TAXONOMY & NOMENCLATURE
(Anderson & Hanley 2001) (Barros et al 2008) (Gaudin & McDonald 2008) (Naish 2005) (Wilson & Reeder 2005)
Describer (Date): Choloepus didactylus (Linnaeus 1758) Syst. Nat., 10 th ed., 1:35
Choloepus hoffmanni (Peters 1858) Peters. 1858. Monatsb. K. Preuss. Akad. Wiss.
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Magnorder: Xenarthra (sloths, anteaters, and armadillos)
Order: Pilosa (sloths and anteaters)
Family: Megalonychidae (includes living and extinct genera)
Genus: Choloepus (Two-toed Sloths)
Species: Choloepus didactylus (Linnaeus' Two-toed Sloth)
Species: Choloepus hoffmanni (Hoffmann's Two-toed Sloth)
Family:
Family: Bradypodidae
Genus: Bradypus (Three-toed sloths)
Species: Bradypus varigatus (Bolivian or Brown-throated sloth)
Bradypus tridactylus (Pale-throated sloth)
Bradypus torquatus (Maned sloth)
Bradypus
pygmaeus (Pygmy sloth) described in 2001 by Anderson & Handley
Taxonomic History and Nomenclature
- Large difference in chromosomes (number and characteristics) within Choloepus may mean there are even more than 2 species
of two-toed sloths
- Two-toed and three-toed sloths were formerly placed in the same family but the two genera have profound behavioral and anatomical differences and are believed to come from two different fossil lineages. They are now placed in separate families.
- 2-toed is larger, faster, and nocturnal. Diet is more varied - eats leaves and fruit. 6 or 7 neck vertebrae and vestigial tail
- 3-toed is smaller, slower and both diurnal and nocturnal. Highly specialized browsers - eat only leaves. 8 or 9 neck vertebrae. Stout tail is 68 mm (2.7 in ) long
- All sloths have 3 claws on their hindlimbs
- Sloths are more closely related to anteaters than armadillos. Members of this group of animals have a reduced number of teeth.
- Anteaters are toothless
- Sloths have only 10 upper teeth and 8 lower teeth
- Anteaters and sloths were once grouped together as "Edentata" meaning 'without teeth'.
- Common names for the 2-toed sloths:
- Choloepus didactylus: Southern two-toed sloth, Linnaeus's two-toed sloth
- Choloepus
hoffmanni: Hoffman's two-toed sloth
- Local names: Prezoso de dos dedos is used in many Spanish-speaking countries ("perezoso" means "lazy"). Preguica real or unau (in Brazil); perico ligero (in Columbia);cucala (in Honduras)
Phylogeny
- Extensive fossil record dates from Eocene - more than 200 genera (Barros et al 2008)
- Found in South, Central, and North America, Caribbean and Antarctica. Size varied from a medium dog to elephant. (Nowak 2008)
- Phylogenetic relationships are not resolved. Gaudin and McDonald postulate four families: Megalonychidae, Megatheriidae, Nothrotheriidae and Mylodontidae. Bradypus is placed as a sister taxon to all other sloths. Choloepus is incorporated into the Megalonychidae.
- At least three genera of giant ground sloths lived in Southern California during the Pleistocene. A Shasta's ground sloth skeleton was recently excavated in Carlsbad (thought to be about 2.1 m (7 ft) long and weighing about 136 - 181 kg (300 - 400 lbs) — bear sized.
- First sloths arrived in North America about 7 million years ago (presumably by swimming between islands from South America)
- By 13,000 years ago, all ground sloths were extinct except for a few populations on Caribbean islands.
DISTRIBUTION & HABITAT
(Emmons 1990) (Gilmore 2008) (Merritt 2006) (Montgomery & Sunquist 1973)
Distribution
- Limited to Central and South America. The 2 species are partially sympatric (overlap) in the Andean regions and western Amazonia. Both overlap with the 3-toed sloth.
- Choloepus didactylus (Columbia - east of Andes, Venezuela, Guianas, Ecuador, Peru and N. Brazil) Ranges from sea level up to 2,438 m (7,999 ft) (Britton 1941).
Native to: Brazil, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname, Venezuela.
- Choloepus hoffmanni 2 disjunct populations. Range from sea level to 3,300 m (10,826 ft) in Panama; up to 1,150 m (ft) in the southern Andes of Venezuela.
