From the tiny, playful marmosets of the Amazon rain
forest to the formidable, cunning baboons of the African savannah,
monkeys hold a particular appeal for humans. Along with apes, monkeys
are a popular attraction in zoos, where their antics and their
reputation for cleverness endear them to visitors. More controversial
is the extensive use of monkeys in biomedical research because of their
anatomical, biochemical, and immunological similarities to humans.
Despite their close relationship to these remarkable animals, humans
pose the greatest danger to the survival of the monkeys, as habitat
destruction and hunting have pushed some species to the brink of
extinction.
Monkeys belong to the order Primates, which also includes the prosimians, apes,
and humans. All monkeys are categorized as either Old World or New
World primates. Although scientists differ in their opinions about some
classifications for the families and subfamilies of monkeys, all
classification schemes recognize the distinction between these two
groups. Most authorities believe that these taxa had a common ancestor
in Africa and that the precursors of the New World monkeys crossed the
South Atlantic and colonized South America during the Eocene epoch
(approximately 38 to 54 million years ago). During this period Africa
and South America were much closer together than they are presently,
with many small islands between them. The animals could have traversed
the oceanic gaps if they became isolated on drifting masses of
vegetation. As the continents drifted farther apart and the small
islands were covered by water, the two groups were separated and
therefore prevented from interbreeding. Their different environments
selected different traits, and the two groups evolved into distinct
taxa. (See also Evolution.)
- Japanese macaques (Macaca fuscata) soak near Nagano, Japan.
Monkeys
are found throughout the tropics of Central and South America, Africa,
and Asia. They are not native to North America, Europe, or Australia,
however. A few species of monkeys, such as the rhesus monkey (Macaca mulatta) of northern China and the Japanese macaque (M. fuscata),
occupy temperate habitats. Most monkeys are arboreal forest dwellers,
but some of the Old World species are terrestrial and occupy open areas
such as savannah grassland.
Reproduction is nonseasonal in
both Old World and New World monkeys, but most species bear their young
during periods of the year that are optimal for survival. Seasonal
timing may be governed by annual wet-dry cycles. Monkeys have gestation
periods ranging from roughly five months to more than seven months.
Most species have single young, though the marmosets and tamarins
sometimes produce twins.
Like apes and humans, monkeys are born
with their eyes open. They are highly dependent on their parents for
care during their early life. In some species, the father provides as
much care as the mother. The young mature at 3 to 4 years of age in
most species. Captive monkeys have been known to live for up to 45
years, but their life span in the wild is probably much shorter.
The
diet of most monkeys is based primarily on plant foods, such as fruits,
flowers, and seeds. Some monkeys, such as the colobines and the
howlers, have specialized digestive tracts that are adapted for
digesting leaves. Other species include insects, eggs, and small
vertebrates in their diets.
Like the apes, most monkeys have
opposable thumbs and great toes. In addition, some species have a
prehensile, or grasping, tail, which they use to cling to branches.
Color vision, acute hearing, and some form of vocalization are also
characteristic of both Old World and New World monkeys.
Old World Monkeys
The
Old World monkeys, along with the apes and humans, belong to the
infraorder Catarrhini. The noses of catarrhines are narrow, and the
nostrils are closely spaced and point downward. (The prefix “cata”
means “down.”) The Old World monkeys are generally larger than New
World monkeys. The tails of catarrhines are never prehensile and may be
very short or absent altogether. Catarrhines are native to most of
sub-Saharan Africa and portions of Asia and the East Indies.
All
Old World monkeys belong to the family Cercopithecidae. Cercopithecids
are diurnal, meaning that they are active primarily during the day. The
forelimbs are usually shorter than the hindlimbs, and the five digits
of the hands and feet have flattened nails rather than the claws found
on some New World monkeys. All cercopithecids have ischial callosities,
or brightly colored patches of tough, leathery skin on their rumps,
which are used in dominance and sexual displays. Cercopithecids exhibit
sexual dimorphism, or physical differences between the sexes, which is
manifested generally in size but sometimes as distinct coloration or,
as in baboons, development of the canine teeth.
The Old World monkeys are divided into two subfamilies, the Cercopithecinae and the Colobinae.
The Cercopithecinae
The
cercopithecines are a highly adaptable group that can occupy a variety
of habitats. Some are arboreal, but others are at home on the forest
floor or in open land. The cercopithecines have a snout that
accommodates the strong chewing muscles and molars required for an
omnivorous diet. Another adaptation of the group is cheek pouches
inside the cheeks, which enable them to store a newly found meal and
therefore avoid having to linger in dangerous feeding places.
