giraffes
Giraffe
The giraffe (Giraffa) is an African artiodactyl 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.
| Giraffe | |
|---|---|
 | |
| Masai giraffe (G. c. tippelskirchi) at the Mikumi National Park, Tanzania | |
| Conservation status | |
 Vulnerable (IUCN 3.1)[1] | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Family: | Giraffidae |
| Genus: | Giraffa Brisson, 1772 |
| Type species | |
| Giraffa camelopardalis | |
| Species | |
 | |
| Range map of extant Giraffa subspecies | |
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.
Etymology
The name "giraffe" has its earliest known origins in the Arabic word zarāfah (زرافة),[2] perhaps borrowed from the animal's Somali name geri.[3] The Arab name is translated as "fast-walker".[4] There were several Middle English spellings, such as jarraf, ziraph, and gerfauntz. The Italian form giraffa arose in the 1590s. The modern English form developed around 1600 from the French girafe.[2] "Camelopard" is an archaic English name for the giraffe deriving from the Ancient Greek for camel and leopard, referring to its camel-like shape and its leopard-like colouring.[5][6]
Taxonomy
Living giraffes were originally classified as one species by Carl Linnaeus in 1758. He gave it the binomial name Cervus camelopardalis. Morten Thrane Brünnich classified the genus Giraffa in 1772.[7] The species name camelopardalis is from Latin.[8]
Evolution
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| Cladogram based on a 2003 study by Hassanin and Douzery.[9] |
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The giraffe is one of only two living genera of the family Giraffidae in the order Artiodactyla, the other being the okapi. The family was once much more extensive, with over 10 fossil genera described. Their closest known relatives may have been the extinct deer-like climacocerids. They, together with the family Antilocapridae (whose only extant species is the pronghorn), have been placed in the superfamily Giraffoidea. These animals may have evolved from the extinct family Palaeomerycidae which might also have been the ancestor of deer.[10]
The elongation of the neck appears to have started early in the giraffe lineage. Comparisons between giraffes and their ancient relatives suggest that vertebrae close to the skull lengthened earlier, followed by lengthening of vertebrae further down.[11] One early giraffid ancestor was Canthumeryx which has been dated variously to have lived 25–20 million years ago (mya), 17–15 mya or 18–14.3 mya and whose deposits have been found in Libya. This animal was medium-sized, slender and antelope-like. Giraffokeryx appeared 15 mya in the Indian subcontinent and resembled an okapi or a small giraffe, and had a longer neck and similar ossicones.[10] Giraffokeryx may have shared a clade with more massively built giraffids like Sivatherium and Bramatherium.[11]
Giraffids like Palaeotragus, Shansitherium and Samotherium appeared 14 mya and lived throughout Africa and Eurasia. These animals had bare ossicones and small cranial sinuses and were longer with broader skulls.[10][11] Paleotragus resembled the okapi and may have been its ancestor.[10] Others find that the okapi lineage diverged earlier, before Giraffokeryx.[11] Samotherium was a particularly important transitional fossil in the giraffe lineage as its cervical vertebrae was intermediate in length and structure between a modern giraffe and an okapi, and was more vertical than the okapi's.[12] Bohlinia, which first appeared in southeastern Europe and lived 9–7 mya was likely a direct ancestor of the giraffe. Bohlinia closely resembled modern giraffes, having a long neck and legs and similar ossicones and dentition.[10]
Bohlinia entered China and northern India in response to climate change. From there, the genus Giraffa evolved and, around 7 mya, entered Africa.[13] Further climate changes caused the extinction of the Asian giraffes, while the African giraffes survived and radiated into several new species. Living giraffes appear to have arisen around 1 mya in eastern Africa during the Pleistocene.[10] Some biologists suggest the modern giraffes descended from G. jumae;[14] others find G. gracilis a more likely candidate.[10] G. jumae was larger and more heavily built while G. gracilis was smaller and more lightly built. The main driver for the evolution of the giraffes is believed to have been the changes from extensive forests to more open habitats, which began 8 mya.[10] During this time, tropical plants disappeared and were replaced by arid C4 plants, and a dry savannah emerged across eastern and northern Africa and western India.[15][16] Some researchers have hypothesised that this new habitat coupled with a different diet, including acacia species, may have exposed giraffe ancestors to toxins that caused higher mutation rates and a higher rate of evolution.[17] The coat patterns of modern giraffes may also have coincided with these habitat changes. Asian giraffes are hypothesised to have had more okapi-like colourations.[10]
