Giraffe

The giraffe (Giraffa) is an African artiodactyl harmony land mammal, the tallest living terrestrial animal and the largest ruminant. It is traditionally considered to be one species, Giraffa camelopardalis, with nine subspecies. However, the existence of up to eight extant giraffe species has been described, based upon research into the mitochondrial and nuclear DNA, as well as morphological measurements of Giraffa. Seven other species are extinct, prehistoric species known from fossils. The giraffe's chief distinguishing characteristics are its extremely long neck and legs, its horn-like ossicones, and its distinctive coat patterns. It is classified under the family Giraffidae, along with its closest extant relative, the okapi. Its scattered range extends from Chad in the north to South Africa in the south, and from Niger in the west to Somalia in the east. Giraffes usually inhabit savannahs and woodlands. Their food source is leaves, fruits and flowers of woody plants, primarily acacia species, which they browse at heights most other herbivores cannot reach. They may be preyed on by lions, leopards, spotted hyenas and African wild dogs. Giraffes live in herds of related females and their offspring, or bachelor herds of unrelated adult males, but are gregarious and may gather in large aggregations. Males establish social hierarchies through "necking", which are combat bouts where the neck is used as a weapon. Dominant males gain mating access to females, which bear the sole responsibility for raising the young.

The giraffe has intrigued various cultures, both ancient and modern, for its peculiar appearance, and has often been featured in paintings, books, and cartoons. It is classified by the International Union for Conservation of Nature as vulnerable to extinction, and has been extirpated from many parts of its former range. Giraffes are still found in numerous national parks and game reserves but estimates as of 2016 indicate that there are approximately 97,500 members of Giraffa in the wild. More than 1,600 were kept in zoos in 2010. The IUCN currently recognises only one species of giraffe with nine subspecies.[20][21] In 2001, a two-species taxonomy was proposed.[22] A 2007 study on the genetics of giraffes, suggested they were six species: the West African, Rothschild's, reticulated, Masai, Angolan, and South African giraffe. The study deduced from genetic differences in nuclear and mitochondrial DNA (mtDNA) that giraffes from these populations are reproductively isolated and rarely interbreed, though no natural obstacles block their mutual access. This includes adjacent populations of Rothschild's, reticulated, and Masai giraffes. The Masai giraffe was also suggested to consist of possibly two species separated by the Rift Valley. Reticulated and Masai giraffes were found to have the highest mtDNA diversity, which is consistent with giraffes originating in eastern Africa. Populations further north are more closely related to the former, while those to the south are more related to the latter. Giraffes appear to select mates of the same coat type, which are imprinted on them as calves.[19]

A 2011 study using detailed analyses of the morphology of giraffes, and application of the phylogenetic species concept, described eight species of living giraffes.[23] A 2016 study also concluded that living giraffes consist of multiple species.[24] The researchers suggested the existence of four species, which have not exchanged genetic information between each other for 1 to 2 million years. Since then, a response to this publication has been published, highlighting seven problems in data interpretation, and concludes "the conclusions should not be accepted unconditionally".[25]

A 2020 study showed that depending on the method chosen, different taxonomic hypotheses recognizing from two to six species, can be considered for the genus Giraffa. That study also found that multi species coalescent methods can lead to taxonomic over-splitting, as those methods delimit geographic structure rather than species. The 3-species hypothesis, which recognises G. camelopardalis, G. giraffa, and G. tippelskirchi, is highly supported by phylogenetic analyses and also corroborated by most population genetic and multi species coalescent analyses.[26]

†Giraffa gracilis
†Giraffa jumae
†Giraffa priscilla
†Giraffa punjabiensis
†Giraffa pygmaea
†Giraffa sivalensis
†Giraffa stillei
G. attica was formerly considered part of Giraffa but was reclassified as Bohlinia attica in 1929

Fully grown giraffes stand 4.3–5.7 m (14.1–18.7 ft) tall, with males taller than females.[44][45][46] The tallest recorded male was 5.88 m (19.3 ft) and the tallest recorded female was 5.17 m (17.0 ft) tall.[44][47] The average weight is 1,192 kg (2,628 lb) for an adult male and 828 kg (1,825 lb) for an adult female[48] with maximum weights of 1,930 kg (4,250 lb) and 1,180 kg (2,600 lb) having been recorded for males and females, respectively.[45][46] Despite its long neck and legs, the giraffe's body is relatively short.[49]:66 Located at both sides of the head, the giraffe's large, bulging eyes give it good all-round vision from its great height.[50]:25 Giraffes see in colour[50]:26 and their senses of hearing and smell are also sharp.[51] The animal can close its muscular nostrils to protect against sandstorms and ants.[50]:27

