The grey wolf (Canis lupus), often known simply as the wolf, is the largest wild member of the Canidae family. Though once abundant over much of Eurasia and North America, the grey wolf inhabits a reduced portion of its former range due to widespread destruction of its territory, human encroachment, and the resulting human-wolf encounters that sparked broad extirpation. Even so, the grey wolf is regarded as being of least concern for extinction by the International Union for Conservation of Nature, when the entire grey wolf population is considered as a whole. Today, wolves are protected in some areas, hunted for sport in others, or may be subject to extermination as perceived threats to livestock and pets.
Grey wolves are social predators that live in nuclear families consisting of a mated pair which monopolises food and breeding rights, followed by their biological offspring and, occasionally, adopted subordinates. They primarily feed on ungulates, which they hunt by wearing them down in short chases. Grey wolves are typically apex predators throughout their range, with only humans and tigers posing significant threats to them.
DNA sequencing and genetic drift studies reaffirm that the grey wolf shares a common ancestry with the domestic dog. A number of other grey wolf subspecies have been identified, though the actual number of subspecies is still open to discussion.
In areas where human cultures and wolves are sympatric, wolves frequently feature in the folklore and mythology of those cultures, both positively and negatively.

Skull of Canis etruscus from the Montevarchi Paleontological Museum
The most likely ancestral candidate of the grey wolf is Canis lepophagus, a small, narrow skulled North American canid of the Miocene era, which may have also given rise to coyotes. Some larger, broader skulled C. lepophagus fossils found in northern Texas may represent the ancestral stock from which true wolves derive. The first true wolves began to appear at the end of the Blancan North American Stage and the onset of the early Irvingtonian. Among them was Canis priscolatrans, a small species closely resembling the red wolf, which colonised Eurasia by crossing the Bering land bridge. The new Eurasian C. priscolatrans population evolved into Canis etruscus, then Canis mosbachensis.[3] This primitive wolf closely resembled the modern southern wolf populations of the Arabian Peninsula and South Asia, which were once distributed in Europe in the early Quaternary glaciation until about 500,000 years ago (see Subspecies).[4] C. mosbachensis evolved in the direction of Canis lupus, and recolonised North America in the late Rancholabrean era. There, a larger canid species called Canis dirus was already established, but it became extinct 8,000 years ago after the large prey it relied on was wiped out. Competition with the newly arrived grey wolves for the smaller and swifter prey that survived may have contributed to its decline. With the extinction of dire wolves, grey wolves became the only large and widespread canid species left.[3]
The North American recolonisation likely occurred in several waves, with the most distinctive populations occurring in the periphery of the range. These populations (C. l. arctos on the high arctic islands, C. l. lycaon in the eastern forests, C. l. baileyi in the far south and C. l. rufus at the continental corner opposite the point of invasion) may represent survivors of early migrations from Eurasia. C. l. baileyi, C. l. lycaon and C. l. rufus display some primitive traits and systematic affinity to one another. Fossil remains from the late Pleistocene of large bodied wolves similar to C. l. arctos and C. l. albus occur in coastal southern California, indicating that large North American grey wolf subspecies were once widespread, and may have been driven southward by glaciation, though wolves no longer reside there. Fossils of small bodied wolves similar to C. l. baileyi have been found in a range encompassing Kansas and southern California. This indicates a late Pleistocene population flux, in which large, Arctic forms of wolf moved farther south, with smaller, warmth adapted wolves expanding as the climate moderated.[5]
Subspecies
Main article: Subspecies of Canis lupus


A Eurasian wolf (Canis lupus lupus), an example of the "northern" wolf clade
As of 2005,[6] 39 subspecies of grey wolf are recognised, including the red wolf and two subspecies of domestic dog, Canis lupus dingo and Canis lupus familiaris. Wolf subspecies are divided into two categories:
"Northern wolves": large sized, large brained wolves with strong carnassials which inhabit North America, Europe and northern Asia.[7]