The giraffe genome is around 2.9 billion base pairs in length compared to the 3.3 billion base pairs of the okapi. Of the proteins in giraffe and okapi genes, 19.4% are identical. The two species are equally distantly related to cattle, suggesting the giraffe's unique characteristics are not because of faster evolution. The divergence of giraffe and okapi lineages dates to around 11.5 mya. A small group of regulatory genes in the giraffe appear to be responsible for the animal's stature and associated circulatory adaptations.[18]
Species and subspecies
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]
There are also seven extinct species of giraffe, listed as the following:
- †Giraffa gracilis
- †Giraffa jumae
- †Giraffa priscilla
- †Giraffa punjabiensis
- †Giraffa pygmaea
- †Giraffa sivalensis
- †Giraffa stillei
G. attica, also extinct, was formerly considered part of Giraffa but was reclassified as Bohlinia attica in 1929.
| One species taxonomy[21][20] | Three species taxonomy[26] | Four species taxonomy[24] | Eight species taxonomy[23] | Description | Image |
|---|---|---|---|---|---|
| Giraffe (G. camelopardalis)[21][20] | Northern giraffe (G. camelopardalis) | Northern giraffe (G. camelopardalis) | Kordofan giraffe (G. antiquorum)[27] | The Kordofan giraffe (G. c. antiquorum) has a distribution which includes southern Chad, the Central African Republic, northern Cameroon, and north-eastern DR Congo.[21] Populations in Cameroon were formerly included in G. c. peralta, but this was incorrect.[28] Compared to the Nubian giraffe, this subspecies has smaller and more irregular spotting patterns. Its spots may be found below the hocks and the insides of the legs. A median lump is present in males.[29]:51–52 Some 2,000 are believed to remain in the wild.[21] Considerable confusion has existed over the status of this species and G. c. peralta in zoos. In 2007, all alleged G. c. peralta in European zoos were shown to be, in fact, G. c. antiquorum.[28] With this correction, about 65 are kept in zoos.[30] The formerly recognised subspecies G. c. congoesis is now considered part of Kordofan species. | |
| Nubian giraffe including Rothschild's giraffe (G. camelopardalis)[20] also known as Baringo giraffe or Ugandan giraffe | The Nubian giraffe (G. c. camelopardalis), is found in eastern South Sudan and south-western Ethiopia, in addition to Kenya and Uganda.[21] It has sharply defined chestnut-coloured spots surrounded by mostly white lines, while undersides lack spotting.[31] The median lump is particularly developed in the male.[29]:51 Around 2,150 are thought to remain in the wild, with another 1,500 individuals belonging to the Rothschild ecotype.[21] With the addition of Rothschild's giraffe to the Nubian subspecies, the Nubian giraffe is very common in captivity, although the original phenotype is rare- a group is kept at Al Ain Zoo in the United Arab Emirates.[32] In 2003, this group numbered 14.[33] The Rothschild's giraffe (G. c. rothschildi) may be an ecotype of G. camelopardalis. Its range includes parts of Uganda and Kenya.[21] Its presence in South Sudan is uncertain.[34] This giraffe has large dark patches that usually have complete margins, but may also have sharp edges. The dark spots may also have paler radiating lines or streaks within them. Spotting does not often reach below the hocks and almost never to the hooves. This ecotype may also develop five "horns".[29]:53 Around 1,500 individuals believed to remain in the wild,[21] and more than 450 are kept in zoos.[30] According to genetic analysis circa September 2016, it is conspecific with the Nubian giraffe (G. c. camelopardalis).[24] | | |||
| West African giraffe (G. peralta),[35][36] also known as Niger giraffe or Nigerian giraffe | The West African giraffe (G. c. peralta) is endemic to south-western Niger.[21] This animal has a lighter pelage than other subspecies,[37]:322 with red lobe-shaped blotches that reach below the hocks. The ossicones are more erect than in other subspecies and males have well-developed median lumps.[29]:52–53 It is the most endangered subspecies within Giraffa, with 400 individuals remaining in the wild.[21] Giraffes in Cameroon were formerly believed to belong to this species, but are actually G. c. antiquorum.[28] This error resulted in some confusion over its status in zoos, but in 2007, it was established that all "G. c. peralta" kept in European zoos actually are G. c. antiquorum. The same 2007 study found that The West African giraffe was more closely related to the Rothschild's giraffe than the Kordofan and its ancestor may have migrated from eastern to northern Africa and then to its current range with the development of the Sahara Desert. At its largest, Lake Chad may have acted as a barrier between West African and Kordofan giraffes during the Holocene (before 5000 BC).[28] | ||||