The giraffe's prehensile tongue is about 45 cm (18 in) long.[45][46] It is purplish-black in colour, perhaps to protect against sunburn, and is useful for grasping foliage, as well as for grooming and cleaning the animal's nose.[50]:27 The upper lip of the giraffe is also prehensile and useful when foraging, and is covered in hair to protect against thorns. The tongue and inside of the mouth are covered in papillae.[7]

The coat has dark blotches or patches (which can be orange, chestnut, brown, or nearly black in colour[51]) separated by light hair (usually white or cream in colour.[51]) Male giraffes become darker as they age.[42] The coat pattern has been claimed to serve as camouflage in the light and shade patterns of savannah woodlands.[43] Giraffe calves inherit some spot pattern traits from their mothers, and variation in some spot traits are correlated with neonatal survival.[52] The skin underneath the dark areas may serve as windows for thermoregulation, being sites for complex blood vessel systems and large sweat glands.[53] Each individual giraffe has a unique coat pattern.[42]

The skin of a giraffe is mostly gray.[48] Its thickness allows the animal to run through thorn bushes without being punctured.[50]:34 The fur may serve as a chemical defence, as its parasite repellents give the animal a characteristic scent. At least 11 main aromatic chemicals are in the fur, although indole and 3-methylindole are responsible for most of the smell. Because the males have a stronger odour than the females, the odour may also have sexual function.[54] Along the animal's neck is a mane made of short, erect hairs.[7] The one-metre (3.3-ft) tail ends in a long, dark tuft of hair and is used as a defense against insects.[50]:36

Skull and ossicones
Both sexes have prominent horn-like structures called ossicones, which are formed from ossified cartilage, covered in skin and fused to the skull at the parietal bones.[42] Being vascularized, the ossicones may have a role in thermoregulation,[53] and are also used in combat between males.[55] Appearance is a reliable guide to the sex or age of a giraffe: the ossicones of females and young are thin and display tufts of hair on top, whereas those of adult males end in knobs and tend to be bald on top.[42] Also, a median lump, which is more prominent in males, emerges at the front of the skull.[7] Males develop calcium deposits that form bumps on their skulls as they age.[51] A giraffe's skull is lightened by multiple sinuses.[49]:70 However, as males age, their skulls become heavier and more club-like, helping them become more dominant in combat.[42] The upper jaw has a grooved palate and lacks front teeth.[50]:26 The giraffe's molars have a rough surface.[50]:27

Legs, locomotion and posture

The front and back legs of a giraffe are about the same length. The radius and ulna of the front legs are articulated by the carpus, which, while structurally equivalent to the human wrist, functions as a knee.[56] It appears that a suspensory ligament allows the lanky legs to support the animal's great weight.[57] The foot of the giraffe reaches a diameter of 30 cm (12 in), and the hoof is 15 cm (5.9 in) high in males and 10 cm (3.9 in) in females.[50]:36 The rear of each hoof is low and the fetlock is close to the ground, allowing the foot to provide additional support to the animal's weight.[7] Giraffes lack dewclaws and interdigital glands. The giraffe's pelvis, though relatively short, has an ilium that is outspread at the upper ends.[7]

A giraffe has only two gaits: walking and galloping. Walking is done by moving the legs on one side of the body at the same time, then doing the same on the other side.[42] When galloping, the hind legs move around the front legs before the latter move forward,[51] and the tail will curl up.[42] The animal relies on the forward and backward motions of its head and neck to maintain balance and the counter momentum while galloping.[37]:327–29 The giraffe can reach a sprint speed of up to 60 km/h (37 mph),[58] and can sustain 50 km/h (31 mph) for several kilometres.[59]