An Arabian wolf (Canis lupus arabs), an example of the "southern" wolf clade
"Southern wolves": native to the Arabian Peninsula and South Asia. They are characterised by their short fur,[8] small brains and weak carnassials. They may represent a relic population of early wolves, as they closely resemble fossil European wolves,[4] and the rate of changes observed in their DNA sequences date them to about 800,000 years, as opposed to the American and European lineages which stretch back only 150,000.[9] The vocalisations of southern wolves have a higher proportion of short, sharp barking,[7] and they seldom howl.[10] It is likely that dogs and dingoes stem from this group.[7][11]
Wolves in Central and East Asia are intermediate in form and size to northern and southern wolves.[4] Differences in brain size are well defined in different wolf populations, with wolves in northern Eurasia having the highest values, North American wolves having slightly smaller brains, and the southern wolves having the smallest. Southern wolves have brains 5–10% smaller than northern wolves.[12] Though different in behaviour and morphology, northern and southern wolves can still interbreed: the Zoological Gardens of London for example once successfully managed to mate a male European wolf to an Indian female, resulting in a cub baring an almost exact likeness to its sire.[13]
Domestication
Main article: Origin of the domestic dog


Skulls of a St. Bernard dog and a lap dog. Though varied in appearance, domestic dog skulls can be consistently distinguished from wolf skulls by their shortened muzzles, broader palates, crowded teeth and the broad, heavy frontal shields at the top of their skulls.[14]
Studies on the genetic distance for mitochondrial DNA on dogs and Eurasian wolves confirmed that wolves are the exclusive ancestral species to dogs. Domestic dogs possess four mtDNA lineages, suggesting four independent domestication events.[15] A later study identified mtDNA evidence suggesting a common origin from a single East Asian gene pool for all dog populations,[16] while another, using a much larger data set of nuclear markers, points to the Middle East as the source of most of the genetic diversity in the domestic dog and a more likely origin of domestication events.[17] A study by the Kunming Institute of Zoology found that the domestic dog is descended from wolves tamed less than 16,300 years ago south of the Yangtse river in China.[18] Morphological comparisons have narrowed the likely ancestral subspecies of grey wolf to wolves of the Middle Eastern and South Asian variety.[4]


Comparative illustration of wolf and dog prints
The actual domestication process is a source of debate. Although it is popularly assumed that dogs are the result of artificial selection, the general intractability of adult wolves to human handling has lead certain experts to theorise that the domestication process occurred through natural selection when Mesolithic human communities began building permanent settlements in which a new ecological niche (middens and landfills) was opened to wolves. These wolves would have formed a commensal relationship with humans, feeding on their waste over many generations, with natural selection favouring assertive wolves with shorter flight distances in human presence, and causing physical changes related to the redundancy of features adapted for hunting big game.[19][20]
Although dogs are the most closely related canids to grey wolves (the sequence divergence between grey wolves and dogs is only 1.8%, as opposed to over 4% between grey wolves, Ethiopian wolves and coyotes),[21] there are a number of physical and behavioural differences. Comparative studies on dog and wolf behaviour and anatomy have shown that dog physiology and most dog behaviours are comparable to those of young wolves, an example of neoteny and pedomorphism.[22] The tympanic bullae are large, confex and almost spherical in wolves, in contrast to dogs whose bullae are smaller, compressed and slightly crumpled.[23] Compared to equally sized dogs, wolves tend to have 20% larger skulls and 10% bigger brains, as well as proportionately larger teeth.[24] The premolars and molars of wolves are much less crowded, and have more complex cusp patterns.[25] Dogs lack a pre-caudal gland, and enter estrus twice yearly, unlike wolves which only do so once annually.[14] The forelegs of wolves are closer to each other than those of dogs, with the former's tracks being further apart. Wolves have larger paws than dogs, and their tails hang straight or in a slight curve toward the body when neutral, whereas dogs carry their tails in a slight curl.[20]
Physical description



