Elephant

Elephants
Temporal range: Pliocene–Recent
African Bush Elephant.jpg
African elephant
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Superorder: Afrotheria
Order: Proboscidea
Family: Elephantidae
Gray, 1821
Genera

Elephants are large mammals of the family Elephantidae and the order Proboscidea. Two species are traditionally recognised, the African elephant (Loxodonta africana) and the Asian elephant (Elephas maximus), although some evidence suggests that African bush elephants and African forest elephants are separate species (L. africana and L. cyclotis respectively). Elephants are scattered throughout sub-Saharan Africa, South Asia, and Southeast Asia. Elephantidae is the only surviving family of the order Proboscidea; other, now extinct, members of the order include deinotheres, gomphotheres, and mastodons. Male African elephants are the largest extant terrestrial animals and can reach a height of 4 m (13 ft) and weigh 7,000 kg (15,000 lb). All elephants have several distinctive features the most notable of which is a long trunk or proboscis, used for many purposes, particularly breathing, lifting water and grasping objects. Their incisors grow into tusks, which can serve as weapons and as tools for moving objects and digging. Elephants' large ear flaps help to control their body temperature. Their pillar-like legs can carry their great weight. African elephants have larger ears and concave backs while Asian elephants have smaller ears and convex or level backs.

Elephants are herbivorous and can be found in different habitats including savannahs, forests, deserts and marshes. They prefer to stay near water. They are considered to be keystone species due to their impact on their environments. Other animals tend to keep their distance; predators such as lions, tigers, hyenas and wild dogs usually target only the young elephants (or "calves"). Females ("cows") tend to live in family groups, which can consist of one female with her calves or several related females with offspring. The groups are led by an individual known as the matriarch, often the oldest cow. Elephants have a fission-fusion society in which multiple family groups come together to socialise. Males ("bulls") leave their family groups when they reach puberty, and may live alone or with other males. Adult bulls mostly interact with family groups when looking for a mate and enter a state of increased testosterone and aggression known as musth, which helps them gain dominance and reproductive success. Calves are the centre of attention in their family groups and rely on their mothers for as long as three years. Elephants can live up to 70 years in the wild. They communicate by touch, sight, smell and sound; elephants use infrasound, and seismic communication over long distances. Elephant intelligence has been compared with that of primates and cetaceans. They appear to have self-awareness and show empathy for dying or dead individuals of their kind.

African elephants are listed as vulnerable by the International Union for Conservation of Nature (IUCN), while the Asian elephant is classed as endangered. One of the biggest threats to elephant populations is the ivory trade, as the animals are poached for their ivory tusks. Other threats to wild elephants include habitat destruction and conflicts with local people. Elephants are used as working animals in Asia. In the past they were used in war; today, they are often put on display in zoos and circuses. Elephants are highly recognisable and have been featured in art, folklore, religion, literature and popular culture.

Etymology

The word "elephant" is based on the Latin elephas (genitive elephantis) ("elephant"), which is the Latinised form of the Greek ἐλέφας (elephas) (genitive ἐλέφαντος (elephantos)),[1] probably from a non-Indo-European language, likely Phoenician.[2] It is attested in Mycenaean Greek as e-re-pa and e-re-pa-to in Linear B syllabic script.[3][4] As in Mycenaean Greek, Homer used the Greek word to mean ivory, but after the time of Herodotus, it also referred to the animal.[1] The word "elephant" appears in Middle English as olyfaunt (c.1300) and was borrowed from Old French oliphant (12th century).[2] The Tamil word is aliyan for elephant.[5] In Swahili elephants are known as Ndovu or Tembo. In Sanskrit the elephant is called hastin,[6] while in Hindi it is known as hāthī .[7] Babylonians called the animal pīru, from which the Middle Persian word for "elephant" pil derives.[8] It was Arabicized as al-fil, and was then borrowed from Arabic into Old Norse as fil (fíl in Icelandic).[9] Loxodonta, the generic name for the African elephants, is Greek for "oblique-sided tooth".[10]

Taxonomy

Classification, species and subspecies

Asian elephant
Comparative morphology of head and forepart of the body of the Asian elephant (1) and the African elephant (2)

Elephants belong to the family Elephantidae, the sole remaining family within the order Proboscidea. Their closest extant relatives are the sirenians (dugongs and manatees) and the hyraxes, with which they share the clade Paenungulata within the superorder Afrotheria.[11] Elephants and sirenians are further grouped in the clade Tethytheria.[12] Traditionally, two species of elephants are recognised; the African elephant (Loxodonta africana) of sub-Saharan Africa, and the Asian elephant (Elephas maximus) of South and Southeast Asia. African elephants have larger ears, a concave back, more wrinkled skin, a sloping abdomen and two finger-like extensions at the tip of the trunk. Asian elephants have smaller ears, a convex or level back, smoother skin, a horizontal abdomen that occasionally sags in the middle and one extension at the tip of the trunk. The looped ridges on the molars are narrower in the Asian elephant while those of the African are more diamond-shaped. The Asian elephant also has dorsal bumps on its head and some patches of depigmentation on its skin.[13] In general, African elephants are larger than their Asian cousins.