A giraffe rests by lying with its body on top of its folded legs.[37]:329 To lie down, the animal kneels on its front legs and then lowers the rest of its body. To get back up, it first gets on its knees and spreads its hind legs to raise its hindquarters. It then straightens its front legs. With each step, the animal swings its head.[50]:31 In captivity, the giraffe sleeps intermittently around 4.6 hours per day, mostly at night.[60] It usually sleeps lying down; however, standing sleeps have been recorded, particularly in older individuals. Intermittent short "deep sleep" phases while lying are characterised by the giraffe bending its neck backwards and resting its head on the hip or thigh, a position believed to indicate paradoxical sleep.[60] If the giraffe wants to bend down to drink, it either spreads its front legs or bends its knees.[42] Giraffes would probably not be competent swimmers as their long legs would be highly cumbersome in the water,[61] although they could possibly float.[62] When swimming, the thorax would be weighed down by the front legs, making it difficult for the animal to move its neck and legs in harmony[61][62] or keep its head above the surface.[61]

Neck

The giraffe has an extremely elongated neck, which can be up to 2–2.4 m (6.6–7.9 ft) in length, accounting for much of the animal's vertical height.[45][46][50]:29[63] The long neck results from a disproportionate lengthening of the cervical vertebrae, not from the addition of more vertebrae. Each cervical vertebra is over 28 cm (11 in) long.[49]:71 They comprise 52–54 per cent of the length of the giraffe's vertebral column, compared with the 27–33 percent typical of similar large ungulates, including the giraffe's closest living relative, the okapi.[17] This elongation largely takes place after birth, perhaps because giraffe mothers would have a difficult time giving birth to young with the same neck proportions as adults.[64] The giraffe's head and neck are held up by large muscles and a strengthened nuchal ligament, which are anchored by long dorsal spines on the anterior thoracic vertebrae, giving the animal a hump.[7][65]

Adult male reticulated giraffe feeding high up on an acacia, in Kenya
The giraffe's neck vertebrae have ball and socket joints.[49]:71 In particular, the atlas–axis joint (C1 and C2) allows the animal to tilt its head vertically and reach more branches with the tongue.[50]:29 The point of articulation between the cervical and thoracic vertebrae of giraffes is shifted to lie between the first and second thoracic vertebrae (T1 and T2), unlike most other ruminants where the articulation is between the seventh cervical vertebra (C7) and T1.[17][64] This allows C7 to contribute directly to increased neck length and has given rise to the suggestion that T1 is actually C8, and that giraffes have added an extra cervical vertebra.[65] However, this proposition is not generally accepted, as T1 has other morphological features, such as an articulating rib, deemed diagnostic of thoracic vertebrae, and because exceptions to the mammalian limit of seven cervical vertebrae are generally characterised by increased neurological anomalies and maladies.[17]

There are several hypotheses regarding the evolutionary origin and maintenance of elongation in giraffe necks.[55] The "competing browsers hypothesis" was originally suggested by Charles Darwin and challenged only recently. It suggests that competitive pressure from smaller browsers, such as kudu, steenbok and impala, encouraged the elongation of the neck, as it enabled giraffes to reach food that competitors could not. This advantage is real, as giraffes can and do feed up to 4.5 m (15 ft) high, while even quite large competitors, such as kudu, can feed up to only about 2 m (6 ft 7 in) high.[66] There is also research suggesting that browsing competition is intense at lower levels, and giraffes feed more efficiently (gaining more leaf biomass with each mouthful) high in the canopy.[67][68] However, scientists disagree about just how much time giraffes spend feeding at levels beyond the reach of other browsers,[14][55][66][69] and a 2010 study found that adult giraffes with longer necks actually suffered higher mortality rates under drought conditions than their shorter-necked counterparts. This study suggests that maintaining a longer neck requires more nutrients, which puts longer-necked giraffes at risk during a food shortage.[70]

Another theory, the sexual selection hypothesis, proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests (see below) to establish dominance and obtain access to sexually receptive females.[14] In support of this theory, necks are longer and heavier for males than females of the same age,[14][55] and the former do not employ other forms of combat.[14] However, one objection is that it fails to explain why female giraffes also have long necks.[71] It has also been proposed that the neck serves to give the animal greater vigilance.[72][73]

Internal systems

In mammals, the left recurrent laryngeal nerve is longer than the right; in the giraffe it is over 30 cm (12 in) longer. These nerves are longer in the giraffe than in any other living animal;[74] the left nerve is over 2 m (6 ft 7 in) long.[75] Each nerve cell in this path begins in the brainstem and passes down the neck along the vagus nerve, then branches off into the recurrent laryngeal nerve which passes back up the neck to the larynx. Thus, these nerve cells have a length of nearly 5 m (16 ft) in the largest giraffes.[74] The structure of a giraffe's brain resembles that of domestic cattle.[50]:31 It is kept cool by evaporative heat loss in the nasal passages.[53] The shape of the skeleton gives the giraffe a small lung volume relative to its mass. Its long neck gives it a large amount of dead space, in spite of its narrow windpipe. These factors increase the resistance to airflow. Nevertheless, the animal can still supply enough oxygen to its tissues and it can increase its respiratory rate and oxygen diffusion when running.[76]

