Fig. 1: Black-and-white photograph of a bottlenose dolphin, alluding to the colour-blindness of whales and seals. These marine mammals lack blue cones, which seems an odd adaptation to a marine environment where the underwater light field is increasingly blue-shifted with depth.
Photograph: R. Krger
When studying the eyes and retinae of various marine mammals, Leo Peichl from the MPI for Brain Research in Frankfurt/M., Gnther Behrmann from the Alfred-Wegener-Institute for Polar and Marine Research in Bremen, and Ronald Krger from the Zoological Institute of Lund University (Sweden) observed a surprising deficit: All 14 investigated species of toothed whales (dolphins), eared seals (sealions) and earless seals consistently lacked the blue cones. The retinae of these species only contained green cones and rod photoreceptors, the latter being important for achromatic vision at low light levels (scotopic 'night' vision). The deficit was demonstrated immunocytochemically with antibodies against the cone visual pigments. This method allows to analyze the cone types in preserved eyes of animals that had stranded or died in zoos.
On the basis of their taxonomically broad sample of species, Peichl, Behrmann and Krger believe that all whales and seals consistently lack blue cones. Whales and seals belong to different mammalian orders. The ancestors of whales were terrestrial artiodactyls (even-toed ungulates), the closest terrestrial relative of the whales is the hippopotamus. Seals have evolved from terrestrial carnivores, among their close relatives are the wolf, ferret and river otter. In all of these terrestrial relatives, the researchers found blue cones. The loss of the blue cones in the marine representatives of such distant mammalian groups strongly argues for convergent evolution and hence for an adaptive advantage of that loss in the marine habitat. The big puzzle is what this advantage could
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Contact: Dr. Leo Peichl
peichl@mpih-frankfurt.mpg.de
49-69-96769-348
Max-Planck-Gesellschaft
19-Apr-2001