Unlike all other New World (platyrrine) monkeys, both male and female howler
monkeys (Alouatta sp.) are obligatory trichromats. In all other platyrrines, only
females can be trichromats, while males are always dichromats, as determined by
multiple behavioral, electrophysiological, and genetic studies. In addition to
obligatory trichromacy, Alouatta has an unusual fovea, with substantially higher
peak cone density in the foveal pit than every other diurnal anthropoid monkey
(both platyrrhines and catarrhines) and great ape yet examined, including humans.
In addition to documenting the general organization of the retinal ganglion cell layer
in Alouatta, the distribution of cones is compared to retinal ganglion cells, to explore
possible relationships between their atypical trichromacy and foveal specialization.
The number and distribution of retinal ganglion cells and displaced amacrine cells
were determined in six flat-mounted retinas from five Alouatta caraya. Ganglion cell
density peaked at 0.5 mm between the fovea and optic nerve head, reaching
40,700?45,200 cells/mm2. Displaced amacrine cell density distribution peaked
between 0.5?1.75 mm from the fovea, reaching mean values between 2,050?3,100
cells/mm2. The mean number of ganglion cells was 1,133,000?79,000 cells and
the mean number of displaced amacrine cells was 537,000?61,800 cells, in
retinas of mean area 641?62 mm2. Ganglion cell and displaced amacrine cell
density distribution in the Alouatta retina was consistent with that observed among
several species of diurnal Anthropoidea, both platyrrhines and catarrhines. The
principal alteration in the Alouatta retina appears not to be in the number of any
retinal cell class, but rather a marked gradient in cone density within the fovea,
which could potentially support high chromatic acuity in a restricted central region.
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