by the Romans 2000 years ago because women bathed with them as a fertility treatment. But in 1980 a team discovered 13 populations of the toad in the remote mountains of the island in Spain, providing a chance to study how the toads have survived the snakes.
Robin Moore, a graduate student in conservation biology at the University of Kent at Canterbury, investigated how the tadpoles respond when they detect the scent of snakes. He found that tadpoles in snake-infested ponds have leaner bodies and stronger tails. This, Moore says, would help the tadpoles sprint faster to escape from hungry snakes. Tadpoles in ponds without snakes have shorter tails and rounder bodies. When Moore put a snake in a bag into a pond with these fatter tadpoles, they adjusted their development to streamline themselves within only 4 weeks in response to the threat. And when the bag with the snake was removed, they equally quickly returned to their couch-potato shape.
Marc Hero, an ecologist at Griffith University in Queensland, Australia, says it is a beautiful study, demonstrating an amazing degree of developmental plasticity in tadpoles. He says the leaner body shape resembles that of tadpoles found in rivers, where they need to be better swimmers. So it may be that the tadpoles had already invented this adaptive trick to cope with ponds that turned into flowing streams after heavy rainfall. When the viperine snakes arrived with Roman bathers, the tadpoles already had a way to cope, he speculates.