The closest relatives of gorillas are chimpanzees and humans,
all of the Homininae having diverged from a common ancestor
about 7 million years ago.[6] Human
gene sequences differ only 1.6% on average from the sequences of
corresponding gorilla genes, but there is further difference in
how many copies each
gene has.[7] Until
recently, gorillas were considered to be a single species, with
three subspecies: the western lowland gorilla, the eastern
lowland gorilla and the mountain gorilla.[8][9] There
is now agreement that there are two species with two subspecies
each. More recently, a third subspecies has been claimed to
exist in one of the species. The separate species and subspecies
developed from a single type of gorilla during the Ice
Age, when their forest habitats shrank and became isolated
from each other.[2]
Primatologists continue
to explore the relationships between various gorilla
populations.[8] The
species and subspecies listed here are the ones upon which most
scientists agree.[citation
needed]
Taxonomy of genus Gorilla[1] |
Phylogeny of superfamily Hominoidea[10](Fig.
4) |
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Hominoidea |
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humans (genus Homo)
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chimpanzees (genus Pan)
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gorillas (genus Gorilla)
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orangutans (genus Pongo)
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gibbons (family Hylobatidae)
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The proposed third subspecies of Gorilla
beringei, which has not yet received a trinomen,
is the Bwindi population of the mountain gorilla, sometimes
called the Bwindi
gorilla.
Some variations that distinguish the classifications of gorilla
include varying density, size, hair color, length, culture, and
facial widths.[2] Now,
over 100,000 western lowland gorillas are thought to exist in
the wild, with 4,000 in zoos; eastern lowland gorillas have a
population of 4,000 in the wild and 24 in zoos.[2] Mountain
gorillas are the most severely endangered, with an estimated
population of about 620 left in the wild and none in zoos.[2]
Physical
characteristics
Gorillas move around by knuckle-walking,
although they sometimes walk bipedally for short distances while
carrying food or in defensive situations.[11] Wild
male gorillas weigh 135 to 180 kg (298 to 397 lb) while adult
females usually weigh half as much as adult males at 68–113 kg
(150–249 lb). Adult males are 1.7 to 1.8 m (5.6 to 5.9 ft) tall,
with an arm span that stretches from 2.3 to 2.6 m (7.5 to
8.5 ft). Female gorillas are shorter with smaller arm spans.[12] Adult
male gorillas are known as silverbacks due to characteristic
silver hair on their backs reaching to the hips. Occasionally, a
silverback of over 1.8 metres (5 ft 11 in) and 230 kg (510 lb)
has been recorded in the wild. Obese gorillas in captivity can
weigh as much as 270 kg (600 lb).[13] Gorilla
facial structure is described as mandibular
prognathism, that is, the mandible protrudes
farther out than the maxilla.
Adult males also have a prominent sagittal
crest.
The eastern gorilla is more darkly colored than the western
gorilla, with the mountain gorilla being the darkest of all. The
mountain gorilla also has the thickest hair. The western lowland
gorilla can be brown or grayish with a reddish forehead. In
addition, gorillas that live in lowland forests are more slender
and agile than the more bulky mountain gorillas. The eastern
gorilla also has a longer face and broader chest than the
western gorilla.[14]
Studies have shown gorilla blood is not reactive to anti-A and
anti-B monoclonal
antibodies, which would, in humans, indicate type
O blood. Due to novel sequences, though, it is different
enough to not conform with the humanABO
blood group system, into which the other great apes fit.[15] Like
humans, gorillas have individual finger prints.[16][17] Their
eye color is dark brown, framed by a black ring around the iris.
Behavior and
ecology
Range and
habitat
Gorillas have a patchy distribution. The range of the two
species is separated by the Congo
River and its tributaries.
The western gorilla lives in west central Africa, while the
eastern gorilla lives in east central Africa. Between the
species and even within the species, gorillas live in a variety
of habitats and elevations. Gorilla habitat ranges from montane
forests to
swamps. Eastern gorillas inhabit montane and submontane forests
between 650 and 4,000 m (2,130 and 13,120 ft) above sea level.[18] Mountain
gorillas live in the montane forests at the higher ends of the
elevation range, while eastern lowland gorillas live in
submontane forests at the lower ends of the elevation range. In
addition, eastern lowland gorillas live in montane bamboo
forests, as well as lowland forests ranging from 600–3,308 m
(1,969–10,853 ft) in elevation.[19] Western
gorillas live in both lowland swamp forests and montane forests,
and elevations ranging from sea level to 1,600 m (5,200 ft).[18] Western
lowland gorillas live in swamp and lowland forests ranging up to
1,600 m (5,200 ft), and Cross River gorillas live in low-lying
and submontane forests ranging from 150–1,600 m (490–5,250 ft).
