The retinae of insectivores have been rarely studied, and
their photoreceptor arrangements and expression patterns of
visual pigments are largely unknown. We have determined
the presence and distribution of cones in three species
of shrews (common shrew Sorex araneus, greater
white-toothed shrew Crocidura russula, dark forest
shrew Crocidura poensis; Soricidae) and in the
lesser hedgehog tenrec Echinops telfairi (Tenrecidae).
Spectral cone types were identified and quantified in flattened
whole retinae by antisera/antibodies recognizing the
middle-to-long-wavelength-sensitive (M/L-)cone opsin and the
short-wavelength-sensitive (S-)cone opsin, respectively. A
combination of immunocytochemistry with conventional histology
was used to assess rod densities and cone/rod ratios. In all
four species the rods dominate at densities of about
230,000–260,000/mm2. M/L- and S-cones are
present, comprising between 2% of the photoreceptors in the
nocturnal Echinops telfairi and 13% in Sorex
araneus that has equal diurnal and nocturnal activity
phases. This suggests dichromatic color vision like in many
other mammals. A striking feature in all four species are
dramatically higher S-cone proportions in ventral than in
dorsal retina (0.5% vs. 2.5–12% in Sorex,
5–15% vs. 30–45% in Crocidura poensis,
3–12% vs. 20–50% in Crocidura russula,
10–30% vs. 40–70% in Echinops). The
functional and comparative aspects of these structural findings are
discussed.