The circulatory system of the giraffe has several adaptations for its great height. Its heart, which can weigh more than 11 kg (25 lb) and measures about 60 cm (2 ft) long, must generate approximately double the blood pressure required for a human to maintain blood flow to the brain. As such, the wall of the heart can be as thick as 7.5 cm (3.0 in).[51] Giraffes have unusually high heart rates for their size, at 150 beats per minute.[49]:76 When the animal lowers its head the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and direct blood into the brain so the animal does not faint.[77] The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered.[78] Conversely, the blood vessels in the lower legs are under great pressure because of the weight of fluid pressing down on them. To solve this problem, the skin of the lower legs is thick and tight; preventing too much blood from pouring into them.[43]

Giraffes have oesophageal muscles that are unusually strong to allow regurgitation of food from the stomach up the neck and into the mouth for rumination.[49]:78 They have four chambered stomachs, as in all ruminants, and the first chamber has adapted to their specialised diet.[7] The intestines of an adult giraffe measure more than 70 m (230 ft) in length and have a relatively small ratio of small to large intestine.[79] The liver of the giraffe is small and compact.[49]:76 A gallbladder is generally present during fetal life, but it may disappear before birth.[7][80][81]

Behaviour and ecology
Habitat and feeding

AFully grown giraffes stand 4.3–5.7 m (14.1–18.7 ft) tall, with males taller than females.[44][45][46] The tallest recorded male was 5.88 m (19.3 ft) and the tallest recorded female was 5.17 m (17.0 ft) tall.[44][47] The average weight is 1,192 kg (2,628 lb) for an adult male and 828 kg (1,825 lb) for an adult female[48] with maximum weights of 1,930 kg (4,250 lb) and 1,180 kg (2,600 lb) having been recorded for males and females, respectively.[45][46] Despite its long neck and legs, the giraffe's body is relatively short.[49]:66 Located at both sides of the head, the giraffe's large, bulging eyes give it good all-round vision from its great height.[50]:25 Giraffes see in colour[50]:26 and their senses of hearing and smell are also sharp.[51] The animal can close its muscular nostrils to protect against sandstorms and ants.[50]:27

The giraffe's prehensile tongue is about 45 cm (18 in) long.[45][46] It is purplish-black in colour, perhaps to protect against sunburn, and is useful for grasping foliage, as well as for grooming and cleaning the animal's nose.[50]:27 The upper lip of the giraffe is also prehensile and useful when foraging, and is covered in hair to protect against thorns. The tongue and inside of the mouth are covered in papillae.[7]

The coat has dark blotches or patches (which can be orange, chestnut, brown, or nearly black in colour[51]) separated by light hair (usually white or cream in colour.[51]) Male giraffes become darker as they age.[42] The coat pattern has been claimed to serve as camouflage in the light and shade patterns of savannah woodlands.[43] Giraffe calves inherit some spot pattern traits from their mothers, and variation in some spot traits are correlated with neonatal survival.[52] The skin underneath the dark areas may serve as windows for thermoregulation, being sites for complex blood vessel systems and large sweat glands.[53] Each individual giraffe has a unique coat pattern.[42]

The skin of a giraffe is mostly gray.[48] Its thickness allows the animal to run through thorn bushes without being punctured.[50]:34 The fur may serve as a chemical defence, as its parasite repellents give the animal a characteristic scent. At least 11 main aromatic chemicals are in the fur, although indole and 3-methylindole are responsible for most of the smell. Because the males have a stronger odour than the females, the odour may also have sexual function.[54] Along the animal's neck is a mane made of short, erect hairs.[7] The one-metre (3.3-ft) tail ends in a long, dark tuft of hair and is used as a defense against insects.[50]:3

