Two-toed Sloths (Choloepus spp.) Fact Sheet: Physical Characteristics

Body measurements

• Adapted for arboreal lifestyle
  • Long curved claws enable the sloth to latch onto tree branches
  • Leathery soles on forefeet and hind feet
  • Reinforced lumbar vertebrae make upside-down lifestyle possible
  • Skeleton, muscles and joint anatomy adapted to support hanging lifestyle
  • Algae growth on fur is good camouflage
  • Stomach is complex for digesting foliage
• Long limbs—forelegs only slightly longer than hind legs (3-toed sloth's forelegs are 35% longer than hind legs)
• Two-toed sloths have hands with two functioning fingers with sharp claws 76-102 mm (3-4 in)
• Three-toed sloths have three fingers on their hands.
• Three claws on hind limbs.
• Head rounded.
• Face is not furred.
• Eyes face forward. Eyelids fleshy. Irises reddish-brown, pupils are round (Meritt)
• Females have 2 mammae

 Species differences

• C. didactylus: Hair uniformly brown. Face often the same color.
• C. hoffmanni: Lighter than C. didactylus. Throat is pale. Chest is darker.
• Gray-brown/tan hair during dry season, covered with lichens, moss and old leaves. When curled up they look like ant or termite nests.
• Tinted green in the wet season due to algal growth. (Trichophylus spp. and Cyanoderma choloepi)
• Unique hairs have 8-11 longitudinal furrows. Unusual growth pattern: hair parts in the middle of abdomen, growing out and down. (Both features may help to slough off water)
• When frightened, young sloth's hair "puffs out", almost doubling its size.
• External coat of long coarse hair and dense smooth undercoat provides good insulation
• Sweat glands are present but none on pads of feet. Sweat glands are large and abundant on snout

Skeleton

•   Elongated limbs and trunk are adaptations to acrobatic, hanging lifestyle
  • Modification of hands and feet into rigid hooks — the 2 digits on each forefoot are closely bound with skin their entire length. (Nowak)
  • Claws are mounted on tips of 2nd and 3rd fingers, which together form the "hook"
  • A wide range of movement possible in several directions from the wrist and between-wrist joints
  • Extreme mobility of limbs allows support for the body in many positions
  • Two-toed sloths possess 6-9 neck vertebrae and can rotate heads 90 degrees
  • Three-toed sloths have 8-9 neck vertebrae and can rotate heads 270 degrees
    • Nearly all other mammals have 7 neck vertebrae
  • Three-toed sloths may have quite small or absent canine-like upper teeth.

Teeth

• 5 simple, peg-like teeth on each side in upper jaw. 4 on each side of lower jaw = 18 total.
• No enamel coating on teeth
• Two large, blade like teeth in front for biting. (Although they look like canines, they are actually derived from other teeth). They are sharpened by rubbing against lower teeth.
• Teeth grow throughout lifetime and have a cupped grinding surface.

Ears

• Small external ears, close to the head, imbedded in fur. Hearing is poor (Nowak 1999)

Vision

• Eyes very mobile — can be partly retracted when eyelids are closed
• Distribution of photoreceptors indicates adaptation to night vision
• No ciliary muscles means near-vision is poor — most are myopic. Rely on other senses to obtain food and make contact with other sloths.
• Convex cornea and thick lens means poor discrimination - Rely little on vision to carry out normal patterns of behavior (Mendel)
• Blink frequently and slowly — often one eye at a time

Internal Organs

• Unlike other mammals, organ including the stomach, spleen and liver are located in different areas, due to upside-down lifestyle.
• Four-chambered stomach is filled with bacteria, which helps ferment the plant matter consumed

Smell

• Olfactory bulbs extremely well-developed

Thermoregulation

• Unlike most mammals — body temperature varies with temperature of environment and is lower than most mammals 33-36 °C (91.4 - 96.8°F). Echidna has lowest body temperature of 28-29°C (82.4 - 84.2°F).
• Fur provides insulation to protect against cooling
• Regulate body temperature by moving about canopy — seeking shade or sun
• Have difficulty maintaining body temperature on rainy days
• They cannot shiver to keep warm as other mammals do because of the unusually low metabolic rates and reduced musculature. They have the lowest muscle mass relative to overall body weight of any mammal.

The eyes of two-toed sloths are adapted for night vision.

Their near vision is thought to be poor.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Adam (1999)
Gilmore et al. (2000)
Goffart (1971)
Mendel (1981)
Meritt (1985)
Nowak (1999)