Gray Wolf skeleton
Anatomy
Grey wolves are slender, powerfully built animals with large, deeply descending ribcages and sloping backs. Their abdomens are pulled in, and their necks heavily muscled. Their limbs are long and robust, with comparatively small paws.[26] The front paws have five toes each, while the back paws have four. The forelimbs are seemingly pressed into the chest, with the elbows pointed inward, and the feet outward.[23] Females tend to have narrower muzzles and foreheads, thinner necks, slightly shorter legs and less massive shoulders than males.[27] Wolves are very strong for their size, possessing sufficient strength to turn over a frozen horse or moose carcass.[28] They are also capable of running at speeds of 56–64 km (34–38 miles) per hour, and can continue running for more than 20 minutes, though not necessarily at that speed.[29] In cold climates, wolves can reduce the flow of blood near their skin to conserve body heat. The warmth of the footpads is regulated independently of the rest of the body, and is maintained at just above tissue-freezing point where the pads came in contact with ice and snow.[30] The intestines of adult wolves measure 460–575 cm, the ratio to body length being 4.13–4.62.[31] The stomach can hold 7–9 kg (15–20 lb) of food[23] and up to 7.5 litres (8 U.S. qt) of water.[32] The liver is relatively large, weighing 0.7–1.9 kg (1.6–4.2 lb) in males and 0.68-0.82 kg (1.5–1.8 lb) in females.[23]


Wolf skull from the Museo del Lupo in Abruzzo National Park. Note the proportionately larger teeth than the dog's.
Wolves' heads are large and heavy, with wide foreheads, strong jaws and long, blunt muzzles. The ears are relatively small and triangular. Wolves usually carry their heads at the same level as their backs, raising their heads only when alert.[26] The sagittal and lambdoid crests are well developed, the former dividing just in front of the bregma into two ridges curving outward to form posterior border of postorbital processes. The interorbital region is moderately elevated and well defined, with distinct longitudinal concavity between raised and thickened postorbital processes.[33] The dental formula is:
Dentition
3.1.4.2
3.1.4.3


Stuffed grey wolf and golden jackal at The Museum of Zoology, St. Petersburg. Note the wolf's larger size and broader muzzle
The teeth are heavy and large, being better suited to bone crushing than those of other extant canids, though not as specialised as those found in hyenas.[34][35] The canine teeth are robust and relatively short (26 mm).[23] The animal can develop a crushing pressure of perhaps 1,500 lbf/in2 compared to 750 lbf/in2 for a German shepherd. This force is sufficient to break open most bones,[36] as well as cut through half inch lassos with one snap.[37]
They generally resemble German shepherds or huskies in bodily configuration, but are distinguishable from them by their orbital angle of 40°–45° rather than 53°–60°,[23] and the greater size of their heads and teeth (see Domestication).[38] Compared to coyotes, wolves are larger and have broader snouts, shorter ears, and a proportionately smaller braincase[23] and lack sweat glands on their pawpads.[39] Compared to golden jackals, wolves are larger and heavier, and have proportionately longer legs, shorter torsos and longer tails.[40] The teeth are overall less trenchant than the jackal's, particularly in the upper molars, which have lower cusps, are broader, and are more terete.[41]
Dimensions
Grey wolves are the largest extant members of the Canidae, excepting certain large breeds of domestic dog.[23] Grey wolf weight and size can vary greatly worldwide, tending to increase proportionally with latitude as predicted by Bergmann's Rule.[42] Adult wolves are 105–160 cm (41–63 in) in length and 80–85 cm (32–34 in) in shoulder height.[43] The tail is ⅔ the length of the head and body,[44] measuring 29–50 cm (11–20 in) in length. The ears are 110–90 mm (44–36 in) in height, and the hind feet 220–250 mm.[43] Wolf weight varies geographically; on average, European wolves may weigh 38.5 kilograms (85 lb), North American wolves 36 kilograms (79 lb), and Indian and Arabian wolves 25 kilograms (55 lb).[45] Females in any given wolf population typically weigh 5–10 lbs less than males.[46] Wolves weighing over 54 kg (120 lbs) are uncommon, though exceptionally large individuals have been recorded in Alaska, Canada,[46] and the former Soviet Union.[43][47] The heaviest recorded grey wolf in North America was killed on 70 Mile River in east-central Alaska on July 12, 1939 and weighed 79 kilograms (170 lb),[46] while the heaviest recorded wolf in Eurasia was killed after World War II in the Kobelyakski Area of the Poltavskij Region, Ukrainian SSR, and weighed 86 kilograms (190 lb).[47]
Fur


