ABSENCE OF SHORT-WAVELENGTH SENSITIVE CONES IN THE RETINAE OF SEALS (CARNIVORA) AND AFRICAN GIANT RATS (RODENTIA)

Most non-primate mammals have two types of cone: short-wavelength sens itive (S) and middle-to-long-wavelength sensitive (M/L) cones. In two species of African giant rats, Cricetomys gambianus and C. emini, and in two species of earless seals, Phoca hispida and P. vitulina, the re tinal cone types and cone distributions were assessed with antibodies specific for the M/L-cone opsin and the S-cone opsin, respectively. Al l four species were found to completely lack S-cones, while M/L-cones were present in low densities. M/L-cone densities, rod densities and c one/rod ratios were determined across the retina. Cone proportions are about 0.3-0.5% in C. gambianus, 0.5-0.8% in C. emini, and 1.5-1.8% in P. hispida. An absence of S-cones has previously been reported in a f ew nocturnal mammals. As earless seals are visually active during nigh t and day, we conclude that an absence of S-cones is not exclusively a ssociated with nocturnality. The functional and comparative aspects ar e discussed.