Lemur - Anatomy and Physiology - Senses

Senses

The sense of smell, or olfaction, is highly important to lemurs and is frequently used in communication. Lemurs have long snouts (compared to the short snouts of haplorrhines) that are traditionally thought to position the nose for better sifting of smells, although long snouts do not necessarily translate into high olfactory acuity since its not the relative size of the nasal cavity that correlates with smell, but the density of olfactory receptors. Instead, the long snouts may facilitate better chewing.

The wet nose, or rhinarium, is a trait shared with other strepsirrhines and many other mammals, but not with haplorrhine primates. Although it is claimed to enhance the sense of smell, it is actually a touch-based sense organ that connects with a well-developed vomeronasal organ (VNO). Since pheromones are usually large, non-volatile molecules, the rhinarium is used to touch a scent-marked object and transfer the pheromone molecules down the philtrum (the nasal mid-line cleft) to the VNO via the nasopalatine ducts that travel through the incisive foramen of the hard palate.

To communicate with smell, which is useful at night, lemurs will scent mark with urine as well as scent glands located on the wrists, inside elbow, genital regions, or the neck. The scrotal skin of most male lemurs has scent glands. Ruffed lemurs (genus Varecia) and male sifakas have a gland at the base of their neck, while the greater bamboo lemur (Prolemur simus) and the ring-tailed lemur have glands inside the upper arms near the axilla. Male ring-tailed lemurs also have scent glands on the inside of their forearms, adjacent to a thorn-like spur, which they use to gouge, and simultaneously, scent-mark tree branches. They will also wipe their tails between their forearms and then engage in "stink fights" by waving their tail as their opponents.

Lemurs (and strepsirrhines in general) are considered to be less visually oriented than the higher primates, since they rely so heavily on their sense of smell and pheromone detection. The fovea on the retina; which yields higher visual acuity, is not well-developed. The postorbital septum (or bony closure behind the eye) in haplorrhine primates is thought to stabilize the eye slightly, allowing for the evolution of the fovea. With only a postorbital bar, lemurs have been unable to develop a fovea. Therefore, regardless of their activity pattern (nocturnal, cathemeral, or diurnal), lemurs exhibit low visual acuity and high retinal summation. Lemurs can see a wider visual field, however, than anthropoid primates due to a slight difference in the angle between the eyes, as shown in the following table:

Although they lack a fovea, some diurnal lemurs have a cone-rich, although less clustered, area centralis. This area centralis has a high rod-to-cone cell ratio in many diurnal species studied thus far, whereas diurnal anthropoids have no rod cells in their fovea. Once again, this suggests lower visual acuity in lemurs than in anthropoids. Furthermore, the rod-to-cone cell ratio can be variable even among diurnal species. For instance, Verreaux's sifaka (Propithecus verreauxi) and the indri (Indri indri) have only a few large cones scattered along their predominantly rod-dominated retina. The eyes of the ring-tailed lemur contain one cone to five rods. Nocturnal lemurs such as mouse lemurs and dwarf lemurs, on the other hand, have retinas made up entirely of rod cells.

Since cone cells make color vision possible, the high prevalence of rod cells in lemur eyes suggest they have not evolved color vision. The most studied lemur, the ring-tailed lemur, has been shown to have blue-yellow vision, but lacks the ability to distinguish red and green hues. Due to polymorphism in opsin genes, which code for color receptivity, trichromatic vision may rarely occur in females of a few lemur species, such as Coquerel's sifaka (Propithecus coquereli) and the red ruffed lemur (Varecia rubra). Most lemurs, therefore, are either monochromats or dichromats.

Most lemurs have retained the tapetum lucidum, a reflective layer of tissue in the eye, which is found in many vertebrates. This trait is absent in haplorrhine primates, and its presence further limits the visual acuity in lemurs. The strepsirrhine choroidal tapetum is unique among mammals because it is made up of crystalline riboflavin, and the resulting optical scattering is what limits visual acuity. Although the tapetum is considered to be ubiquitous in lemurs, there appear to be exceptions among true lemurs, such as the black lemur and the common brown lemur, as well as the ruffed lemurs. Since the riboflavins in the tapetum have a tendency to dissolve and vanish when processed for histological investigation, however, the exceptions are still debatable.

Lemurs also have a third eyelid known as a nictitating membrane, whereas most other primates have a lesser developed plica semilunaris. The nictitating membrane keeps the cornea moist and clean by sweeping across the eye.