Swedish zoologist Carl Linnaeus first described the genus Elephas and an elephant from Sri Lanka (then known as Ceylon) under the binomial Elephas maximus in 1758. In 1798, Georges Cuvier classified the Indian elephant under the binomial Elephas indicus. Dutch zoologist Coenraad Jacob Temminck described the Sumatran elephant in 1847 under the binomial Elephas sumatranus. English zoologist Frederick Nutter Chasen classified all three as subspecies of the Asian elephant in 1940.[14] Asian elephants vary geographically in their colour and amount of depigmentation. The Sri Lankan elephant (Elephas maximus maximus) inhabits Sri Lanka, the Indian elephant (E. m. indicus) is native to mainland Asia (on the Indian subcontinent and Indochina), and the Sumatran elephant (E. m. sumatranus) is found in Sumatra.[13] One disputed subspecies, the Borneo elephant, lives in northern Borneo and is smaller than all the other subspecies. It has larger ears, a longer tail, and straighter tusks than the typical elephant. Sri Lankan zoologist Paules Edward Pieris Deraniyagala described it in 1950 under the trinomial Elephas maximus borneensis, taking as his type an illustration in National Geographic.[15] It was subsequently subsumed under either E. m. indicus or E. m. sumatranus. Results of a 2003 genetic analysis indicate its ancestors separated from the mainland population about 300,000 years ago.[16] A 2008 study found that Borneo elephants are not indigenous to the island but were brought there before 1521 by the Sultan of Sulu from Java, where elephants are now extinct.[15]

African forest elephant

The African elephant was first named by German naturalist Johann Friedrich Blumenbach in 1797 as Elephas africana.[17] The genus Loxodonta was commonly believed to have been named by Georges Cuvier in 1825. Cuvier spelled it Loxodonte and an anonymous author romanised the spelling to Loxodonta; the International Code of Zoological Nomenclature recognises this as the proper authority.[18] In 1942, 18 subspecies of African elephant were recognised by Henry Fairfield Osborn, but further morphological data has reduced the number of classified subspecies,[19] and by the 1990s, only two were recognised, the savannah or bush elephant (L. a. africana) and the forest elephant (L. a. cyclotis);[20] the latter has smaller and more rounded ears and thinner and straighter tusks, and is limited to the forested areas of western and Central Africa.[21] A 2000 study argued for the elevation of the two forms into separate species (L. africana and L. cyclotis respectively) based on differences in skull morphology.[22] DNA studies published in 2001 and 2007 also suggested they were distinct species,[23][24] while studies in 2002 and 2005 concluded that they were the same species.[25][26] Further studies (2010 and 2011) have supported African savannah and forest elephants' status as separate species.[27][28] As of 2011, the taxonomic designations of African elephants were still debated.[28] The third edition of Mammal Species of the World lists the two forms as full species[18] and does not list any subspecies in its entry for Loxodonta africana.[18] This approach is not taken by the United Nations Environment Programme's World Conservation Monitoring Centre nor by the IUCN, both of which list L. cyclotis as a synonym of L. africana.[29][30] Some evidence suggests that elephants of western Africa are a separate species,[31] although this is disputed.[26][28] The pygmy elephants of the Congo Basin, which have been suggested to be a separate species (Loxodonta pumilio) are probably forest elephants whose small size and/or early maturity are due to environmental conditions.[32]

Evolution and extinct relatives

Over 161 extinct members and three major evolutionary radiations of the order Proboscidea have been recorded. The earliest proboscids, the African Eritherium and Phosphatherium of the late Paleocene, heralded the first radiation.[33] The Eocene included Numidotherium, Moeritherium and Barytherium from Africa. These animals were relatively small and aquatic. Later on, genera such as Phiomia and Palaeomastodon arose; the latter likely inhabited forests and open woodlands. Proboscidean diversity declined during the Oligocene.[34] One notable species of this epoch was Eritreum melakeghebrekristosi of the Horn of Africa, which may have been an ancestor to several later species.[35] The beginning of the Miocene saw the second diversification, with the appearance of the deinotheres and the mammutids. The former were related to Barytherium, lived in Africa and Eurasia,[36] while the latter may have descended from Eritreum[35] and spread to North America.[36]

Mounted skeleton of early proboscid Moeritherium in Japan

The second radiation was represented by the emergence of the gomphotheres in the Miocene,[36] which likely evolved from Eritreum[35] and originated in Africa, spreading to every continent except Australia and Antarctica. Members of this group included Gomphotherium and Platybelodon.[36] The third radiation started in the late Miocene and led to the arrival of the elephantids, which descended from, and slowly replaced, the gomphotheres.[37] The African Primelephas gomphotheroides gave rise to Loxodonta, Mammuthus and Elephas. Loxodonta branched off earliest, around the Miocene and Pliocene boundary, while Mammuthus and Elephas diverged later during the early Pliocene. Loxodonta remained in Africa, while Mammuthus and Elephas spread to Eurasia, and the former reached North America. At the same time, the stegodontids, another proboscidean group descended from gomphotheres, spread throughout Asia, including the Indian subcontinent, China, southeast Asia and Japan. Mammutids continued to evolve into new species, such as the American mastodon.[38]

Comparison of woolly mammoth (left) and American mastodon

At the beginning of the Pleistocene, elephantids experienced a high rate of speciation. Loxodonta atlantica became the most common species in northern and southern Africa but was replaced by Elephas iolensis later in the Pleistocene. Only when Elephas disappeared from Africa did Loxodonta become dominant once again, this time in the form of the modern species. Elephas diversified into new species in Asia, such as E. hysudricus and E. platycephus;[39] the latter the likely ancestor of the modern Asian elephant.[40] Mammuthus evolved into several species, including the well-known woolly mammoth.[39] In the Late Pleistocene, most proboscidean species vanished during the Quaternary glaciation which killed off 50% of genera weighing over 5 kg (11 lb) worldwide.[41]