Baboons, drills, and mandrills.
Baboons (genus Papio)
are terrestrial monkeys found predominantly in open or rocky terrain in
Africa. They have doglike faces and walk on all fours. Baboons are
boisterous, cunning, and often fierce animals that have been known to
raid human settlements for food and occasionally attack humans (see Baboon). The drills and mandrills (genus Mandrillus)
of West Africa are large monkeys characterized by prominent ridges on
each side of the nasal bones. Mandrills provide an excellent example of
the sexual dimorphism found in some primates: the male's fearsome face
is an extraordinary combination of a brilliant blue muzzle and bright
crimson snout, while the female's face is a dull brown. This difference
in body coloration between males and females, called sexual
dichromatism, is found particularly in species in which the male must
go through an elaborate courtship period to win a mate.
Macaques.
These monkeys (genus Macaca)
are primarily Asian. Many live mostly on the ground, and, unlike most
monkeys, some can swim. Many are ill-tempered and aggressive. The
rhesus monkey (M. mulatta), a macaque of northern India and China, is often exhibited in zoos and used widely in medical research. The Barbary ape (M. sylvanus)
of northern Africa is also a macaque. A colony of these monkeys was
established and has been maintained for many years on the Rock of
Gibraltar by the British.
Guenons.
These are common, medium-sized monkeys (genus Cercopithecus)
of Africa. Primarily arboreal, most species spend their days chattering
in treetops. Guenons characteristically have long arms, legs, and
tails; small, round heads; and whiskers and beards. Many have brightly
colored coats and faces.
Mangabeys.
The mangabeys (genus Cercocebus)
are large, slender-bodied monkeys of African forests. Less aggressive
than baboons and macaques, mangabeys are unique among monkeys in being
virtually mute. They communicate by complex facial signals, which
include fluttering eyelids. In species with white or light-colored
eyelids, the fluttering signals are visible even in the deep shade of
an African rain forest. Mangabeys form social groups that may include
more than one family.
The Colobinae
The
colobine subfamily includes the colobus monkeys of Africa and the
langurs of Asia. The colobines are sometimes called leaf monkeys
because of their primarily folivorous diet. Their stomachs, like those
of cows, consist of several compartments. This adaptation enables them
to digest the tough cellulose in leaves. (See also Evolution.)
Colobus monkeys.
These monkeys are divided into three genera. The black and white colobus monkeys belong to the genus Colobus.
The extraordinary coat, or pelage, of these monkeys was prized in the
international fur trade of the 19th century, which led to a drastic
decline in their population that continued with human destruction of
their habitat in the 20th century. The red colobus monkeys of the genus
Piliocolobus have also declined in numbers because of
deforestation and hunting. Although colobus monkeys generally inhabit
rain forests, some species of the red colobus are found in mangroves
and floodplains; in Senegal and Gambia, they are often found in
savannah woodlands. The olive colobus monkey, Procolobus verus,
is the sole member of its genus. In this species, newborns are carried
in the mother's mouth during the first weeks of life, which is unique
among monkeys and apes.
Langurs.
These
are long-limbed, long-tailed monkeys of Asia. They are classified in
seven genera and occupy diverse habitats that range from the desert
edge to the snowline of the Himalayas. One species of large langurs is
suspected by some naturalists of being the legendary “abominable
snowman.” The legend presumably arose from the observation of partially
melted, and thus enlarged, footprints in the snow, which appear to have
been made by a much larger primate.
The langurs of the genus Presbytis
are arboreal forest-dwellers found on the islands of Sumatra, Java, and
Borneo; one species also inhabits parts of the Malay Peninsula. The
Hanuman langur (Semnopithecus entellus), the sole member of its
genus, is well-adapted to a range of environments and inhabits the
Indian subcontinent in semi-desert regions, in tropical forests, and in
alpine scrubland. It is considered sacred by the Hindus of India and is
fed regularly by the people of Jodhpur. The brow-ridged langurs (genus Trachypithecus)
are found in the forests of the Indian subcontinent and in parts of
Southeast Asia. The strikingly colored douc langurs (genus Pygathrix),
which are endemic to Vietnam, Laos, and Cambodia, were pushed to the
brink of extinction by years of war in the region and by intensive
hunting. Hunting also endangers the snub-nosed langurs (genus Rhinopithecus),
which inhabit the mountains of China, Tibet, and northern Vietnam,
because their coats are valued for their supposed medicinal purposes.
The pig-tailed langur (Simias concolor), found only on the
Mentawai Islands off the coast of western Sumatra, is considered one of
the most seriously endangered mammals in the world. Its large size
makes it particularly conspicuous to hunters.
