The influence of visual and tactile stimulation on growth and metamorphosis in anuran larvae

1. Sensory modalities that allow tadpoles to assess their environment, and subsequently mediate their development, are not well understood. 2. By putting clay model tadpoles into the tanks with live tadpoles we have enhanced tactile and visual stimuli for tadpoles of three species (Rana sylvatica, Bufo americanus and Xenopus laevis) in a controlled fashion. The goal was to determine whether visual and tactile cues in the absence of chemical signals influenced tadpole growth and development. 3. The response to enhanced visual and tactile stimuli was strong in Rana, intermediate in Xenopus, but absent in Bufo tadpoles. Rana tadpoles that experienced both stimuli enhanced developed the fastest and metamorphosed at the smallest body size. Development was slower in the treatments with only one stimulus enhanced, and slowest in the controls. 4. Our results suggest that tadpoles use both vision and mechanoreception for environment assessment, and that they are able to modify their growth and developmental rates in response to sensory enrichment. 5. Tadpoles exposed to the combination of visual and tactile stimulation showed the highest whole-body content of the stress hormone corticosterone, suggesting that the enhanced stimuli were experienced as stressful. Corticosterone is known to synergize with thyroid hormone to promote metamorphosis.