Proboscideans experienced several evolutionary trends, such as an increase in size, which led to many giant species that stood up to 4 m (13 ft) tall.[42] As with other megaherbivores, including the extinct sauropod dinosaurs, the large size of elephants likely developed to allow them to survive on vegetation with low nutritional value.[43] Their limbs grew longer and the feet shorter and broader. Early proboscideans developed longer mandibles and smaller craniums, while more advanced ones developed shorter mandibles, which shifted the head's centre of gravity. The skull grew larger, especially the cranium, while the neck shortened to provide better support for the skull. The increase in size led to the development and elongation of the mobile trunk to provide reach. The number of premolars, incisors and canines decreased.[42] The cheek teeth (molars and premolars) became larger and more specialized, especially after elephants started to switch from C3-plants to C4-grasses, which caused their teeth to undergo a three-fold increase in teeth height as well as substantial multiplication of lamellae after about five million years ago. Only in the last million year or so did they return to a diet mainly consisting of C3 trees and shrubs.[44][45] The upper second incisors grew into tusks, which varied in shape from straight, to curved (either upward or downward), to spiralled, depending on the species. Some proboscideans developed tusks from their lower incisors.[42] Elephants retain certain features from their aquatic ancestry such as their middle ear anatomy and the internal testes of the males.[46]

There has been some debate over the relationship of Mammuthus to Loxodonta or Elephas. Some DNA studies suggest Mammuthus is more closely related to the former,[47][48] while others point to the latter.[12] However, analysis of the complete mitochondrial genome profile of the woolly mammoth (sequenced in 2005) supports Mammuthus being more closely related to Elephas.[23][27][49] Morphological evidence supports Mammuthus and Elephas as sister taxa, while comparisons of protein albumin and collagen have concluded that all three genera are equally related to each other.[50] Some scientists believe a cloned mammoth embryo could one day be implanted in an Asian elephant's womb.[51]

Dwarf species

Skeleton of a Cretan dwarf elephant

Several species of proboscideans lived on islands and experienced insular dwarfism. This occurred primarily during the Pleistocene, when some elephant populations became isolated by fluctuating sea levels, although dwarf elephants did exist earlier in the Pliocene. These elephants likely grew smaller on islands due to a lack of large or viable predator populations and limited resources. By contrast, small mammals such as rodents develop gigantism in these conditions. Dwarf proboscideans are known to have lived in Indonesia, the Channel Islands of California, and several islands of the Mediterranean.[52]

Elephas celebensis of Sulawesi is believed to have descended from Elephas planifrons. Elephas falconeri of Malta and Sicily was only 1 m (3 ft), and had probably evolved from the straight-tusked elephant. Other descendants of the straight-tusked elephant existed in Cyprus. Dwarf elephants of uncertain descent lived in Crete, Cyclades and Dodecanese, while dwarf mammoths are known to have lived in Sardinia.[52] The Columbian mammoth colonised the Channel Islands and evolved into the pygmy mammoth. This species reached a height of 1.2–1.8 m (4–6 ft) and weighed 200–2,000 kg (440–4,410 lb). A population of small woolly mammoths survived on Wrangel Island, now 87 miles north of the Siberian coast, as recently as 4,000 years ago.[52] After their discovery in 1993, they were considered dwarf mammoths.[53] This classification has been re-evaluated and since the Second International Mammoth Conference in 1999, these animals are no longer considered to be true "dwarf mammoths".[54]

Anatomy and morphology

African elephant skeleton

Elephants are the largest living terrestrial animals. African elephants stand 3–4 m (10–13 ft) and weigh 4,000–7,000 kg (8,800–15,400 lb) while Asian elephants stand 2–3.5 m (7–11 ft) and weigh 3,000–5,000 kg (6,600–11,000 lb).[13] In both cases, males are larger than females.[14][17] Among African elephants, the forest form is smaller than the savannah form.[21] The skeleton of the elephant is made up of 326–351 bones.[55] The vertebrae are connected by tight joints, which limit the backbone's flexibility. African elephants have 21 pairs of ribs, while Asian elephants have 19 or 20 pairs.[56]

An elephant's skull is resilient enough to withstand the forces generated by the leverage of the tusks and head-to-head collisions. The back of the skull is flattened and spread out, creating arches that protect the brain in every direction.[57] The skull contains air cavities (sinuses) that reduce the weight of the skull while maintaining overall strength. These cavities give the inside of the skull a honeycomb-like appearance. The cranium is particularly large and provides enough room for the attachment of muscles to support the entire head. The lower jaw is solid and heavy.[55] Because of the size of the head, the neck is relatively short to provide better support.[42] Lacking a lacrimal apparatus, the eye relies on the harderian gland to keep it moist. A durable nictitating membrane protects the eye globe. The animal's field of vision is compromised by the location and limited mobility of the eyes.[58] Elephants are considered dichromats[59] and they can see well in dim light but not in bright light.[60] The core body temperature averages 35.9 °C (97 °F), similar to a human. Like all mammals, an elephant can raise or lower its temperature a few degrees from the average in response to extreme environmental conditions.[61]

Ears

African elephant with ears spread in a threat or attentive position; note the visible blood vessels