Food and
foraging
Gorillas moving in habitat
A gorilla's day is synchronized, divided between rest periods
and travel or feeding periods. Diets differ between and within
species. Mountain gorillas mostly eat foliage, such as leaves,
stems, pith, and shoots, while fruit makes up a very small part
of their diets.[20] Mountain
gorilla food is widely distributed and both individuals and
groups do not have to compete with each other. Their home ranges
average 3–15 km2 (1.16–5.79 mi2),
and their movements range around 500 m (0.31 mi) or less on an
average day.[20] Despite
eating a few species in each habitat, mountain gorillas have
flexible diets and can live in a variety of habitats.[20]
Eastern lowland gorillas have more diverse diets, which vary
seasonally. Leaves and pith are commonly eaten, but fruits can
make up as much as 25% of their diets. Since fruit is less
available, lowland gorillas must travel farther each day, and
their home ranges vary from 2.7–6.5 km2 (1.04
to 2.51 mi2), with day ranges
154–2,280 m (0.096–1.417 mi). Eastern lowland gorillas will also
eat insects, preferably ants.[21] Western
lowland gorillas depend on fruits more than the others and they
are more dispersed across their range.[22] They
travel even further than the other gorilla subspecies, at
1,105 m (0.687 mi) per day on average, and have larger home
ranges of 7–14 km2 (2.70–5.41 mi2).[22] Western
lowland gorillas have less access to terrestrial herbs, although
they can access aquatic herbs in some areas. Termites and ants
are also eaten.
Gorillas rarely drink water "because they consume succulent
vegetation that is comprised of almost half water as well as
morning dew",[23] although
both mountain and lowland gorillas have been observed drinking.
One possible predator of gorillas is the leopard.
Gorilla remains have been found in leopard scat, but this may be
the result of scavenging.[24] When
the group is attacked by humans, leopards, or other gorillas, an
individual silverback will protect the group, even at the cost
of his own life.[25] George
Schaller reported, "[a] silverback gorilla and a leopard were
both found dead from mutually inflicted wounds".[26]
Social structure
Silverback gorilla with one of his females
Gorillas live in groups called troops. Troops tend to be made of
one adult male or silverback, multiple adult females and their
offspring.[27][28][29] However,
multiple-male troops also exist.[28] A
silverback is typically more than 12 years of age, and is named
for the distinctive patch of silver hair on its back, which
comes with maturity. Silverbacks also have large canine teeth
which also come with maturity. Both males and females tend to
emigrate from their natal groups. For mountain gorillas, females
disperse from their natal troops more than males.[27][30] Mountain
gorillas and western lowland gorillas also commonly transfer to
second new groups.[27] Mature
males tend to also leave their groups and establish their own
troops by attracting emigrating females. However, male mountain
gorillas sometimes stay in their natal troops and become
subordinate to the silverback. If the silverback dies, these
males may be able to become dominant or mate with the females.
This behavior has not been observed in eastern lowland gorillas.
In a single male group, when the silverback dies, the females
and their offspring disperse and find a new troop.[30][31] Without
a silverback to protect them, the infants will likely fall
victim to infanticide.
Joining a new group is likely to be a tactic against this.[30][32] However,
while gorilla troops usually disband after the silverback dies,
female eastern lowlands gorillas and their offspring have been
recorded staying together until a new silverback transfers into
the group. This likely serves as protection from leopards.[31]
The silverback is the center of the troop's attention, making
all the decisions, mediating conflicts, determining the
movements of the group, leading the others to feeding sites and
taking responsibility for the safety and well-being of the
troop. Younger males subordinate to the silverback, known as
blackbacks, may serve as backup protection. Blackbacks are aged
between 8 and 12 years of age[29] and
lack the silver back hair. The bond a silverback has with his
females forms the core of gorilla social life. Bonds between
them are maintained by grooming and staying close together.[33] Females
form strong relationships with males to gain mating
opportunities and protection from predators and infanticidal
outside males.[34] However,
aggressive behaviors between males and females do occur, but
rarely lead to serious injury. Relationships between females may
vary. Maternally related females in a troop tend to be friendly
towards each other and associate closely. Otherwise, females
have few friendly encounters and commonly act aggressively
towards each other.[27] Females
may fight for social access to males and a male may intervene.[33] Male
gorillas have weak social bonds, particularly in multiple-male
groups with apparent dominance hierarchies and strong
competition for mates. Males in all-male groups, though, tend to
have friendly interactions and socialize through play, grooming
and staying together,[29] and
occasionally they even engage in homosexual interactions.[35]
Nesting
Gorilla night nest constructed in a tree.