A member of the leaf-monkey family that deserves special mention is the peculiar-looking proboscis monkey (Nasalis larvatus)
of Borneo with its long, banana-shaped nose. The males grow to be quite
large, weighing as much as 50 pounds (23 kilograms). Proboscis monkeys
are usually found near fresh water, often in lowland rain forests or
mangrove swamps. Proboscis monkeys are strong swimmers, perhaps the
best among the primates. Clear-cutting of mangroves has endangered the
species.
New World Monkeys
The
New World, or neotropical, monkeys belong to the infraorder
Platyrrhini. They are distinguished from the Old World primates
primarily by their noses, which are broad and have widely spaced,
outwardly directed nostrils. (“Platy” means “flat,” and “rhin” means
“nose.”) Most New World monkeys are fairly small, with tails that are
usually long and sometimes prehensile. This adaptive trait enables
these arboreal monkeys to hang by their tails from tree branches while
feeding. Unlike their Old World cousins, the New World monkeys lack
ischial callosities. With the exception of the owl monkey, the New
World monkeys are diurnal. They are found only in tropical regions of
Central and South America.
The classification of New World
monkeys at the family level has been a source of continuing debate
among researchers. The most commonly accepted scheme classifies these
primates in two families: the Callitrichidae, which includes the
marmosets and tamarins, and the Cebidae, which includes the other
genera.
The Callitrichidae
The
family Callitrichidae contains the smallest monkeys in the world: the
marmosets and the tamarins. They are found only in the rain forests of
Central and South America. The coat is soft and sometimes silky, often
with characteristic tufts adorning the head. Callitrichids move along
horizontal branches in a quadrupedal run that is often interrupted by
leaps. All of the digits except for the great toe have claws instead of
the flattened nails found in other primates. The callitrichids do not
have opposable thumbs.
Callitrichids are either monogamous or
polyandrous, meaning that a female can have one or more than one mate
depending on the species. Females produce one to three young a year,
and twins are common. Males assist during birth and carry the young on
their backs, transferring them to the mother for feeding.
Marmosets.
These monkeys of the Amazon Basin belong to the genus Callithrix, though some authorities place the pygmy marmoset in its own genus, Cebuella.
The pygmy marmoset is the smallest known monkey, weighing approximately
4 ounces (113 grams) as an adult. In 1998 scientists announced that
they had discovered a new marmoset species in the Brazilian Amazon: the
dwarf marmoset, Callithrix humilis, weighing about 12 ounces
(340 grams). Marmosets are omnivorous; their diet includes insects,
spiders, fruits, small vertebrates, birds' eggs, and tree sap.
Goeldi's marmoset (Callimico goeldii)
is placed in the family Callitrichidae in many classification schemes,
but other authorities classify it with the Cebidae. Some contend that
the species is neither a callitrichid nor a cebid because it exhibits
characteristics of both taxa. Its facial appearance and claws are
similar to those of the callitrichids, but its dental formula and skull
structure resemble those of the cebids. This rare monkey is also
unusual in that it is often found in the understory of the forest
rather than in the canopy, where the other New World monkeys are found.
The Callimico is endangered because of habitat destruction and hunting.
Tamarins.
The tamarins are classified in the genera Saguinus and Leontopithecus. Saguinus
has a wide geographic range, inhabiting tropical forests, open
woodlands, and secondary growth forests in parts of Colombia, Ecuador,
Peru, Bolivia, Brazil, the Guianas, and southern Central America. The
diet is similar to that of the marmosets, consisting of fruit, insects,
spiders, small vertebrates, and tree sap. Many tamarin species are
polyandrous—the female has several mates within the family troop.
The genus Leontopithecus
contains the three lion tamarin species, which are found only in the
lowland rain forests of southeastern Brazil. The name lion refers to
the distinctive mane on the shoulders of these monkeys. Despite the
establishment of captive-breeding programs for each species, the golden
lion tamarin (L. rosalia), the golden-headed lion tamarin (L. chrysomelas), and the black lion tamarin (L. chrysopygus) remain highly endangered.
The Cebidae
The
highly diverse monkeys of the family Cebidae inhabit rain forests from
southern Mexico to northern Argentina. They are characterized by long
limbs and digits with nails rather than claws. Most cebids, which range
in size from approximately 61/2
to 33 pounds (3 to 15 kilograms), are considerably larger than any
callitrichid. All species except the uakaris have a long, heavily
furred tail; in some genera the tail is prehensile with a soft,
sensitive underside that is naked near the tip. All species other than
the night monkeys are diurnal. The diet is generally omnivorous and
based primarily on fruits and insects, but the howler monkeys are
folivorous.