Elephant ears have thick bases with thin tips. The ear flaps, or pinnae, contain numerous blood vessels called capillaries. Warm blood flows into the capillaries, helping to release excess body heat into the environment. This occurs when the pinnae are still, and the animal can enhance the effect by flapping them. Larger ear surfaces contain more capillaries, and more heat can be released. Of all the elephants, African bush elephants live in the hottest climates, and have the largest ear flaps.[62] Elephants are capable of hearing at low frequencies and are most sensitive at 1 kHz.[63]

Trunk

The trunk, or proboscis, is a fusion of the nose and upper lip, although in early fetal life, the upper lip and trunk are separated.[42] The trunk is elongated and specialised to become the elephant's most important and versatile appendage. It contains up to 150,000 separate muscle fascicles, with no bone and little fat. These paired muscles consist of two major types: superficial (surface) and internal. The former are divided into dorsals, ventrals and laterals, while the latter are divided into transverse and radiating muscles. The muscles of the trunk connect to a bony opening in the skull. The nasal septum is composed of tiny muscle units that stretch horizontally between the nostrils. Cartilage divides the nostrils at the base.[64] As a muscular hydrostat, the trunk moves by precisely coordinated muscle contractions. The muscles work both with and against each other. A unique proboscis nerve – formed by the maxillary and facial nerves – runs along both sides of the trunk.[65]

African elephant with its trunk raised, a behaviour often adopted when trumpeting
Asian elephant drinking water with trunk

Elephant trunks have multiple functions, including breathing, olfaction, touching, grasping, and sound production.[42] The animal's sense of smell may be four times as sensitive as that of a bloodhound.[66] The trunk's ability to make powerful twisting and coiling movements allows it to collect food, wrestle with conspecifics,[67] and lift up to 350 kg (770 lb).[42] It can be used for delicate tasks, such as wiping an eye and checking an orifice,[67] and is capable of cracking a peanut shell without breaking the seed.[42] With its trunk, an elephant can reach items at heights of up to 7 m (23 ft) and dig for water under mud or sand.[67] Individuals may show lateral preference when grasping with their trunks: some prefer to twist them to the left, others to the right.[65] Elephants can suck up water both to drink and to spray on their bodies.[42] An adult Asian elephant is capable of holding 8.5 L (2.2 US gal) of water in its trunk.[64] They will also spray dust or grass on themselves.[42] When underwater, the elephant uses its trunk as a snorkel.[46]

The African elephant has two finger-like extensions at the tip of the trunk that allow it to grasp and bring food to its mouth. The Asian elephant has only one, and relies more on wrapping around a food item and squeezing it into its mouth.[13] Asian elephants have more muscle coordination and can perform more complex tasks.[64] Losing the trunk would be detrimental to an elephant's survival,[42] although in rare cases individuals have survived with shortened ones. One elephant has been observed to graze by kneeling on its front legs, raising on its hind legs and taking in grass with its lips.[64] Floppy trunk syndrome is a condition of trunk paralysis in African bush elephants caused by the degradation of the peripheral nerves and muscles beginning at the tip.[68]

Teeth

Closeup of the cheek teeth of a dead juvenile bush elephant

Elephants usually have 26 teeth: the incisors, known as the tusks, 12 deciduous premolars, and 12 molars. Unlike most mammals, which grow baby teeth and then replace them with a single permanent set of adult teeth, elephants are polyphyodonts that have cycles of tooth rotation throughout their lives. The chewing teeth are replaced six times in a typical elephant's lifetime. Teeth are not replaced by new ones emerging from the jaws vertically as in most mammals. Instead, new teeth grow in at the back of the mouth and move forward to push out the old ones. The first chewing tooth on each side of the jaw falls out when the elephant is two to three years old. The second set of chewing teeth falls out when the elephant is four to six years old. The third set is lost at 9–15 years of age, and set four lasts until 18–28 years of age. The fifth set of teeth lasts until the elephant is in its early 40s. The sixth (and usually final) set must last the elephant the rest of its life. Elephant teeth have loop-shaped dental ridges, which are thicker and more diamond-shaped in African elephants.[69]

Tusks

Asian elephant eating tree bark, using its tusks to peel it off.

The tusks of an elephant are modified incisors in the upper jaw. They replace deciduous milk teeth when the animal reaches 6–12 months of age and grow continuously at about 17 cm (7 in) a year. A newly developed tusk has a smooth enamel cap that eventually wears off. The dentine is known as ivory and its cross-section consists of crisscrossing line patterns, known as "engine turning", which create diamond-shaped areas. As a piece of living tissue, a tusk is relatively soft; it is as hard as the mineral calcite. Much of the incisor can be seen externally, while the rest is fastened to a socket in the skull. At least one-third of the tusk contains the pulp and some have nerves stretching to the tip. Thus it would be difficult to remove it without harming the animal. When removed, ivory begins to dry up and crack if not kept cool and moist. Tusks serve multiple purposes. They are used for digging for water, salt, and roots; debarking or marking trees; and for moving trees and branches when clearing a path. When fighting, they are used to attack and defend, and to protect the trunk.[70]

Like humans, who are typically right- or left-handed, elephants are usually right- or left-tusked. The dominant tusk, called the master tusk, is generally more worn down, as it is shorter with a rounder tip. For the African elephants, tusks are present in both males and females, and are around the same length in both sexes, reaching up to 3 m (10 ft),[70] but those of males tend to be thicker.[71] In the Asian species, only the males have large tusks. Female Asians have very small ones, or none at all.[70] Tuskless males exist and are particularly common among Sri Lankan elephants.[72] Asian males can have tusks as long as Africans', but they are usually slimmer and lighter; the largest recorded was 3.02 m (10 ft) long and weighed 39 kg (86 lb). Hunting for elephant ivory in Africa[73] and Asia[74] has led to natural selection for shorter tusks.[75]