- Northernmost population ranges from Nicaragua south into western Venezuela
- Southern population is found from north-central Peru, through extreme southwestern Brazil (southwestern Amazon and probably Acre) to central Bolivia.
- There is a doubtful record for this species from the Aripuana, Mato Grosso State, Brazil (F. Avilla-Pires, pers. commun). The range of the species within Brazil is unclear and further surveys are needed.
Native to: Bolivia, Brazil, Colombia, Costa Rica, Ecuador, Honduras, Nicaragua, Panama, Peru, Venezuela
Habitat:
- Limited to humid, warm, well-established tropical and cloud forests
- Trees with interlacing crowns allow for lateral movement without descending to ground (descend approximately weekly to defecate)
- Prefer crowns of trees with lianas to provide cover from predators and shelter from sun during the day.
- Trees also provide body support for sleeping — no nest building
- Other factors that determine habitat selection
- Physiological state of animal and plants
- Recent history of movement and feeding
- Social inheritance of mother's home range and tree preference
- Plant species diversity where the animal lives
- Seasonal changes in plant composition & availability
Home Range:
-
Varies between 1.2 and 6.5 ha for C. didactylus (Taube, 1997)
PHYSICAL CHARACTERISTICS
(Adam 1999) (Gilmore et al 2000) (Goffart 1971) (Mendel 1981) (Meritt 1985) (Nowak 1999)
Body Weight: 4 to 9 kg (10 to 20 lbs); Newborns weigh about 350 to 454 grams (12 oz )
Body Length: Adults: 540 to 740 mm (21 to 29 inch long)
Tail Length: (C. didactylus) 23 ± 7 mm (about 1 inch )
General
- 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 inches)
- 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
Pelage/ Skin
- 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 sp. 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.
BEHAVIOR & ECOLOGY
(Gilmore 2008) (Meritt 1985)
(Montgomery & Sunquist 1978) (Ramprashad 1985) (Sunquist & Montgomery 1973)
Activity Cycle
- Slow moving, solitary, arboreal, herbivores
- Two-toed sloth is exclusively nocturnal (three-toed sloth is active day and night)
- Activity begins about one hour following sunset
- Active about 11 hours throughout night
- Activity ceases about 2 hours before dawn
- Average 7.6 hours of activity/day
- 55% of time is spent in bouts of continuous activity lasting 2 or more hours
- Sleep 15 to 18 hours per day
- Approximately 6 hours each day are spent foraging
- Most two-toed sloths change to a different tree each night
- Does everything hanging upside down — eating, sleeping, mating and even giving birth!
- Basal metabolic rates lie between 40 and 60% of that expected from their mass
Social Groups
- Typically solitary; females occasionally feed in the same tree
- Young will stay with mother for nine to twelve months, depending on the species (Taube 2001)
Aggression
- Charges suspected aggressor, pulls objects to mouth with forearm and bites — sharp teeth are like canines of carnivores
- Three-toed sloth is more docile
- Their front teeth are not sharp so they don't bite defensively
- They are often taken for pets because of mild disposition
Communication
Tactile
- Mutual grooming observed only during infant/mother relationship —
mother vigorously licks head, face and ano-genital region for first few weeks of infant's life. Stimulates young to pass urine and feces. (Meritt 1985)
- Self grooming/scratching with fore claws
Vocalization
- Generally silent; hiss in defense; low bleats in distress
- Sloth infants separated from their mothers will let out a loud bleat lasting 30-90 seconds
- Distress call is low-pitched
- Three-toed young's call is a rising whistle (Montgomery & Sunquist 1974)
- Sloth ear appears designed for low-frequency sounds (Ramprashad 1985)
Olfaction/Scent Marking
- Excellent sense of smell. Olfactory bulbs well-developed
- Males scent mark on tree branches from a gland near the anus
Locomotion
- Slow and deliberate
- Agile in trees, with claws hooking onto branches
- Climbing speed estimated at 1.8 to 2.4 meters/ minute (6 to 8 feet/ minute or 480 feet/ hour ) under duress 274.3 meters/hour (900 feet/hour)
- Slower on the ground: 16.1 meters/hour (53 feet/hour)
- Physically incapable of truly walking. Drag themselves with claws and forelimbs for short distances, usually on the way from one tree to another.
- Excellent swimmers. May drop from a tree into a river and swim to shore.