Night monkeys.
The night, or owl, monkeys of South America (genus Aotus),
which are also called douroucoulis, are the only nocturnal monkeys in
the world. With large eyes and excellent night vision, night monkeys
are well adapted to their nocturnal niche. They move rapidly along tree
limbs and exhibit remarkable acrobatics, though the tail is not
prehensile. Night monkeys are threatened by habitat loss, hunting, and
capture for use in biomedical research.
Squirrel monkeys.
These monkeys (genus Saimiri)
are found in forests and in some cultivated areas of South America and
along the Pacific coast of Costa Rica and Panama. Squirrel monkeys are
among the most vocal of the primates and are thought to form larger
troops than other New World species; troops containing about 300
individuals have been reported. Squirrel monkeys are more abundant than
other primates in the Amazon region, but deforestation remains a threat
to their populations.
Howler monkeys.
The howlers (genus Alouatta)
are among the largest of New World primates. Head and body length is
roughly 24 to 36 inches (61 to 91 centimeters), and the tail is about
the same length; adult weights range from 9 to 22 pounds (4 to 10
kilograms). The howler is best known for its remarkable voice, which is
the result of the angle of the large lower jaw and a greatly enlarged
hyoid bone. Its calls, which can be heard 2 to 3 miles (3 to 5
kilometers) away, have been described as deep howls and gave these
monkeys their name. The howler is distinguished from other New World
monkeys by its specialized stomach, similar to that found in colobine
monkeys, which is adapted for digesting leaves.
Titi monkeys.
In much of their behavior and appearance, the titis (genus Callicebus)
resemble the night monkeys. The titis are diurnal, however, and their
preferred quadruped gait is slower and more cautious than the swift,
acrobatic movements of the night monkeys. Titis live in small family
troops consisting of a monogamous adult pair and their offspring. When
titi monkeys sit side by side, their long tails often intertwine.
Spider monkeys.
Spider monkeys (genus Ateles)
are exceptionally acrobatic, exceeded in their agility perhaps only by
the gibbons. The limbs and prehensile tail of the spider monkey are
unusually long relative to its body length. The tail is often used as a
fifth arm to pick up objects and swing through trees in the uppermost
canopies of rain and montane forests. Their large size and noisy
behavior make spider monkeys particularly vulnerable to hunters.
Capuchins.
Perhaps the most familiar of the New World monkeys are the capuchins (genus Cebus).
These vivacious and highly intelligent animals are probably the most
common monkeys in captivity in the United States and Europe. Capuchins
are favored by organ-grinders and are often used in films and
television. All of the capuchins are adaptable and flexible in habitat
selection, and one species, C. apella, has the largest range of any New World monkey.
Wooly monkeys.
As their name implies, these monkeys (genus Lagothrix)
of central South America have a distinctive coat: short, thick, and
woolly, with a heavy undercoat. They are fairly large among American
monkeys, weighing between 12 and 24 pounds (5.4 and 10.9 kilograms).
Although they are primarily arboreal, they frequently descend to the
forest floor and walk upright, balancing themselves with their arms.
They use their extremely long, prehensile tail as an brace while
standing.
Muriqui.
The muriqui (genus Brachyteles), or wooly spider monkey, is one of the rarest and most endangered primates. The sole species, B. arachnoides,
is found only in the Atlantic coast forests of southeastern Brazil. The
muriqui is the largest of the New World monkeys, with an average weight
of 26 to 33 pounds (12 to 15 kilograms). Its thumbs and hands are
particularly well-adapted for swinging from branch to branch, enabling
the muriqui to travel more rapidly than other arboreal primates.
Uakaris.
The uakaris (genus Cacajao)
are the only South American monkeys with short tails. The face and the
top of the head are naked and colored red, white, or black depending on
the species. The habitat of uakaris is more restricted than that of
most South American primates; they are found mainly along small rivers
and lakes. They are quieter than most monkeys and occupy the highest
reaches of the canopy.
Sakis and bearded sakis.
The sakis (genus Pithecia)
are fairly common in the middle levels and underbrush of forests of the
Amazon Basin and the Guianas. They generally live in small family
groups, though some appear to be solitary. Despite their nervous and
melancholy appearance, they have gentle personalities and make good
pets when tamed. The bearded sakis (genus Chiropotes) are not
as well known as the sakis. Unlike the sakis, they occupy the canopy of
rain forests. They are distinguished by a full, thick beard.
