Ring-tailed Lemur - Anatomy and Physiology

Anatomy and Physiology

The ring-tailed lemur is a relatively large lemur. Its average weight is 2.2 kilograms (4.9 lb). Its head–body length ranges between 39 and 46 cm (15 and 18 in), its tail length is 56 and 63 cm (22 and 25 in), and its total length is 95 and 110 cm (37 and 43 in). Other measurements include a hind foot length of 102 and 113 mm (4.0 and 4.4 in), ear length of 40 and 48 mm (1.6 and 1.9 in), and cranium length of 78 and 88 mm (3.1 and 3.5 in).

The species has a slender frame and narrow face, fox-like muzzle. The ring-tailed lemur's trademark—a long, bushy tail—is ringed in alternating black and white transverse stripes, numbering 12 or 13 white rings and 13 or 14 black rings, and always ending in a black tip. The total number of rings nearly matches the approximate number of caudal vertebrae (~25). Its tail is longer than its body and is not prehensile. Instead, it is only used for balance, communication, and group cohesion.

The pelage (fur) is so dense that it can clog electric clippers. The ventral (chest) coat and throat are white or cream. The dorsal (back) coat varies from gray to rosy-brown, sometimes with a brown pygal patch around the tail region, where the fur grades to pale gray or grayish brown. The dorsal coloration is slightly darker around the neck and crown. The hair on the throat, cheeks, and ears is white or off-white and also less dense, allowing the dark skin underneath to show through. The muzzle is dark grayish and the nose is black, and the eyes are encompassed by black triangular patches. Facial vibrissae (whiskers) are developed and found above the lips (mystacal), on the cheeks (genal), and on the eyebrow (superciliary). Vibrissae are also found slightly above the wrist on the underside of the forearm. The ears are relatively large compared to other lemurs and are covered in hair, which has only small tufts if any. Although slight pattern variations in the facial region may be seen between individuals, there are no obvious differences between the sexes.

Unlike most diurnal primates, but like all strepsirhine primates, the ring-tailed lemur has a tapetum lucidum, or reflective layer behind the retina of the eye, that enhances night vision. The tapetum is highly visible in this species because the pigmentation of the ocular fundus (back surface of the eye), which is present in—but varies between—all lemurs, is very spotty. The ring-tailed lemur also has a rudimentary foveal depression on the retina. Another shared characteristic with the other strepsirrhine primates is the rhinarium, a moist, naked, glandular nose supported by the upper jaw and protruding beyond the chin. The rhinarium continues down where it divides the upper lip. The upper lip is attached to the premaxilla, preventing the lip from protruding and thus requiring the lemur to lap water rather than using suction.

Scent glands on a male: the brachial glands on the upper chest (left), and antebrachial gland and spur on the forearm (right)

The skin of the ring-tailed lemur is dark gray or black in color, even in places where the fur is white. It is exposed on the nose, palms, soles, eyelids, lips, and genitalia. The skin is smooth, but the leathery texture of the hands and feet facilitate terrestrial movement. The anus, located at the joint of the tail, is covered when the tail is lowered. The area around the anus (circumanal area) and the perineum are covered in fur. In males, the scrotum lacks fur, is covered in small, horny spines, and the two sacs of the scrotum are divided. The penis is nearly cylindrical in shape and is covered in small spines, as well as having two pairs of larger spines on both sides. Males have a relatively small baculum (penis bone) compared to their size. The scrotum, penis, and prepuce are usually coated with a foul-smelling secretion. Females have a thick, elongated clitoris that protrudes from the labia of the vulva. The opening of the urethra is closer to the clitoris than the vagina, forming a "drip tip."

Females have two pairs of mammary glands (four nipples), but only one pair is functional. The anterior pair (closest to the head) are very close to the axillae (armpit). Furless scent glands are present on both males and females. Both genders have small, dark antebrachial (forearm) glands measuring 1 cm long and located on the inner surface of the forearm nearly 25 cm (9.8 in) above the wrist joint. (This trait is shared between the Lemur and Hapalemur genera.) The gland is soft and compressible, bears fine dermal ridges (like fingerprints), and is connected to the palm by a fine, 2 mm–high, hairless strip. However, only the male has a horny spur that overlays this scent gland. The spur develops with age through the accumulation of secretions from an underlying gland that may connect through the skin through as many as a thousand minuscule ducts. The males also have brachial (arm) glands on the axillary surface of their shoulders (near the armpit). The brachial gland is larger than the antebrachial gland, covered in short hair around the periphery, and has a naked crescent-shaped orifice near the center. The gland secretes a foul-smelling, brown, sticky substance. The brachial gland is barely developed if present at all in females. Both genders also have apocrine and sebaceous glands in their genital or perianal regions, which are covered in fur.

Its fingers are slender, padded, mostly lacking webbing, and semi-dexterous with flat, human-like nails. The thumb is both short and widely separated from the other fingers. Despite being set at a right angle to the palm, the thumb is not opposable since the ball of the joint is fixed in place. As with all strepsirrhines, the hand is ectaxonic (the axis passes through the fourth digit) rather than mesaxonic (the axis passing through the third digit) as seen in monkeys and apes. The fourth digit is the longest, and only slightly longer than the second digit. Likewise, the fifth digit is only slightly longer than the second. The palms are long and leathery, and like other primates, they have dermal ridges to improve grip. The feet are semi-digitigrade and more specialized than the hands. The big toe is opposable and is smaller than the big toe of other lemurs, which are more arboreal. The second toe is short, has a small terminal pad, and has a toilet-claw (sometimes referred to as a grooming claw) specialized for personal grooming, specifically to rake through fur that is unreachable by the mouth. The toilet-claw is a trait shared among nearly all prosimian primates. Unlike other lemurs, the ring-tailed lemur's heel is not covered by fur.

Like other lemurs, the ring-tailed lemur has a claw-like nail (toilet-claw) on its second toe (left) and dermal ridges on its hands to improve its grip (center). Unlike other lemurs, it lacks fur on its heel (right).