Thermal activation of escape swimming in post-hatching Xenopus laevis frog larvae.

Survival requires the selection of appropriate behavioural responses in the face of danger. With respect to the threat of predation, both the decision to escape and the underlying neuronal mechanisms have been extensively studied, but processes that trigger evasion of abiotic stressors, which are potentially hazardous to survival, are less well understood. Here, we document the interplay between rhythmic locomotory and 'C-start' escape swimming in Xenopus frog larvae when exposed to hyperthermic conditions. As temperature rises, swim cycle frequency increases while swim bout duration decreases, until swimming can no longer be initiated by sensory stimuli. Above a critical higher temperature, more intense sequences of spontaneous high amplitude C-start escape activity occur. Each C-start is followed by a few cycles of fast rhythmic swimming in which activity alternates between the two sides. The initial, high amplitude ventral root burst of an escape sequence propagates rostrocaudally approximately threefold faster than subsequent cycles. The high conduction velocity of this initial burst is consistent with the activation of a Mauthner neuron, one of a pair of giant reticulospinal neurons in fish and amphibians. In support of the involvement of a Mauthner neuron, unilateral lesions of the caudal hindbrain eliminated escape activity on the operated side, but activity remained on the un-operated side. Behaviourally, tadpoles responded to temperature ramps with a sequence of C-start responses in which the body arced through approximately 130 degrees in 22 ms, followed by high frequency swimming. These results suggest that high temperature activates the Mauthner neurons to trigger C-start escape behaviour.