Skin

An Asian elephant after wallowing; the mud may act as a sunscreen

An elephant's skin is generally very tough, at 2.5 cm (1 in) thick on the back and parts of the head. The skin around the mouth, anus and inside of the ear is considerably thinner. Elephants typically have grey skin, but African elephants look brown or reddish after wallowing in coloured mud. Asian elephants have some patches of depigmentation, particularly on the forehead and ears and the areas around them. Calves have brownish or reddish hair, especially on the head and back. As elephants mature, their hair darkens and becomes sparser, but dense concentrations of hair and bristles remain on the end of the tail as well as the chin, genitals and the areas around the eyes and ear openings. Normally the skin of an Asian elephant is covered with more hair than its African counterpart.[76]

An elephant uses mud as a sunscreen, protecting its skin from ultraviolet light. Although tough, an elephant's skin is very sensitive. Without regular mud baths to protect it from burning, insect bites, and moisture loss, an elephant's skin suffers serious damage. After bathing, the elephant will usually use its trunk to blow dust onto its body and this dries into a protective crust. Elephants have difficulty releasing heat through the skin because of their low surface-area-to-volume ratio, which is many times smaller than that of a human. They have even been observed lifting up their legs, presumably in an effort to expose their soles to the air.[76]

Legs, locomotion and posture

An Asian elephant walking

To support the animal's weight, an elephant's limbs are positioned more vertically under the body than in most other mammals. The long bones of the limbs have cancellous bone in place of medullary cavities. This strengthens the bones while still allowing haematopoiesis.[77] Both the front and hind limbs can support an elephant's weight, although 60% is borne by the front.[78] Since the limb bones are placed on top of each other and under the body, an elephant can stand still for long periods of time without using much energy. Elephants are incapable of rotating their front legs, as the ulna and radius are fixed in pronation; the "palm" of the manus faces backward.[77] The pronator quadratus and the pronator teres are either reduced or absent.[79] The circular feet of an elephant have soft tissues or "cushion pads" beneath the manus or pes, which distribute the weight of the animal.[78] They appear to have a sesamoid, an extra "toe" similar in placement to a giant panda's extra "thumb", that also helps in weight distribution.[80] As many as five toenails can be found on both the front and hind feet.[13]

Elephants can move both forwards and backwards, but cannot trot, jump, or gallop. They use only two gaits when moving on land, the walk and a faster gait similar to running.[77] In walking, the legs act as pendulums, with the hips and shoulders rising and falling while the foot is planted on the ground. With no "aerial phase", the fast gait does not meet all the criteria of running, although the elephant uses its legs much like other running animals, with the hips and shoulders falling and then rising while the feet are on the ground.[81] Fast-moving elephants appear to 'run' with their front legs, but 'walk' with their hind legs and can reach a top speed of 18 km/h (11 mph).[82] At this speed, most other quadrupeds are well into a gallop, even accounting for leg length. Spring-like kinetics could explain the difference between the motion of elephants and other animals.[83] During locomotion, the cushion pads expand and contract, and reduce both the pain and noise that would come from a very heavy animal moving.[78] Elephants are capable swimmers. They have been recorded swimming for up to six hours without touching the bottom, and have travelled as far as 48 km (30 mi) at a stretch and at speeds of up to 2.1 km/h (1 mph).[84]

Internal and sexual organs

African elephant heart in a jar

The brain of an elephant weighs 4.5–5.5 kg (10–12 lb) compared to 1.6 kg (4 lb) for a human brain. While the elephant brain is larger overall, it is proportionally smaller. At birth, an elephant's brain already weighs 30–40% of its adult weight. The cerebrum and cerebellum are well developed, and the temporal lobes are so large that they bulge out laterally.[61] The throat of an elephant appears to contain a pouch where it can store water for later use.[42]

The heart of an elephant weighs 12–21 kg (26–46 lb). It has a double-pointed apex, an unusual trait among mammals.[61] When standing, the elephant's heart beats approximately 30 times per minute. Unlike many other animals, the heart rate speeds up by 8 to 10 beats per minute when the elephant is lying down.[85] The lungs are attached to the diaphragm, and breathing relies mainly on the diaphragm rather than the expansion of the ribcage.[61] Connective tissue exists in place of the pleural cavity. This may allow the animal to deal with the pressure differences when its body is underwater and its trunk is breaking the surface for air,[46] although this explanation has been questioned.[86] Another possible function for this adaptation is that it helps the animal suck up water through the trunk.[46] Elephants inhale mostly through the trunk, although some air goes through the mouth. They have a hindgut fermentation system, and their large and small intestines together reach 35 m (115 ft) in length. The majority of an elephant's food intake goes undigested despite the process lasting up to a day.[61]

A male elephant's testes are located internally near the kidneys. The elephant's penis can reach a length of 100 cm (39 in) and a diameter of 16 cm (6 in) at the base. It is S-shaped when fully erect and has a Y-shaped orifice. The female has a well-developed clitoris at up to 40 cm (16 in). The vulva is located between the hind legs instead of near the tail as in most mammals. Determining pregnancy status can be difficult due to the animal's large abdominal cavity. The female's mammary glands occupy the space between the front legs, which puts the suckling calf within reach of the female's trunk.[61] Elephants have a unique organ, the temporal gland, located in both sides of the head. This organ is associated with sexual behaviour, and males secrete a fluid from it when in musth.[87] Females have also been observed with secretions from the temporal glands.[66]

