Strepsirrhini - Anatomy and Physiology

Anatomy and Physiology

All lemuriforms possess a specialized dental structure called a toothcomb, with the exception of the aye-aye, in which the structure has been modified into two continually growing (hypselodont) incisors (or canine teeth), similar to those of rodents. The toothcomb consists of either two or four procumbent lower incisors and procumbent lower canine teeth followed by a canine-shaped premolar. It is used to comb the fur during oral grooming. Shed hairs that accumulate between the teeth of the toothcomb are removed by the sublingua or "under-tongue". Lemuriforms also possess a grooming claw on the second digit of each foot for scratching. Adapiforms did not possess a toothcomb. Instead, their lower incisors varied in orientation—from somewhat procumbent to somewhat vertical—and the lower canines were projected upwards and were often prominent. Adapiforms may have had a grooming claw, but there is little evidence of this.

Like all primates, strepsirrhine orbits (eye sockets) have a postorbital bar, a protective ring of bone created by a connection between the frontal and zygomatic bones. Both living and extinct strepsirrhines lack a thin wall of bone behind the eye, referred to as postorbital closure, which is only seen in haplorhine primates. Although the eyes of strepsirrhines point forward, giving stereoscopic vision, the orbits do not face fully forward. Among living strepsirrhines, most or all species are thought to possess a reflective layer behind the retina of the eye, called a tapetum lucidum, which improves vision in low light, but they lack a fovea, which improves day vision. This differs from tarsiers, which lack a tapetum lucidum but possess a fovea.

Strepsirrhine primates have a brain relatively comparable to or slightly larger in size than most mammals. Compared to simians, however, they have a relatively small brain-to-body size ratio. Strepsirrhines are also traditionally noted for their unfused mandibular symphysis (two halves of the lower jaw), however, fusion of the mandibular symphysis was common in adapiforms, notably Notharctus. Also, several extinct giant lemurs exhibited a fused mandibular symphysis.

Many nocturnal species have large, independently movable ears, although there are significant differences in sizes and shapes of the ear between species. The structure of the middle and inner ear of strepsirrhines differs between the lemurs and lorisoids. In lemurs, the tympanic cavity, which surrounds the middle ear, is expanded. This leaves the ectotympanic ring, which supports the eardrum, free within the auditory bulla. This trait is also seen in adapiforms. In lorisoids, however, the tympanic cavity is smaller and the ectotympanic ring becomes attached to the edge of the auditory bulla. The tympanic cavity in lorisoids also has two accessory air spaces, which are not present in lemurs. Both lorisoids and cheirogaleid lemurs have replaced the internal carotid artery with an enlarged ascending pharyngeal artery.

Sexual dichromatism (different coloration patterns between males and females) can be seen in most brown lemur species, but otherwise lemurs show very little if any difference in body size or weight between sexes. This lack of sexual dimorphism is not characteristic of all strepsirrhines. Some adapiforms were sexually dimorphic, with males bearing a larger sagittal crest (a ridge of bone on the top of the skull to which jaw muscles attach) and canine teeth. Lorisoids exhibit some sexual dimorphism, but males are typically no more than 20 percent larger than females.