Turning Bias in Tadpoles

After surfacing to breathe air, most tadpoles descend by turning sharply to the left or right. We have used this behavior to examine whether tadpoles of two anuran species have innate turning biases, i.e., a handedness. Rana catesbeiana (bullfrog) tadpoles showed an overall bias to turn to the left, although there was variation in the intensity of this handedness among populations. This is the first documentation of behavioral laterality in an anuran larva. Xenopus laevis (clawed frog) larvae, on the other hand, showed no bias, turning equally often to the right and the left. This phylogenetic difference is consistent with the fact that ranid tadpoles, including R. catesbeiana, are externally asymmetric, whereas pipid tadpoles, including X. laevis, are not. Ranid tadpoles have a single sinistral spiracle through which water that enters the mouth is expelled from the body. Pipid tadpoles, in contrast, have dual symmetrical spiracles, one on each side of their body. A functional/adaptive explanation is offered for why Rana tadpoles, with their left-sided spiracle, should turn to the left after surfacing to breathe air. However, it is also possible that the handedness in spiracle position and turning are not functionally linked. This issue can be resolved by examining tadpole turning bias in anuran families whose larvae have a single, midline spiracle.