Gorillas construct nests for daytime and night use. Nests tend
to be simple aggregations of branches and leaves about 2 to 5 ft
(0.61 to 1.52 m) in diameter and are constructed by individuals.
Gorillas, unlike chimpanzees or orangutans, tend to sleep in
nests on the ground. The young nest with their mothers, but
construct nests after three years of age, initially close to
those of their mothers.[36] Gorilla
nests are distributed arbitrarily and use of tree species for
site and construction appears to be opportunistic.[37] Nest-building
by great apes is now considered to be not just animal
architecture, but as an important instance of tool
use.[37]
Reproduction and parenting
Young gorilla riding on mother
Females mature at 10–12 years (earlier in captivity); males at
11–13 years. A female’s first ovulatory cycle occurs when she is
six years of age, and is followed by a two-year period of
adolescent infertility.[38] The
estrous cycle last 30–33 days, with outward ovulation signs
subtle compared to those of chimpanzees. The gestation period
lasts 8.5 months. Female mountain gorillas first give birth at
10 years of age and have four-year interbirth intervals.[38] Males
can be fertile before reaching adulthood. Gorillas mate year
round.[39]
Females will purse their lips and slowly approach a male while
making eye contact. This serves to urge the male to mount her.
If the male does not respond, then she will try to attract his
attention by reaching towards him or slapping the ground.[40] In
multiple-male groups, solicitation indicates female preference,
but females can be forced to mate with multiple males.[40] Males
incite copulation by approaching a female and displaying at her
or touching her and giving a "train grunt".[39] Recently,
gorillas have been observed engaging in face-to-face
sex, a trait once considered unique to humans and bonobos
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A gorilla's lifespan is normally between 35 and 40 years,
although zoo gorillas may live for 50 years or more. Colo,
a female Western Gorilla at the Columbus
Zoo and Aquarium is
the oldest known gorilla, at 57 years of age (as of her birthday
on 22 December 2013).[45]
Communication
Twenty-five distinct vocalizations are
recognized, many of which are used primarily for group
communication within dense vegetation. Sounds classified as
grunts and barks are heard most frequently while traveling, and
indicate the whereabouts of individual group members.[46] They
may also be used during social interactions when discipline is
required. Screams and roars signal alarm or warning, and are
produced most often by silverbacks. Deep, rumbling belches
suggest contentment and are heard frequently during feeding and
resting periods. They are the most common form of intragroup
communication.[47] Severe aggression is
rare in stable groups, but when two mountain gorilla groups
meet, the two silverbacks can sometimes engage in a fight to the
death, using their canines to cause deep, gaping injuries.[47]
For this reason, conflicts are most often resolved by displays
and other threat behaviors that are intended to intimidate
without becoming physical. The ritualized charge
display is unique to gorillas. The entire sequence has nine
steps: (1) progressively quickening hooting, (2) symbolic
feeding, (3) rising bipedally, (4) throwing vegetation, (5)
chest-beating with cupped hands, (6) one leg kick, (7) sideways
running, two-legged to four-legged, (8) slapping and tearing
vegetation, and (9) thumping the ground with palms to end
display.[48]
Intelligence
Gorillas are considered highly intelligent. A few individuals in
captivity, such as Koko,
have been taught a subset of sign
language. Like the other great
apes, gorillas can laugh, grieve, have "rich emotional
lives", develop strong family bonds, make and use tools, and
think about the past and future.[49] Some
researchers believe gorillas have spiritual feelings or
religious sentiments.[2] They
have been shown to have cultures in different areas revolving
around different methods of food preparation, and will show
individual color
preferences.[2]
Tool use
A female gorilla exhibiting tool use by using a tree
trunk as a support whilst fishing herbs
The following observations were made by a team led by Thomas
Breuer of the Wildlife
Conservation Society in
September 2005. Gorillas are now known to use tools in
the wild. A female gorilla in the Nouabalé-Ndoki
National Park in
theRepublic
of Congo was
recorded using a stick as if to gauge the depth of water whilst
crossing a swamp.
A second female was seen using a tree stump as a bridge and also
as a support whilst fishing in the swamp. This means all of thegreat
apes are now
known to use tools.[50]
In September 2005, a two-and-a-half-year-old gorilla in the Republic
of Congo was
discovered using rocks to smash open palm nuts inside a game
sanctuary.[51] While
this was the first such observation for a gorilla, over 40 years
previously, chimpanzees had
been seen using tools in the wild 'fishing' for termites. Great apes are
endowed with semiprecision grips, and have been able to use both
simple tools and even weapons, by improvising a club from a
convenient fallen branch, for example.
