Assessing and interpreting lateralised behaviours in anuran larvae.
We review here what is known about lateralised behaviours in tadpoles, focusing on turning biases and methods for testing them. Two testing protocols have been used that are specific for explosive turns which are likely to be Mauthner cell-mediated. They involve recording the turns made by tadpoles: (1) upon descent after they take a breath of air, and (2) after they are mechanically startled. Turning bias has also been explored for tadpoles: (3) in a T-maze, (4) as they exit a tube into a larger arena, and (5) as they turn away from a barrier or object brought towards their snout. Slower turns observed under the last three protocols may not be regulated by the same neuromotor pathways as the faster turns elicited in the first two protocols. Turning biases do not occur in tadpoles of all species. In species where turning biases have been observed during explosive turns, the biases are typically towards the left side. The majority of tadpoles have a single asymmetric left-sided spiracle, yet lateralised behaviours in these tadpoles do not appear to be obligatorily linked to this morphological asymmetry. Tadpole turning biases are usually of the order of 60-90% towards the preferred side and are rarely absolute for individuals, and never for populations. Turning biases, when present, usually appear shortly after hatching and disappear before metamorphosis. Turning biases that appear during metamorphosis may depend on appendicular rather than axial muscle, and thus are fundamentally different behaviours; i.e., involving different neuromuscular components. Lateralised behaviours may occur in anurans before they reach the free-living larval stage. This could include bias in the side towards which a frog embryo coils its tail within the egg case, or the side towards which a newly hatched tadpole leans when lying on the bottom How lateralised behaviours of anurans that change with ontogeny relate to each other is not known Although it has been suggested that turning biases are absent in the more archaic frog genera only two 'archaic' genera have been examined Xenopus and Bombina and the data are too few to discern any clear phylogenetic patterns Lastly we present a model suggesting why tadpoles might have turning biases in situations where maximum speed is paramount This model assumes that the fastest neuromotor programs will be 'hardwired' in the central nervous system The model predicts that the amount of morphological asymmetry in the Mauthner neurons will be directly proportional to the amount of turning bias seen in tadpoles.
Wassersug
Richard J
RJ
Department of Anatomy and Neurobiology, Dalhousie University, Halifax, Nova Scotia, Canada. Tadpole@is.dal.ca
Yamashita
Masamichi
M