Skull and ossicones
Both sexes have prominent horn-like structures called ossicones, which are formed from ossified cartilage, covered in skin and fused to the skull at the parietal bones.[42] Being vascularized, the ossicones may have a role in thermoregulation,[53] and are also used in combat between males.[55] Appearance is a reliable guide to the sex or age of a giraffe: the ossicones of females and young are thin and display tufts of hair on top, whereas those of adult males end in knobs and tend to be bald on top.[42] Also, a median lump, which is more prominent in males, emerges at the front of the skull.[7] Males develop calcium deposits that form bumps on their skulls as they age.[51] A giraffe's skull is lightened by multiple sinuses.[49]:70 However, as males age, their skulls become heavier and more club-like, helping them become more dominant in combat.[42] The upper jaw has a grooved palate and lacks front teeth.[50]:26 The giraffe's molars have a rough surface.[50]:27

Legs, locomotion and posture

The front and back legs of a giraffe are about the same length. The radius and ulna of the front legs are articulated by the carpus, which, while structurally equivalent to the human wrist, functions as a knee.[56] It appears that a suspensory ligament allows the lanky legs to support the animal's great weight.[57] The foot of the giraffe reaches a diameter of 30 cm (12 in), and the hoof is 15 cm (5.9 in) high in males and 10 cm (3.9 in) in females.[50]:36 The rear of each hoof is low and the fetlock is close to the ground, allowing the foot to provide additional support to the animal's weight.[7] Giraffes lack dewclaws and interdigital glands. The giraffe's pelvis, though relatively short, has an ilium that is outspread at the upper ends.[7]

A giraffe has only two gaits: walking and galloping. Walking is done by moving the legs on one side of the body at the same time, then doing the same on the other side.[42] When galloping, the hind legs move around the front legs before the latter move forward,[51] and the tail will curl up.[42] The animal relies on the forward and backward motions of its head and neck to maintain balance and the counter momentum while galloping.[37]:327–29 The giraffe can reach a sprint speed of up to 60 km/h (37 mph),[58] and can sustain 50 km/h (31 mph) for several kilometres.[59]

A giraffe rests by lying with its body on top of its folded legs.[37]:329 To lie down, the animal kneels on its front legs and then lowers the rest of its body. To get back up, it first gets on its knees and spreads its hind legs to raise its hindquarters. It then straightens its front legs. With each step, the animal swings its head.[50]:31 In captivity, the giraffe sleeps intermittently around 4.6 hours per day, mostly at night.[60] It usually sleeps lying down; however, standing sleeps have been recorded, particularly in older individuals. Intermittent short "deep sleep" phases while lying are characterised by the giraffe bending its neck backwards and resting its head on the hip or thigh, a position believed to indicate paradoxical sleep.[60] If the giraffe wants to bend down to drink, it either spreads its front legs or bends its knees.[42] Giraffes would probably not be competent swimmers as their long legs would be highly cumbersome in the water,[61] although they could possibly float.[62] When swimming, the thorax would be weighed down by the front legs, making it difficult for the animal to move its neck and legs in harmony[61][62] or keep its head above the surface.[61]

Neck

The giraffe (right) and its close relative the okapi (left) both have seven cervical vertebrae
The giraffe has an extremely elongated neck, which can be up to 2–2.4 m (6.6–7.9 ft) in length, accounting for much of the animal's vertical height.[45][46][50]:29[63] The long neck results from a disproportionate lengthening of the cervical vertebrae, not from the addition of more vertebrae. Each cervical vertebra is over 28 cm (11 in) long.[49]:71 They comprise 52–54 per cent of the length of the giraffe's vertebral column, compared with the 27–33 percent typical of similar large ungulates, including the giraffe's closest living relative, the okapi.[17] This elongation largely takes place after birth, perhaps because giraffe mothers would have a difficult time giving birth to young with the same neck proportions as adults.[64] The giraffe's head and neck are held up by large muscles and a strengthened nuchal ligament, which are anchored by long dorsal spines on the anterior thoracic vertebrae, giving the animal a hump.[7][65]

Adult male reticulated giraffe feeding high up on an acacia, in Kenya
The giraffe's neck vertebrae have ball and socket joints.[49]:71 In particular, the atlas–axis joint (C1 and C2) allows the animal to tilt its head vertically and reach more branches with the tongue.[50]:29 The point of articulation between the cervical and thoracic vertebrae of giraffes is shifted to lie between the first and second thoracic vertebrae (T1 and T2), unlike most other ruminants where the articulation is between the seventh cervical vertebra (C7) and T1.[17][64] This allows C7 to contribute directly to increased neck length and has given rise to the suggestion that T1 is actually C8, and that giraffes have added an extra cervical vertebra.[65] However, this proposition is not generally accepted, as T1 has other morphological features, such as an articulating rib, deemed diagnostic of thoracic vertebrae, and because exceptions to the mammalian limit of seven cervical vertebrae are generally characterised by increased neurological anomalies and maladies.[17]