Behaviour and life history

Ecology and activities

File:Elephant eating Yala Sri Lanka.ogv
An Asian elephant feeding on grass
An African elephant using its prehensile trunk for foraging

The African bush elephant can be found in habitats as diverse as dry savannahs, deserts, marshes, and lake shores, and in elevations from sea level to mountain areas above the snow line. Forest elephants mainly live in equatorial forests, but will enter gallery forests and ecotones between forests and savannahs.[21] Asian elephants prefer areas with a mix of grasses, low woody plants and trees, primarily inhabiting dry thorn-scrub forests in southern India and Sri Lanka and evergreen forests in Malaya.[14] Elephants are herbivorous and will eat leaves, twigs, fruit, bark, grass and roots.[21] They are born with sterile intestines, and require bacteria obtained from their mothers feces to digest vegetation.[88] African elephants are mostly browsers while Asian elephants are mainly grazers. They can consume as much as 150 kg (330 lb) of food and 40 L (11 US gal) of water in a day. Elephants tend to stay near water sources.[21] Major feeding bouts take place in the morning, afternoon and night. At midday, elephants rest under trees and may doze off while standing. Sleeping occurs at night while the animal is lying down.[77][89] Elephants average 3–4 hours of sleep per day.[90] Both males and family groups typically move 10–20 km (6–12 mi) a day, but distances as far as 90–180 km (56–112 mi) have been recorded in the Etosha region of Namibia.[91] Elephants go on seasonal migrations in search of food, water and mates. At Chobe National Park, Botswana, herds travel 325 km (202 mi) to visit the river when the local waterholes dry up.[92]

Because of their large size, elephants have a huge impact on their environments and are considered keystone species. Their habit of uprooting trees and undergrowth can transform savannah into grasslands; when they dig for water during drought, they create waterholes that can be used by other animals. They can enlarge waterholes when they bathe and wallow in them. At Mount Elgon, elephants excavate caves that are used by ungulates, hyraxes, bats, birds and insects.[93] Elephants are important seed dispersers; African forest elephants ingest and defecate seeds, with either no effect or a positive effect on germination. The seeds are typically dispersed in large amounts over great distances.[94] In Asian forests, large seeds require giant herbivores like elephants and rhinoceros for transport and dispersal. This ecological niche cannot be filled by the next largest herbivore, the tapir.[95] Because most of the food elephants eat goes undigested, their dung can provide food for other animals, such as dung beetles and monkeys.[93] Elephants can have a negative impact on ecosystems. At Murchison Falls National Park in Uganda, the overabundance of elephants has threatened several species of small birds that depend on woodlands. Their weight can compact the soil, which causes the rain to run off, leading to erosion.[89]

Elephants do not digest much of their food. Other animals, such as this baboon, may pick through elephant dung looking for undigested seeds.

Elephants typically coexist peacefully with other herbivores, which will usually stay out of their way. Some aggressive interactions between elephants and rhinoceros have been recorded. At Aberdare National Park, Kenya, a rhino attacked an elephant calf and was killed by the other elephants in the group.[89] At Hluhluwe–Umfolozi Game Reserve, South Africa, introduced young orphan elephants went on a killing spree that claimed the lives of 36 rhinos during the 1990s, but ended with the introduction of older males.[96] The size of adult elephants makes them nearly invulnerable to predators,[14] though there are rare reports of adult elephants falling prey to tigers.[97] Calves may be preyed on by lions, spotted hyenas, and wild dogs in Africa[17] and tigers in Asia.[14] The lions of Savuti, Botswana, have adapted to hunting juvenile elephants during the dry season, and a pride of 30 lions has been recorded killing juvenile individuals between the ages of four and eleven years.[98] Elephants appear to distinguish between the growls of larger predators like tigers and smaller ones like leopards (which have not been recorded killing calves); the latter they react less fearfully and more aggressively to.[99] Elephants tend to have high numbers of parasites, particularly nematodes, compared to other herbivores. This is due to lower predation pressures that would otherwise kill off many of the individuals with significant parasite loads.[100]

Social organisation

A family of African elephants: note the protected position of the calves in the middle of the group
A family of elephants bathing, a behaviour which reinforces social bonding

Female elephants spend their entire lives in tight-knit matrilineal family groups, some of which are made up of more than ten members, including three pairs of mothers with offspring, and are led by the matriarch which is often the eldest female.[101] She remains leader of the group until death[17] or if she no longer has the energy for the role;[102] a study on zoo elephants showed that when the matriarch died, the levels of faecal corticosterone ('stress hormone') dramatically increased in the surviving elephants.[103] When her tenure is over, the matriarch's eldest daughter takes her place; this occurs even if her sister is present.[17] The older matriarchs tend to be more effective decision-makers.[104]

The social circle of the female elephant does not necessarily end with the small family unit. In the case of elephants in Amboseli National Park, Kenya, a female's life involves interaction with other families, clans, and subpopulations. Families may associate and bond with each other, forming what are known as bond groups. These are typically made of two family groups. During the dry season, elephant families may cluster together and form another level of social organisation known as the clan. Groups within these clans do not form strong bonds, but they defend their dry-season ranges against other clans. There are typically nine groups in a clan. The Amboseli elephant population is further divided into the "central" and "peripheral" subpopulations.[101]

