Plasticity of Reactions to Light in Frogs and a Possible Role for the Pineal Eye

Abstract

THE reaction of frogs to light varies considerably. Negative phototaxis as high as 80 per cent has been reported with tests in a two-choice jumping stand or Y maze¹; in another Y maze the distribution between positive and negative responses was about even². In yet other tests a strong positive reaction to light was seen³, sometimes reaching the 90 per cent level⁴. The importance of the exact behavioural situation used to assess the level of positive or negative responses to light can be demonstrated with a sequential choice apparatus (Fig. 1). In this, different batches of Rana temporaria showed some variability in the initial choice between a black or white surrounded water compartment. But, once in the water, there was a 5–1 probability, at the least, that at the second choice point the dark area would be selected, even if the first response had been to the lighter side. Much higher negative phototaxis scores are therefore obtained if second choice is taken into account. The first response appears to sample only part of a behavioural sequence.