There are several hypotheses regarding the evolutionary origin and maintenance of elongation in giraffe necks.[55] The "competing browsers hypothesis" was originally suggested by Charles Darwin and challenged only recently. It suggests that competitive pressure from smaller browsers, such as kudu, steenbok and impala, encouraged the elongation of the neck, as it enabled giraffes to reach food that competitors could not. This advantage is real, as giraffes can and do feed up to 4.5 m (15 ft) high, while even quite large competitors, such as kudu, can feed up to only about 2 m (6 ft 7 in) high.[66] There is also research suggesting that browsing competition is intense at lower levels, and giraffes feed more efficiently (gaining more leaf biomass with each mouthful) high in the canopy.[67][68] However, scientists disagree about just how much time giraffes spend feeding at levels beyond the reach of other browsers,[14][55][66][69] and a 2010 study found that adult giraffes with longer necks actually suffered higher mortality rates under drought conditions than their shorter-necked counterparts. This study suggests that maintaining a longer neck requires more nutrients, which puts longer-necked giraffes at risk during a food shortage.[70]

Another theory, the sexual selection hypothesis, proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests (see below) to establish dominance and obtain access to sexually receptive females.[14] In support of this theory, necks are longer and heavier for males than females of the same age,[14][55] and the former do not employ other forms of combat.[14] However, one objection is that it fails to explain why female giraffes also have long necks.[71] It has also been proposed that the neck serves to give the animal greater vigilance.[72][73]

Internal systems

In mammals, the left recurrent laryngeal nerve is longer than the right; in the giraffe it is over 30 cm (12 in) longer. These nerves are longer in the giraffe than in any other living animal;[74] the left nerve is over 2 m (6 ft 7 in) long.[75] Each nerve cell in this path begins in the brainstem and passes down the neck along the vagus nerve, then branches off into the recurrent laryngeal nerve which passes back up the neck to the larynx. Thus, these nerve cells have a length of nearly 5 m (16 ft) in the largest giraffes.[74] The structure of a giraffe's brain resembles that of domestic cattle.[50]:31 It is kept cool by evaporative heat loss in the nasal passages.[53] The shape of the skeleton gives the giraffe a small lung volume relative to its mass. Its long neck gives it a large amount of dead space, in spite of its narrow windpipe. These factors increase the resistance to airflow. Nevertheless, the animal can still supply enough oxygen to its tissues and it can increase its respiratory rate and oxygen diffusion when running.[76]

The circulatory system of the giraffe has several adaptations for its great height. Its heart, which can weigh more than 11 kg (25 lb) and measures about 60 cm (2 ft) long, must generate approximately double the blood pressure required for a human to maintain blood flow to the brain. As such, the wall of the heart can be as thick as 7.5 cm (3.0 in).[51] Giraffes have unusually high heart rates for their size, at 150 beats per minute.[49]:76 When the animal lowers its head the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and direct blood into the brain so the animal does not faint.[77] The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered.[78] Conversely, the blood vessels in the lower legs are under great pressure because of the weight of fluid pressing down on them. To solve this problem, the skin of the lower legs is thick and tight; preventing too much blood from pouring into them.[43]

Giraffes have oesophageal muscles that are unusually strong to allow regurgitation of food from the stomach up the neck and into the mouth for rumination.[49]:78 They have four chambered stomachs, as in all ruminants, and the first chamber has adapted to their specialised diet.[7] The intestines of an adult giraffe measure more than 70 m (230 ft) in length and have a relatively small ratio of small to large intestine.[79] The liver of the giraffe is small and compact.[49]:76 A gallbladder is generally present during fetal life, but it may disappear before birth.[7][80][81]

Behaviour and ecology
Habitat and feeding

Giraffes usually inhabit savannahs and open woodlands. They prefer Acacieae, Commiphora, Combretum and open Terminalia woodlands over denser environments like Brachystegia woodlands.[37]:322 The Angolan giraffe can be found in desert environments.[82] Giraffes browse on the twigs of trees, preferring trees of the subfamily Acacieae and the genera Commiphora and Terminalia,[4] which are important sources of calcium and protein to sustain the giraffe's growth rate.[10] They also feed on shrubs, grass and fruit.[37]:324 A giraffe eats around 34 kg (75 lb) of foliage daily.[42] When stressed, giraffes may chew the bark off branches. Although herbivorous, the giraffe has been known to visit carcasses and lick dried meat off bones.[37]:325

