Phototactic responses to spectrally dominant stimuli and use of colour vision by adult anuran amphibians: A comparative survey

Abstract

A compartive survey of 127 secies of adult anuran amphibians, drawn from all six sub-orders and all but one of the seventeen families, revealed three basic phototactic responses to spectrally dominant stimuli: (1) a U-shaped response curve high at the ends of the spectrum (violet and red) and low in the middle (eleven species from six families); (2) a blue-mode response (107 species from thirteen families); and (3) random or unclassifiable responses (nine species from five families). The lack of taxonomic correlates suggests that the phototactic types evolved in anuran ancestors, and the lack of ecological correlates rejects Muntz's (1962) ‘escape-to-water’ functional hypothesis of the blue-mode phototaxis. Special ‘colour-vs-brightness’ (CvB) tests confirm that the blue-mode response involves true colour vision (sixteen species), but fails to provide such evidence for the U-shaped response (two species), which may therefore be merely a spectral sensitivity phenomenon. No effect on spectral responses was found according to intraspecific geographic variation, sex, ambient temperature, type of locomotion, time of day or time of year. There may be an effect of the adaptational state of the eye which demands fuller investigation. Of 116 species also tested for response to intensities of white light ( Jaeger & Hailman 1973), 96 per cent obey the correlation of photonegativity with U-shaped response, photopositivity with blue-mode response and unimodal intensity preference with unclassifiable spectral response. Extending our previous model ( Jaeger & Hailman 1973), we propose that every species has a preferred intensity, termed the optimum ambient illumination (OAI), and every species shows all three kinds of spectral responses. When ambient illumination is brighter than the species' OAI, the animal becomes photonegative and shows the U-shaped spectral response; when ambient is dimmer, the animal becomes photopositive and shows the blue-mode colour-preference; and when ambient is at the OAI, the animal strives to maintain that intensity while being indifferent to colours. A given species may appear to be photonegative or photopositive when the range of experimental intensities fails to include the species' OAI. The true adaptive differences among anuran species in phototactic behaviour lies only in the set-point of the OAI, which is at low intensities for nocturnal or fossorial species, and high for diurnal or open-habitat species. Aquatic species moving to the surface of a pond, terrestrial species moving toward breaks in the forest canopy, and arboreal species moving towards the crowns or branch-tips of trees all achieve a higher ambient illumination by photo-positive movements toward the blue light of sky. Finally, if our model be correct, Muntz's (1966) receptor-coding scheme is not adequate to explain the interrelations between responses to intensities and colours, so the physiological mechanism of phototaxis remains an open question.