THE RELATIONSHIP OF LOCOMOTION TO DIFFERENTIAL PREDATION ON PSEUDACRIS TRISERIATA

Abstract

Laboratory experiments, using the garter snake Thamnophis sirtalis as predator and the chorus frog Pseudacris triseriata as prey, show that metamorphosing anurans are captured more frequently by T. sirtalis than either premetamorphic tadpoles or postmetamorphic frogs. Stomach contents of wild-caught Thamnophis have similarly revealed more anuran prey in transformation than at earlier or later stages. The high susceptibility of transforming Pseudacris to snake predation appears to be a direct consequence of a body form not conducive to effective locomotion. The sustained swimming ability of premetamorphic and metamorphic P. triseriata was examined in the laboratory. It was found that until the eruption of the forelimbs, the larger a tadpole is, the longer it can maintain its position in a steady current. The length of the tail appears to be a positive factor in sustained swimming. Large hindlimbs do not appear to assist swimming and may increase drag. Tadpoles that have begun to metamorphose (forelimbs visible) demonstrate a greatly diminished ability to hold their position in a current. The maximum jumping distance of metamorphic and postmetamorphic P. triseriata was also examined in the laboratory. Individuals which have just begun to metamorphose cannot hop as far as individuals which have advanced through metamorphosis and lost their tails. Amputation of the tails from early metamorphic individuals demonstrates the retarding role that the tail has on saltatory locomotion. Additional features, such as maturation of the anterior appendages, may also account for an increased jumping ability observed in P. triseriata as they progress through metamorphosis. Anurans spend a disproportionately short time in metamorphic transformation. The rapidity of this transformation appears to be an adaptation minimizing the amount of time anurans spend in this most vulnerable form.