| Reticulated giraffe (G. reticulata),[38] also known as Somali giraffe | The reticulated giraffe (G. c. reticulata) is native to north-eastern Kenya, southern Ethiopia, and Somalia.[21] Its distinctive coat pattern consists of sharp-edged, reddish brown polygonal patches divided by a network of thin white lines. Spots may or may not extend below the hocks, and a median lump is present in males.[29]:53 An estimated 8,660 individuals remain in the wild,[21] and based on International Species Information System records, more than 450 are kept in zoos.[30] | ||||
| Southern giraffe (G. giraffa) | Southern giraffe (G. giraffa) | Angolan giraffe (G. angolensis), also known as Namibian giraffe | The Angolan giraffe (G. c. angolensis) is found in northern Namibia, south-western Zambia, Botswana, and western Zimbabwe.[21] A 2009 genetic study on this subspecies suggested the northern Namib Desert and Etosha National Park populations form a separate subspecies.[39] This species has large brown blotches with edges that are either somewhat notched or have angular extensions. The spotting pattern extends throughout the legs but not the upper part of the face. The neck and rump patches tend to be fairly small. The species also has a white ear patch.[29]:51 About 13,000 animals are estimated to remain in the wild;[21] and about 20 are kept in zoos.[30] | ||
| South African giraffe (G. giraffa)[40] also known as Cape giraffe | The South African giraffe (G. c. giraffa) is found in northern South Africa, southern Botswana, southern Zimbabwe, and south-western Mozambique.[21] It has dark, somewhat rounded patches "with some fine projections" on a tawny background colour. The spots extend down the legs and get smaller. The median lump of males is less developed.[29]:52 A maximum of 31,500 are estimated to remain in the wild,[21] and around 45 are kept in zoos.[30] | ||||
| Masai giraffe (G. tippelskirchi) | Masai giraffe (G. tippelskirchi) | Masai giraffe (G. tippelskirchi),[41] also known as Kilimanjaro giraffe | The Masai giraffe (G. c. tippelskirchi) can be found in central and southern Kenya and in Tanzania.[21] It has distinctive, irregular, jagged, star-like blotches which extend to the hooves. A median lump is usually present in males.[29]:54[42] A total of 32,550 are thought to remain in the wild,[21] and about 100 are kept in zoos.[30] | ||
| Thornicroft's giraffe ("G. thornicrofti", after Harry Scott Thornicroft),[43] also known as Luangwa giraffe, or Rhodesian giraffe | The Thornicroft's giraffe (G. c. thornicrofti) is restricted to the Luangwa Valley in eastern Zambia.[21] The patches are notched and somewhat star-shaped, and may or may not extend across the legs. The median lump of males is underdeveloped.[29]:54 No more than 550 remain in the wild,[21] with none kept in zoos.[30] | ||||
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Appearance and anatomy
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] When standing among trees and bushes, they are hard to see at even a few metres distance. However, adult giraffes move about to gain the best view of an approaching predator, relying on their size and ability to defend themselves rather than on camouflage, which may be more important for calves.[10] 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] or tan.[54] 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.[55] 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.[56] 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.[57] It appears that a suspensory ligament allows the lanky legs to support the animal's great weight.[58] 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),[59] and can sustain 50 km/h (31 mph) for several kilometres.[60]
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.[61] 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.[61] 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,[62] although they could possibly float.[63] 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[62][63] or keep its head above the surface.[62]
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[64] 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.[65] 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][66]
