Abstract

In this thesis, I have examined the behaviour and some of its neural underpinnings of a ‘model’ animal, the tadpoles and froglets of Xenopus laevis, at different levels of description and detail. At a macroscopic level, I investigated the animals’ movements in a very simple space. Zooming in, I looked at locomotion in freely and fictively swimming animals as well as at some of the sensory and motor consequences of locomotion. For many of these projects, I tested not only one particular developmental stage but a range of stages, allowing me to test for changes in behaviour with development. Methodologically, I employed video tracking to quantify movements in space over a longer period of time, as well as at a higher temporal and spatial resolution for short periods to record head movements during swimming. Semi-intact in vitro preparations of tadpoles were used to examine fictive locomotion and its consequences using electrophysiological recordings of peripheral nerves. Movements in space remained fairly similar over development, from small tadpoles to froglets, with all animals following the walls in a square environment, although the strength of wall following (WF) increased with growth. Tentacles, which are putatively mechanosensory appendages that large tadpoles temporarily possess, did not play any role for the strength of WF. WF was passive at all developmental stages, meaning that the animals never actively turned at a convex curvature to follow the wall, but instead went straight and left the wall. This implies that WF is unlikely to serve a defensive or spatial function. Looking specifically at locomotion in tadpoles showed that these animals commonly swim at 20 - 40 mm/s forward speeds, and move their heads left to right at up to 2500°/s angular velocities. These velocities decrease with development, probably because swimming frequency also decreases, from about 8 to about 5 Hz. Developmentally appropriate swimming frequencies are also seen in fictive swimming when the animals are deprived of normal sensory feedback. The mechanisms behind the developmental decrease in swimming frequency remain to be elucidated; biomechanical factors might well play a role. The left- right head oscillations during swimming also represent vestibular self-stimulation, which reaches amplitudes that are much higher than any of the stimuli used in sensory vestibular experiments. Another consequence of locomotion was observed in large tadpoles with tentacles: These tentacles are retracted during swimming, via a locomotor corollary discharge from the spinal cord. What I have shown in this thesis is first, that navigational behaviour of X. laevis in a simple laboratory setting seems to be mainly driven and constrained by the environment. Second, I have quantified head movements during swimming and therefore vestibular reafference, and found a developmental decrease in the swimming frequency. Finally, I uncovered an unusual effect of locomotion, namely the retraction of the tentacles during swimming. Together, these studies deepen the understanding of behaviour and its consequences in X. laevis.