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L. Peichl, R.R. Dubielzig, A. Kübber–Heiss, C. Schubert, P.K. Ahnelt; Retinal Cone Types in Brown Bears and the Polar Bear Indicate Dichromatic Color Vision . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4539.
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Purpose: Most mammals are cone dichromats, having short–wave sensitive (S–)cones and middle–to–long–wave sensitive (L–)cones. In contrast, the seals are L–cone monochromats lacking the S–cones, probably as an adaptation to their marine visual environment (Peichl et al., 2001, EJN 13: 1520). Do the bears, terrestrial relatives of the seals, possess both cone types? Does the semi–marine polar bear, living in a rather colorless arctic environment, still possess both cone types? Methods: We have studied the polar bear (Thalarctos maritimus) and four subspecies of brown bears (Ursus arctos), viz. European brown bear, Kamtschatka bear, grizzly and Kodiac bear. Eyes were obtained at autopsies of zoo animals and were fixed in formalin. Retinal spectral cone types were assessed using the L–opsin antiserum JH492, the S–opsin antiserum JH455 (both provided by J. Nathans), and the S–opsin antiserum sc–14363 (Santa Cruz Inc.). Results: All bears possessed substantial cone populations, and all possessed L–cones and S–cones. Peak cone density in the Kamtschatka bear was above 30.000/mm², midperipheral cone densities in all bears were above 10.000/mm². In all bears the S–cones formed an unusually high proportion of the cones, varying regionally between 15% and 25%, and reaching 30% at some locations. Double labelling for L– and S–opsin in the European brown bear and polar bear revealed no opsin co–expression, each cone was either a pure L–cone or a pure S–cone. Conclusions: With their L– and S–cones, bears have the potential for dichromatic color vision. The polar bear, which is overall well–adapted to its semi–marine lifestyle and its rather colorless habitat of snow and ice, also has retained both cone opsins. This suggests different visual demands and adaptive pressures on polar bears and seals. Overall cone densities and S–cone proportions in the ursids studied are higher than in most other mammals (overviews: Ahnelt & Kolb, 2000, Prog Ret Eye Res 19: 711; Calderone et al., 2003, BBE 62: 182), indicating an adaptation to specific modes of diurnal vision.
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