The distributions of rod and cone photoreceptors
have been determined in the retina of the California ground
squirrel, Spermophilus beecheyi. Retinas were
fixed by perfusion and the rods and cones were detected
with indirect immunofluorescence using opsin antibodies.
Local densities were determined at 2-mm intervals across
the entire retina, from which total numbers of each receptor
type were estimated and isodensity distributions were constructed.
The ground squirrel retina contains 7.5 million cones and
1.27 million rods. The peak density for the cones (49,550/mm2)
is found in a horizontal strip of central retina 2 mm ventral
to the elongated optic nerve head, falling gradually to
half this value in the dorsal and ventral retinal periphery.
Of the cones, there are 14 M cones for every S cone. S
cone density is relatively flat across most of the retina,
reaching a peak (4500/mm2) at the temporal end
of the visual streak. There is one exception to this, however:
S cone density climbs dramatically at the extreme dorso-nasal
retinal margin (20,000/mm2), where the local
ratio of S to M cones equals 1. Rod density is lowest in
the visual streak, where the rods comprise less than 5%
of the local photoreceptor population, increasing conspicuously
in the ventral retina, where the rods achieve 30% of the
local photoreceptor population (13,000/mm2).
The functional importance of the change in S to M cone
ratio at the dorsal circumference of the retina is compromised
by the extremely limited portion of the visual field subserved
by this retinal region. The significance for vision, if
any, remains to be determined. By contrast, the change
in rod/cone ratio between the dorsal and ventral halves
of the retina indicates a conspicuous asymmetry in the
ground squirrel's visual system, suggesting a specialization
for maximizing visual sensitivity under dim levels of illumination
in the superior visual field.