Disruptive selection: a tail color polymorphism in Acris tadpoles in response to differential predation

Abstract

Previous workers have noted that cricket frog tadpoles of the genus Acris have black tail tips. My initial collections of Acris crepitans tadpoles from various localities in Kansas revealed a polymorphism in tail color associated with habitat type: ponds primarily have black-tailed tadpoles, whereas lakes and creeks have mostly plain-tailed forms. Collections of potential predators from these localities showed that the black-tailed pond populations co-occur with a high density of the aeshnid dragonfly larva, Anax Junius, and led to the hypothesis that the black tail functions as a deflection mechanism to divert the attack of the Anax larva to the tail of the tadpole and away from the more vulnerable head and body. Plain-tailed tadpoles are found primarily in lakes and creeks where fish are the major predators. Tail damage data from natural populations and data from predator–prey experiments support the hypothesis. Disruptive selection is most likely the mechanism responsible for maintenance of this polymorphism. Gene exchange occurs as adult frogs migrate from one habitat type to another, but selection on tadpoles by different predator regimes is habitat specific.