During the wet season, food is abundant and giraffes are more spread out, while during the dry season, they gather around the remaining evergreen trees and bushes.[4] Mothers tend to feed in open areas, presumably to make it easier to detect predators, although this may reduce their feeding efficiency.[69] As a ruminant, the giraffe first chews its food, then swallows it for processing and then visibly passes the half-digested cud up the neck and back into the mouth to chew again.[49]:78–79 It is common for a giraffe to salivate while feeding.[50]:27 The giraffe requires less food than many other herbivores because the foliage it eats has more concentrated nutrients and it has a more efficient digestive system.[4] The animal's faeces come in the form of small pellets.[7] When it has access to water, a giraffe drinks at intervals no longer than three days.[42]

Giraffes have a great effect on the trees that they feed on, delaying the growth of young trees for some years and giving "waistlines" to trees that are too tall. Feeding is at its highest during the first and last hours of daytime. Between these hours, giraffes mostly stand and ruminate. Rumination is the dominant activity during the night, when it is mostly done lying down.[42]

Social life

Giraffes are usually found in groups that vary in size and composition according to ecological, anthropogenic, temporal, and social factors.[83] Traditionally, the composition of these groups had been described as open and ever-changing.[84] For research purposes, a "group" has been defined as "a collection of individuals that are less than a kilometre apart and moving in the same general direction."[85] More recent studies have found that giraffes have long-term social associations and may form groups or pairs based on kinship, sex or other factors. These groups may regularly associate with one another in larger communities or sub-communities within a fission–fusion society.[86][87][88] The number of giraffes in a group can range up to 66 individuals.[88][83] Masai giraffe extending its tongue to feed, in Tanzania. Its tongue, lips and palate are tough enough to deal with sharp thorns in trees.
Giraffes usually inhabit savannahs and open woodlands. They prefer Acacieae, Commiphora, Combretum and open Terminalia woodlands over denser environments like Brachystegia woodlands.[37]:322 The Angolan giraffe can be found in desert environments.[82] Giraffes browse on the twigs of trees, preferring trees of the subfamily Acacieae and the genera Commiphora and Terminalia,[4] which are important sources of calcium and protein to sustain the giraffe's growth rate.[10] They also feed on shrubs, grass and fruit.[37]:324 A giraffe eats around 34 kg (75 lb) of foliage daily.[42] When stressed, giraffes may chew the bark off branches. Although herbivorous, the giraffe has been known to visit carcasses and lick dried meat off bones.[37]:325

During the wet season, food is abundant and giraffes are more spread out, while during the dry season, they gather around the remaining evergreen trees and bushes.[4] Mothers tend to feed in open areas, presumably to make it easier to detect predators, although this may reduce their feeding efficiency.[69] As a ruminant, the giraffe first chews its food, then swallows it for processing and then visibly passes the half-digested cud up the neck and back into the mouth to chew again.[49]:78–79 It is common for a giraffe to salivate while feeding.[50]:27 The giraffe requires less food than many other herbivores because the foliage it eats has more concentrated nutrients and it has a more efficient digestive system.[4] The animal's faeces come in the form of small pellets.[7] When it has access to water, a giraffe drinks at intervals no longer than three days.[42]

Giraffes have a great effect on the trees that they feed on, delaying the growth of young trees for some years and giving "waistlines" to trees that are too tall. Feeding is at its highest during the first and last hours of daytime. Between these hours, giraffes mostly stand and ruminate. Rumination is the dominant activity during the night, when it is mostly done lying down.[42]

Social life

Giraffes are usually found in groups that vary in size and composition according to ecological, anthropogenic, temporal, and social factors.[83] Traditionally, the composition of these groups had been described as open and ever-changing.[84] For research purposes, a "group" has been defined as "a collection of individuals that are less than a kilometre apart and moving in the same general direction."[85] More recent studies have found that giraffes have long-term social associations and may form groups or pairs based on kinship, sex or other factors. These groups may regularly associate with one another in larger communities or sub-communities within a fission–fusion society.[86][87][88] The number of giraffes in a group can range up to 66 individuals.[88][83]