The Dolphin Institute - Dolphin Research Summaries

(C) 1976 University of Hawaii

A set of four experiments was performed on a bottlenosed dolphin, Tursiops truncatus, in order to test its abilities to perform discriminations of purity and wavelength, as well as to determine its spectral sensitivity in air and in water, and under low and high intensity light conditions. The stimuli were projected monochromatic lights produced by Ealing TFP interference filters (blue at 485 nm, green an 547 nm and red at 632 nm) and achromatic light. Two experiments were attempts to train purity and wavelength discriminations while intensity was randomly varied, first by using a purity cue in a spatial reversal problem, then by using variation of size and duration of S- as secondary cues in Go-No purity and wavelength discriminations. Neither experiment succeeded in training a purity or a wavelength discrimination. The other two experiments yielded spectral sensitivities under several conditions for the dolphin, the first obtaining intensity difference thresholds to the three monochromatic lights in air and in water under low light intensity conditions, the second obtaining absolute thresholds to the three monochromatic lights in water under low and high intensity conditions. The three sets of sensitivity values obtained under low intensity light conditions were fitted by a Dartnall function peaking at 495 nm. The one set of sensitivity values obtained under high intensity light conditions was fitted by a Dartnall function at 500 nm. Human spectral sensitivity functions obtained in the same underwater conditions indicated a predominantly photopic spectral sensitivity function under high intensity light conditions, and also with the difference thresholds under low intensity light conditions. The absolute thresholds in the low intensity condition were best fitted by a mesopic-shaped spectral sensitivity function, but still peaking at 507 nm. The dolphin function was 0.5 log units more sensitive than the human function in low intensity light, and 0.2 log units less sensitive in high intensity light, probably because of the higher density of rods and the lower density of cones in the dolphin eye relative to the human eye. It was concluded that the spectral sensitivity functions obtained for the dolphin represented predominantly cone function at high light intensities and predominantly rod function at low light intensities. The rod and cone functions were so similar that the cones were considered to be means of extending the intensity range of which the dolphin’s eye could function, rather than a source of color vision.

Madsen, C. (1976). Tests for color discrimination and spectral sensitivity in the bottlenosed dolphin, Tursiops truncatus. Unpublished doctoral dissertation, University of Hawaii, Honolulu.

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