Interactions with humans
Studies
The word "gorilla" comes from the history of Hanno
the Navigator, (c. 500 BC) a Carthaginian explorer
on an expedition on the west African coast.
They encountered "a savage people, the greater part of whom were
women, whose bodies were hairy, and who our interpreters called
Gorillae".[52] The
word was then later used as the species name, though it is
unknown whether what these ancient Carthaginians encountered
were truly gorillas, another species of ape or monkeys, or
humans.[8]
American physician and missionary Thomas
Staughton Savage obtained
the first specimens (the skull and other bones) during his time
in Liberia in Africa.[3] The
first scientific description of gorillas dates back to an
article by Savage and the naturalist Jeffries
Wyman in 1847 in Proceedings
of the Boston Society of Natural History,[53][54] where Troglodytes
gorilla is
described, now known as the western
gorilla. Other species of gorilla are described in the next
few years.[8]
Explorer Paul
Du Chaillu was
the first westerner to see a live gorilla during his travel
through western equatorial Africa from
1856 to 1859. He brought dead specimens to the UK in 1861.[55][56][57]
The first systematic study was not conducted until the 1920s,
when Carl
Akeley of the American
Museum of Natural History traveled
to Africa to hunt for an animal to be shot and stuffed. On his
first trip, he was accompanied by his friends Mary
Bradley, a mystery writer, her husband, and their young
daughter Alice, who would later write science fiction under the
pseudonym James
Tiptree Jr. After
their trip, Mary Bradley wrote On
the Gorilla Trail. She later became an advocate for the
conservation of gorillas, and wrote several more books (mainly
for children). In the late 1920s and early 1930s, Robert
Yerkes and his
wife Ava helped further the study of gorillas when they sent
Harold Bigham to Africa. Yerkes also wrote a book in 1929 about
the great apes.
Drawing of French explorer Paul
Du Chaillu at
close quarters with a gorilla
After World
War II, George
Schaller was one
of the first researchers to go into the field and study
primates. In 1959, he conducted a systematic study of the
mountain gorilla in the wild and published his work. Years
later, at the behest of Louis
Leakey and the National
Geographic, Dian
Fossey conducted
a much longer and more comprehensive study of the mountain
gorilla. When she published her work, many misconceptions and
myths about gorillas were finally disproved, including the myth
that gorillas are violent.
Western lowland gorillas (Gorilla
gorilla gorilla) are believed to be one of the zoonotic origins
of HIV/AIDS.
The SIVgor Simian
immunodeficiency virus that
infects them is similar to a certain strain of HIV-1.[58][59][60][61]
Conservation
status
The eastern gorilla is listed as endangered on
the IUCN
Red List,[62] with
the mountain gorilla listed as Critically
Endangered.[63] The
western gorilla and its subspecies are also listed as Critically
Endangered.[64] Threats
to gorilla survival include habitat
destruction and poaching for
the bushmeat trade.
In 2004, a population of several hundred gorillas in the Odzala
National Park, Republic
of Congo was
essentially wiped out by the Ebola
virus.[65] A
2006 study published in Science concluded
more than 5,000 gorillas may have died in recent outbreaks of
the Ebola virus in central Africa. The researchers indicated in
conjunction with commercial hunting of these apes, the virus
creates "a recipe for rapid ecological extinction."[66] Conservation
efforts include the Great
Apes Survival Project, a partnership between the United
Nations Environment Programme and
the UNESCO,
and also an international treaty, the Agreement
on the Conservation of Gorillas and Their Habitats,
concluded under UNEP-administered Convention
on Migratory Species. The Gorilla
Agreement is the
first legally binding instrument exclusively targeting gorilla
conservation; it came into effect on 1 June 2008.
Genome
sequencing
The gorilla became the next-to-last great ape genus to have its
genome sequenced. This has given scientists further insight into
the evolution and origin of humans. Despite the chimpanzees
being the closest extant relatives of humans, 15% of the human
genome was found to be more like that of the gorilla.[67] In
addition, 30% of the gorilla genome "is closer to human or
chimpanzee than the latter are to each other; this is rarer
around coding genes, indicating pervasive selection throughout
great ape evolution, and has functional consequences in gene
expression."[68] Analysis
of the gorilla genome has cast doubt on the idea that the rapid
evolution of hearing genes gave rise to language in humans, as
it also occurred in gorillas.[69]
Cultural
references
Since they came to the attention of western society in the
1860s,[57] gorillas
have been a recurring element of many aspects of popular culture
and media. For example, gorillas have featured prominently in
monstrous fantasy films such as King
Kong, and pulp
fictionsuch as the stories of Tarzan and Conan
the Barbarian have
featured gorillas as physical opponents to the titular
protagonists.
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