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][65] 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.[66] 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.[56] 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.[67] 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.[68][69] However, scientists disagree about just how much time giraffes spend feeding at levels beyond the reach of other browsers,[14][56][67][70] 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.[71]
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][56] 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.[72] It has also been proposed that the neck serves to give the animal greater vigilance.[73][74]
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;[75] the left nerve is over 2 m (6 ft 7 in) long.[76] 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.[75] 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.[77]
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.[78] 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.[79] 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.[80] 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][81][82]
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.[83] 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.[70] 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
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Giraffes are usually found in groups that vary in size and composition according to ecological, anthropogenic, temporal, and social factors.[84] Traditionally, the composition of these groups had been described as open and ever-changing.[85] 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."[86] 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.[87][88][89] The number of giraffes in a group can range up to 66 individuals.[89][84]
Giraffe groups tend to be sex-segregated[89] although mixed-sex groups made of adult females and young males are known to occur. Particularly stable giraffe groups are those made of mothers and their young,[86] which can last weeks or months.[90] Social cohesion in these groups is maintained by the bonds formed between calves.[37]:330[86] Female association appears to be based on space-use and individuals may be matrilineally related.[89] In general, females are more selective than males in who they associate with in regards to individuals of the same sex.[88] Young males also form groups and will engage in playfights. However, as they get older males become more solitary but may also associate in pairs or with female groups.[89][90] Giraffes are not territorial,[7] but they have home ranges that vary according to rainfall and proximity to human settlements.[91] Male giraffes occasionally wander far from areas that they normally frequent.[37]:329
Although generally quiet and non-vocal, giraffes have been heard to communicate using various sounds. During courtship, males emit loud coughs.[42] Females call their young by bellowing. Calves will emit snorts, bleats, mooing and mewing sounds. Giraffes also snore, hiss, moan, grunt and make flute-like sounds.[42][92] During nighttime, giraffes appear to hum to each other above the infrasound range for purposes which are unclear.[92]
Reproduction and parental care
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Reproduction in giraffes is broadly polygamous: a few older males mate with the fertile females. Male giraffes assess female fertility by tasting the female's urine to detect oestrus, in a multi-step process known as the flehmen response.[86][90] Males prefer young adult females over juveniles and older adults.[86] Once an oestrous female is detected, the male will attempt to court her. When courting, dominant males will keep subordinate ones at bay.[90] A courting male may lick a female's tail, rest his head and neck on her body or nudge her with his horns. During copulation, the male stands on his hind legs with his head held up and his front legs resting on the female's sides.[42]
Giraffe gestation lasts 400–460 days, after which a single calf is normally born, although twins occur on rare occasions.[93] The mother gives birth standing up. The calf emerges head and front legs first, having broken through the fetal membranes, and falls to the ground, severing the umbilical cord.[7] The mother then grooms the newborn and helps it stand up.[50]:40 A newborn giraffe is 1.7–2 m (5.6–6.6 ft) tall.[44][45][46] Within a few hours of birth, the calf can run around and is almost indistinguishable from a one-week-old. However, for the first 1–3 weeks, it spends most of its time hiding;[94] its coat pattern providing camouflage. The ossicones, which have lain flat while it was in the womb, become erect within a few days.[42]