- Range: can cover 38.1 meters (125 feet) of forest turf each day
Interspecies Interaction
- Preyed upon by harpy eagles, anacondas, jaguars, ocelots and, of course, humans; excellent camouflage and slow movement help them elude predators
- Several species of pyramid moths occasionally inhabit fur (far more common on Bradypus)
- Sloths are hosts for several mosquito-borne virus diseases: St. Louis encephalitis and Venezuelan encephalitis
- Sloths are unusual virus hosts— show no symptoms of many diseases although high amounts of virus can be found in their blood
- By defecating at the base of their host cecropia tree, the sloth provides the tree with fertilizer
DIET & FEEDING
(Gilmore 2000)(Goffart 1971) (Parra 1978) (Sunquist & Montgomery 1978)
- Folivores or generalized herbivores — will eat leaves, twigs, buds, fruit, and occasionally rodents, and insects
- Favorite trees (*indicates species favored by all species)
- Dipteryx panamensis*
- Sapium caudatum
- Terminalia amazonica
- Spondius nigrescens
- Trattinickia aspera
- Chrysophyllum panamensis
- Anacardium excelsium*
- Entirely nocturnal (feed only at night)
- Eat in every position. Pull a leafy branch to mouth, break it off and then hold stem between palm and claw, turning and pushing the stem forward to eat the leaves
- Large thick tongue, densely covered with sharp, minute, backward-directed spines
- Slow metabolic rate means they can survive on a small amount of nourishment. They are quite light weight for a mammal their size, which is helpful when harvesting leaves from long, thin branches.
- Seldom drink. Moisture is obtained from plant matter and dew on leaves
- Metabolic rate is only about 40 to 60% of that of other mammals this size (Gilmore 2000)
- Food has low energy content — may contain poisonous compounds that require low rate of absorption for detoxification
- Digestion occurs in a large 4-chambered stomach (like ruminants) but unlike ruminants who possess long intestines, the sloth intestine is short
(even shorter than in carnivores)
- Bacteria hydrolyze the plant cellulose
- Digestion slows in lower temperatures — increased mortality has been observed in cold, rainy months (Sunquist & Montgomery)
- Passage of food through gut takes 6-21 days. (Usually takes hours for other herbivores); process is similar in tortoise (Parra 1978)
- Young leaves digested at highest rates. Only mature leaves of certain species can be digested quickly enough to avoid starvation
- Due to its slow metabolism and high-cellulose diet, defecation and urination occur only once a week. Descends to ground to deposit mass of fecal pellets and urinate (occur simultaneously)
- Weight loss due to elimination has exceeded 30% of body weight but this is exceptional
REPRODUCTION & DEVELOPMENT
(Eisenberg & Maliniak 1985) (Meritt 1985) (Nowak 1999) (Taube 2001)
Courtship (Based on captive observations)
- Mating occurs throughout the year, though some observers detect a marked mating season in March and April
- Females in estrus appear to initiate mating (Meritt 2985)
Reproductive Rate
- Inter-birth intervals:
- C. hoffmanni about 15 months: young independent at 10 months
- C. didactylus about 16 months; young independent at 12 months
- Females do not show a decline in fecundity as they age (Nowak 1999)
Gestation
- Similar in both Choleopus species: 10 months (Taube 1985)
- Bradypus gestation about 6 months
Life Stages
Birth
- Litter size: one
- Weight: 340 - 400 grams (about 12 ounces)
- Length: 25.4 cm (10 inches)
- Mother gives birth on ground or in upside down, hanging position; infant grabs onto her fur and makes its way to her chest to nurse
- Milk is higher in fat (6.9 %) and protein (61%)
than cow's milk
Infant (< 1 year old)
- Infant is born alert and strong, eyes open teeth present, claws fully formed. Gripping reflex enables it to climb to mother's abdomen. Nuzzling and suckling by newborn stimulates release of milk — usually within 48 hours
- Newborn nurses for 6015 days for male infant, 27 days for female
- Play behavior observed at 15-19 days
- First hangs upside down at 20-25 days, and regularly feeds away from mother at 5 months
- Young of all species cease nursing at about a month old, but may take leaves even earlier
- Young are carried on mother's abdomen for six to nine months and feed on leaves they can reach from the position
- By 6 months, elimination occurs in adult manner
- May keep a close association for up to two years
Adult
- Two-toed sloths reach sexual maturity at approximately three years of age (females) and four to five years old (males)
- External genitalia small and inconspicuous (sexing is difficult)
Longevity (Nowak 1999)