Some elephant populations in India and Sri Lanka have similar basic social organisations. There appear to be cohesive family units and loose aggregations. They have been observed to have "nursing units" and "juvenile-care units". In southern India, elephant populations may contain family groups, bond groups and possibly clans. Family groups tend to be small, consisting of one or two adult females and their offspring. A group containing more than two adult females plus offspring is known as a "joint family". Malay elephant populations have even smaller family units, and do not have any social organisation higher than a family or bond group. Groups of African forest elephants typically consist of one adult female with one to three offspring. These groups appear to interact with each other, especially at forest clearings.[101]

Lone bull: Adult male elephants spend much of their time alone or in single-sex groups

The social life of the adult male is very different. As he matures, a male spends more time at the edge of his group and associates with outside males or even other families. At Amboseli, young males spend over 80% of their time away from their families when they are 14–15. The adult females of the group start to show aggression towards the male, which encourages him to permanently leave. When males do leave, they either live alone or with other males. The former is typical of bulls in dense forests. Asian males are usually solitary, but occasionally form groups of two or more individuals; the largest consisted of seven bulls. Larger bull groups consisting of over 10 members occur only among African bush elephants, the largest of which numbered up to 144 individuals.[105] A dominance hierarchy exists among males, whether they range socially or solitarily. Dominance depends on the age, size and sexual condition.[105] Old bulls appear to control the aggression of younger ones and prevent them from forming "gangs".[106] Adult males and females come together for reproduction. Bulls appear to associate with family groups if an oestrous cow is present.[105]

Male elephants sparing

Sexual behaviour

Musth

Bull in musth

Adult males enter a state of increased testosterone known as musth. In a population in southern India, males first enter musth at the age of 15, but it is not very intense until they are older than 25. At Amboseli, bulls under 24 do not go into musth, while half of those aged 25–35 and all those over 35 do. Young bulls appear to enter musth during the dry season (January–May), while older bulls go through it during the wet season (June–December). The main characteristic of a bull's musth is a fluid secreted from the temporal gland that runs down the side of his face. He may urinate with his penis still in his sheath, which causes the urine to spray on his hind legs. Behaviours associated with musth include walking with the head held high and swinging, picking at the ground with the tusks, marking, rumbling and waving only one ear at a time. This can last from a day to four months.[107]

Males become extremely aggressive during musth. Among both musth and nonmusth bulls, size is the determining factor in agonistic encounters. In contests between individuals from the two groups, musth bulls win the majority of the time, even when the non-musth bull is larger. A male may stop showing signs of musth when he encounters a musth male of higher rank. Those of equal rank tend to avoid each other. Agonistic encounters typically consist of threat displays, chases and minor sparring with the tusks. Serious fights are rare.[107]

Mating

Bull mating with a member of a female group

Elephants are polygynous breeders,[108] and copulations are most frequent during the peak of the wet season.[109] A cow in oestrus releases chemical signals (pheromones) in her urine and vaginal secretions to signal her readiness to mate. A bull will follow a potential mate and assess her condition with the flehmen response, which requires the male to collect a chemical sample with his trunk and bring it to the vomeronasal organ.[110] The oestrous cycle of a cow lasts 14–16 weeks with a 4–6-week follicular phase and an 8–10-week luteal phase. While most mammals have one surge of luteinizing hormone during the follicular phase, elephants have two. The first (or anovulatory) surge, could signal to males that the female is in oestrus by changing her scent, but ovulation does not occur until the second (or ovulatory) surge.[111] Fertility rates in cows decline around 45–50 years of age.[102]

Bulls engage in a behaviour known as mate-guarding, where they follow oestrous females and defend them from other males. Most mate-guarding is done by musth males, and females actively seek to be guarded by them, particularly older ones.[112] Thus these bulls have more reproductive success.[105] Musth appears to signal to females the condition of the male, as weak or injured males do not have normal musths.[113] For young females, the approach of an older bull can be intimidating, so her relatives stay nearby to provide support and reassurance.[114] During copulation, the male lays his trunk over the female's back.[115] The penis is very mobile, being able to move independently of the pelvis.[116] Prior to mounting, it curves forward and upward. Copulation lasts about 45 seconds and does not involve pelvic thrusting or ejaculatory pause.[117]

Homosexual behaviour is frequent in both sexes. As in heterosexual interactions, this involves mounting. Male elephants sometimes stimulate each other by playfighting and "championships" may form between old bulls and younger males. Female same-sex behaviours have been documented only in captivity where they are known to masturbate one another with their trunks.[118]

Birthing and calves

An African elephant mother bathing with her calf

Gestation in elephants typically lasts around two years with interbirth intervals usually lasting four to five years. Births tend to take place during the wet season.[119] Calves are born 85 cm (33 in) tall and weigh around 120 kg (260 lb).[114] Typically, only a single young is born, but twins sometimes occur.[120][121] The relatively long pregnancy is maintained by five corpus luteums (as opposed to one in most mammals) and gives the foetus more time to develop, particularly the brain and trunk.[120] As such, newborn elephants are precocial and quickly stand and walk to follow their mother and family herd.[122] A new calf is usually the centre of attention for herd members. Adults and most of the other young will gather around the newborn, touching and caressing it with their trunks. For the first few days, the mother is intolerant of other herd members near her young. Alloparenting – where a calf is cared for by someone other than its mother – takes place in some family groups. Allomothers are typically two to twelve years old.[114] When a predator is near, the family group gathers together with the calves in the centre.[123]

