262
W.-H. U et al.
the existence of this triallelic system, heterozygous NWM females are trichromatic,
though males and homozygous females are dichromatic. Recently Shyue et al. (1998)
and Boissinot et al. (1998) found that the triallelic system also exists in each of the
three additional species studied: the capuchin, the tamarin, and the saki monkey.
The capuchin and tamarin belong to the same family (Cebidae) as do the squirrel
monkey and marmoset, but the saki monkey belongs to the other NWM family,
Atelidae (or Pitheciidae, see above) and is one ofthe NWM species most divergent
from the squirrel monkey and marmoset. Thus, the triallelic system appears to exist
in the majority of NWMs.
The owl monkeys (a NWM genus) are the only nocturnal higher primate. Their
blue opsin gene has been found to be defective (Jacobs et al. 1996b). So, they possess only one functional color vision gene (the X-linked opsin gene) and are monochromatic.
3.3 Prosimians
Recent studies of color vision in prosimians have clarified several issues. It was
previously claimed that the ringtail lemur (Lemur catta), which is a diurnal
pro simian, could make color discriminations, though the capacity was far from
acute (Mervis 1974; Blakeslee and Jacobs 1985). However, Jacobs and Deegan
(1993) found that ringtail lemurs and brown lemurs (also diurnal) have only a
single class of photopigment in the middle to long wavelengths and a short wavelength pigment. They therefore concluded that both of these diurnal prosimians are
dichromatic and speculated that the limited ability of ringtail lemurs to discriminate colors might have resulted from the ability of lemurs to jointly utilize signals
from cones and rods. (Note that a mammal with a short wavelength pigment and a
middle/long wavelength pigment cannot perceive the whole visible color spectrum
and especially cannot distinguish between red and green, but can see only blue and
yellow hues.)
Bushbabies are nocturnal prosimians. In apparent accord with this lifestyle, the
early literature contained repeated claims that the bushbaby retina contains only
rods, but no cones (see the review by Deegan and Jacobs 1996). Indeed, no cones
were detected in a study using micro spectrophotometric measurements of the absorbance properties of individual photo receptors (Petry and Harosi 1990). However, labeling the photoreceptors in Galago garnetti with cone-specific antibodies,
Wikler and Rakic (1990) estimated the proportion of cones in the bushbaby retina
to be from -1 % to -3%, depending on the retina location. Recently, Deegan and
Jacobs (1996) found that the cones of the thick-tailed bushbaby (Otolemur
crassicaudatus) contains a single type of photopigment with spectral peak near 545
nanometer (nm).
As color vision is presumably of no use to a strictly nocturnal animal, it has
been commonly thought that color pigment genes in an animal with a long history
of nocturnal life should have evolved rapidly and would become degenerate or nonfunctional because of relaxation in their functional constraints. In fact, the blue
W.-H. U et al.
the existence of this triallelic system, heterozygous NWM females are trichromatic,
though males and homozygous females are dichromatic. Recently Shyue et al. (1998)
and Boissinot et al. (1998) found that the triallelic system also exists in each of the
three additional species studied: the capuchin, the tamarin, and the saki monkey.
The capuchin and tamarin belong to the same family (Cebidae) as do the squirrel
monkey and marmoset, but the saki monkey belongs to the other NWM family,
Atelidae (or Pitheciidae, see above) and is one ofthe NWM species most divergent
from the squirrel monkey and marmoset. Thus, the triallelic system appears to exist
in the majority of NWMs.
The owl monkeys (a NWM genus) are the only nocturnal higher primate. Their
blue opsin gene has been found to be defective (Jacobs et al. 1996b). So, they possess only one functional color vision gene (the X-linked opsin gene) and are monochromatic.
3.3 Prosimians
Recent studies of color vision in prosimians have clarified several issues. It was
previously claimed that the ringtail lemur (Lemur catta), which is a diurnal
pro simian, could make color discriminations, though the capacity was far from
acute (Mervis 1974; Blakeslee and Jacobs 1985). However, Jacobs and Deegan
(1993) found that ringtail lemurs and brown lemurs (also diurnal) have only a
single class of photopigment in the middle to long wavelengths and a short wavelength pigment. They therefore concluded that both of these diurnal prosimians are
dichromatic and speculated that the limited ability of ringtail lemurs to discriminate colors might have resulted from the ability of lemurs to jointly utilize signals
from cones and rods. (Note that a mammal with a short wavelength pigment and a
middle/long wavelength pigment cannot perceive the whole visible color spectrum
and especially cannot distinguish between red and green, but can see only blue and
yellow hues.)
Bushbabies are nocturnal prosimians. In apparent accord with this lifestyle, the
early literature contained repeated claims that the bushbaby retina contains only
rods, but no cones (see the review by Deegan and Jacobs 1996). Indeed, no cones
were detected in a study using micro spectrophotometric measurements of the absorbance properties of individual photo receptors (Petry and Harosi 1990). However, labeling the photoreceptors in Galago garnetti with cone-specific antibodies,
Wikler and Rakic (1990) estimated the proportion of cones in the bushbaby retina
to be from -1 % to -3%, depending on the retina location. Recently, Deegan and
Jacobs (1996) found that the cones of the thick-tailed bushbaby (Otolemur
crassicaudatus) contains a single type of photopigment with spectral peak near 545
nanometer (nm).
As color vision is presumably of no use to a strictly nocturnal animal, it has
been commonly thought that color pigment genes in an animal with a long history
of nocturnal life should have evolved rapidly and would become degenerate or nonfunctional because of relaxation in their functional constraints. In fact, the blue