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Mothers with calves will gather in nursery herds, moving or browsing together. Mothers in such a group may sometimes leave their calves with one female while they forage and drink elsewhere. This is known as a "calving pool".[94] Adult males play almost no role in raising the young,[37]:337 although they appear to have friendly interactions.[86] Calves are at risk of predation, and a mother giraffe will stand over her calf and kick at an approaching predator.[42] Females watching calving pools will only alert their own young if they detect a disturbance, although the others will take notice and follow.[94]
The length time in which offspring stay with their mother varies, though it can last until the female's next calving.[94] Likewise, calves may suckle for only a month[37]:335 or as long as a year.[42][90] Females become sexually mature when they are four years old, while males become mature at four or five years. Spermatogenesis in male giraffes begins at three to four years of age.[95] Males must wait until they are at least seven years old to gain the opportunity to mate.[42][50]:40
Necking
Male giraffes use their necks as weapons in combat, a behaviour known as "necking". Necking is used to establish dominance and males that win necking bouts have greater reproductive success.[14] This behaviour occurs at low or high intensity. In low intensity necking, the combatants rub and lean against each other. The male that can hold itself more erect wins the bout. In high intensity necking, the combatants will spread their front legs and swing their necks at each other, attempting to land blows with their ossicones. The contestants will try to dodge each other's blows and then get ready to counter. The power of a blow depends on the weight of the skull and the arc of the swing.[42] A necking duel can last more than half an hour, depending on how well matched the combatants are.[37]:331 Although most fights do not lead to serious injury, there have been records of broken jaws, broken necks, and even deaths.[14]
After a duel, it is common for two male giraffes to caress and court each other. Such interactions between males have been found to be more frequent than heterosexual coupling.[96] In one study, up to 94 percent of observed mounting incidents took place between males. The proportion of same-sex activities varied from 30–75 percent. Only one percent of same-sex mounting incidents occurred between females.[97]
Mortality and health
Giraffes have high adult survival probability,[98][99] and an unusually long lifespan compared to other ruminants, up to 38 years.[100] Because of their size, eyesight and powerful kicks, adult giraffes are usually not subject to predation,[42] although lions may regularly prey on individuals up to 550 kg (1,210 lb).[101] Giraffes are the most common food source for the big cats in Kruger National Park, comprising nearly a third of the meat consumed, although only a small portion of the giraffes were probably killed by predators, as a majority of the consumed giraffes appeared to be scavenged.[102][103] Nile crocodiles can also be a threat to giraffes when they bend down to drink.[50] Calves are much more vulnerable than adults and are additionally preyed on by leopards, spotted hyenas and wild dogs.[51] A quarter to a half of giraffe calves reach adulthood.[98][104] Calf survival varies according to the season of birth, with calves born during the dry season having higher survival rates.[105]
The local, seasonal presence of large herds of migratory wildebeests and zebras reduces predation pressure on giraffe calves and increases their survival probability.[106] In turn, it has been suggested that other ungulates may benefit from associating with giraffes as their height allows them to spot predators from further away. Zebras were found to glean information on predation risk from giraffe body language and spend less time scanning the environment when giraffes are present.[107]
Some parasites feed on giraffes. They are often hosts for ticks, especially in the area around the genitals, which has thinner skin than other areas. Tick species that commonly feed on giraffes are those of genera Hyalomma, Amblyomma and Rhipicephalus. Giraffes may rely on red-billed and yellow-billed oxpeckers to clean them of ticks and alert them to danger. Giraffes host numerous species of internal parasite and are susceptible to various diseases. They were victims of the (now eradicated) viral illness rinderpest.[7] Giraffes can also suffer from a skin disorder, which comes in the form of wrinkles, lesions or raw fissures. In Tanzania, it appears to be caused by a nematode, and may be further affected by secondary infections. As much as 79% of giraffes show signs of the disease in Ruaha National Park, but it did not cause mortality in Tarangire and is less prevalent in areas with fertile soils.[108][109][110]
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