For the first few days, the newborn is unsteady on its feet, and needs the support of its mother. It relies on touch, smell and hearing, as its eyesight is poor. It has little precise control over its trunk, which wiggles around and may cause it to trip. By its second week of life, the calf can walk more firmly and has more control over its trunk. After its first month, a calf can pick up, hold and put objects in its mouth, but cannot suck water through the trunk and must drink directly through the mouth. It is still dependent on its mother and keeps close to her.[122]

For its first three months, a calf relies entirely on milk from its mother for nutrition after which it begins to forage for vegetation and can use its trunk to collect water. At the same time, improvements in lip and leg coordination occur. Calves continue to suckle at the same rate as before until their sixth month, after which they become more independent when feeding. By nine months, mouth, trunk and foot coordination is perfected. After a year, a calf's abilities to groom, drink, and feed itself are fully developed. It still needs its mother for nutrition and protection from predators for at least another year. Suckling bouts tend to last 2–4 min/hr for a calf younger than a year and it continues to suckle until it reaches three years of age or older. Suckling after two years may serve to maintain growth rate, body condition and reproductive ability.[122] Play behaviour in calves differs between the sexes; females run or chase each other, while males play-fight. The former are sexually mature by the age of nine years[114] while the latter become mature around 14–15 years.[105] Adulthood starts at about 18 years of age in both sexes.[124][125] Elephants have long lifespans, reaching 60–70 years of age.[69] Lin Wang, a captive male Asian elephant, lived for 86 years.[126]

Communication

Asian elephants greeting each other by inter-twining their trunks

Touching is an important form of communication among elephants. Individuals greet each other by stroking or wrapping their trunks; the latter also occurs during mild competition. Older elephants use trunk-slaps, kicks and shoves to discipline younger ones. Individuals of any age and sex will touch each other's mouths, temporal glands and genitals, particularly during meetings or when excited. This allows individuals to pick up chemical cues. Touching is especially important for mother–calf communication. When moving, elephant mothers will touch their calves with their trunks or feet when side-by-side or with their tails if the calf is behind them. If a calf wants to rest, it will press against its mother's front legs and when it wants to suckle, it will touch her breast or leg.[127]

Visual displays mostly occur in agonistic situations. Elephants will try to appear more threatening by raising their heads and spreading their ears. They may add to the display by shaking their heads and snapping their ears, as well as throwing dust and vegetation. They are usually bluffing when performing these actions. Excited elephants may raise their trunks. Submissive ones will lower their heads and trunks, as well as flatten their ears against their necks, while those that accept a challenge will position their ears in a V shape.[128]

Elephants produce several sounds, usually through the larynx, though some may be modified by the trunk. Perhaps the most well known is the trumpet, which is made during excitement, distress or aggression.[129] Fighting elephants may roar or squeal, and wounded ones may bellow.[130] Rumbles are produced during mild arousal[131] and some appear to be infrasonic.[132] Infrasonic calls are important, particularly for long-distance communication,[129] in both Asian and African elephants. For Asian elephants, these calls have a frequency of 14–24 Hz, with sound pressure levels of 85–90 dB and last 10–15 seconds.[132] For African elephants, calls range from 15–35 Hz with sound pressure levels as high as 117 dB, allowing communication for many kilometres, with a possible maximum range of around 10 km (6 mi).[133]

At Amboseli, several different infrasonic calls have been identified. A greeting rumble is emitted by members of a family group after having been separated for several hours. Contact calls are soft, unmodulated sounds made by individuals that have been separated from their group and may be responded to with a "contact answer" call that starts out loud, but becomes softer. A "let's go" soft rumble is emitted by the matriarch to signal to the other herd members that it is time to move to another spot. Bulls in musth emit a distinctive, low-frequency pulsated rumble nicknamed the "motorcycle". Musth rumbles may be answered by the "female chorus", a low-frequency, modulated chorus produced by several cows. A loud postcopulatory call may be made by an oestrous cow after mating. When a cow has mated, her family may produce calls of excitement known as the "mating pandemonium".[131]

Elephants are known to communicate with seismics, vibrations produced by impacts on the earth's surface or acoustical waves that travel through it. They appear to rely on their leg and shoulder bones to transmit the signals to the middle ear. When detecting seismic signals, the animals lean forward and put more weight on their larger front feet; this is known as the "freezing behaviour". Elephants possess several adaptations suited for seismic communication. The cushion pads of the feet contain cartilaginous nodes and have similarities to the acoustic fat found in marine mammals like toothed whales and sirenians. A unique sphincter-like muscle around the ear canal constricts the passageway, thereby dampening acoustic signals and allowing the animal to hear more seismic signals.[134] Elephants appear to use seismics for a number of purposes. An individual running or mock charging can create seismic signals that can be heard at great distances.[135] When detecting the seismics of an alarm call signalling danger from predators, elephants enter a defensive posture and family groups will pack together. Seismic waveforms produced by locomotion appear to travel distances of up to 32 km (20 mi) while those from vocalisations travel 16 km (10 mi).[136]

Intelligence and cognition

File:Insightful-Problem-Solving-in-an-Asian-Elephant-pone.0023251.s005.ogv
Elephant rolling a block to allow it to reach food