Abstract:
We used immunocytochemistry to determine the presence and topographical density distributions
of rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) in the
four-striped field mouse (Rhabdomys pumilio) and the Namaqua rock mouse (Micaelamys
namaquensis). Both species possessed duplex retinas that were rod dominated. In R. pumilio,
the density of both cones and rods were high (cone to rod ratio: 1:1.23) and reflected the
species' fundamentally diurnal, but largely crepuscular lifestyle. Similarly, the ratio of cones
to rods in M. namaquensis (1:12.4) reflected its nocturnal lifestyle. Similar rod density peaks
were observed (R. pumilio: ~84467/mm2; M. namaquensis: ~81088/mm2), but a density gradient
yielded higher values in the central (~56618/mm2) rather than in the peripheral retinal
region (~32689/mm2) in R. pumilio. Two separate cone types (S-cones and M/L-cones)
were identified implying dichromatic color vision in the study species. In M. namaquensis,
both cone populations showed a centro-peripheral density gradient and a consistent S- to
M/L-cone ratio (~1:7.8). In R. pumilio, S cones showed a centro-peripheral gradient (S- to
M/L-cone ratio; central: 1:7.8; peripheral: 1:6.8) which appeared to form a visual streak, and
a specialized area of M/L-cones (S- to M/L-cone ratio: 1:15) was observed inferior to the
optic nerve. The number of photoreceptors per linear degree of visual angle, estimated from
peak photoreceptor densities and eye size, were four cones and 15 rods per degree in M.
namaquensis and 11 cones and 12 rods per degree in R. pumilio. Thus, in nocturnal M.
namaquensis rods provide much finer image sampling than cones, whereas in diurnal/crepuscular
R. pumilio both photoreceptor types provide fine image sampling. IpRGCs were comparably sparse in R. pumilio (total = 1012) and M. namaquensis (total = 862), but were
homogeneously distributed in M. namaquensis and densest in the dorso-nasal quadrant in
R. pumilio. The adaptive significance of the latter needs further investigation.