Gray wolves molt some of their coats in late spring or early summer.
Grey wolves have very dense and fluffy winter fur, with short undefur and long, coarse guard hairs.[26] Most of the underfur and some of the guard hairs are shed in the spring and grow back in the autumn period.[45] The longest hairs occur on the back, particularly on the front quarters and neck. Especially long hairs are found on the shoulders, and almost form a crest on the upper part of the neck. The hairs on the cheeks are elongated and form tufts. The ears are covered in short hairs which stongly project from the fur. Short, elastic and closely adjacent hairs are present on the limbs from the elbows down to the calcaneal tendons.[48] The winter fur is highly resistant to cold; wolves in northern climates can rest comfortably in open areas at −40° by placing their muzzles between the rear legs and covering their faces with their tail. Wolf fur provides better insulation than dog fur, and, as with wolverines, it does not collect ice when warm breath is condensed against it.[45] In warm climates, the fur is coarser and scarcer than in northern wolves.[26] Female wolves tend to have smoother furred limbs than males, and generally develop the smoothest overall coats as they age. Older wolves generally have more white hairs in the tip of the tail, along the nose and on the forehead. The winter fur is retained longest in lactating females, though with some hair loss around their nipples.[27] Hair length on the middle of the back is 60–70 mm. Hair length of the guard hairs on the shoulders generally does not exceed 90 mm, but can reach 110–130 mm.[49]
Coat colour ranges from almost pure white through various shades of blond, cream, and ochre to greys, browns, and blacks.[50] Differences in coat colour between sexes are largely absent,[51] though females may have redder tones.[52] Fur colour does not seem to serve any camouflage purpose, with some scientists concluding that the blended colors have more to do with emphasizing certain gestures during interaction.[53] Black coloured wolves (which occur through wolf-dog hybridisation) rarely occur in Eurasia, where interactions with domestic dogs has been reduced over the past thousand years due to the depletion of wild wolf populations.[54] They are more common in North America; about half of the wolves in the reintroduced wolf population in Wyoming's Yellowstone National Park are black.[54] In southern Canada and Minnesota the black phase is more common than the white, though grey coloured wolves predominate.[50]
Senses
Their sense of smell is relatively weakly developed when compared to that of some hunting dog breeds, being able to detect carrion upwind no farther than 2–3 km. Because of this, they rarely capture hidden hares or birds, though they can easily follow fresh tracks.[55] Their auditory perception is however very sharp, being able to hear up to a frequency of 26kH,[56] and is greater than that of foxes. Their hearing is sharp enough to register the fall of leaves in the autumn period.[55] The legend that wolves fear the sound of string instruments may have a basis in fact, as captive wolves in the Regent's Park Zoo were shown to exhibit signs of intense distress when hearing low minor chords.[57] Their eyesight is not as powerful as that of dogs, though their night vision is the most advanced of the Canidae.[55]
Behaviour

Social structure


A wolf pack in Yellowstone National Park
In popular literature, wolf packs are often portrayed as strictly hierarchical social structures with a breeding "alpha" pair which climbs the social ladder through fighting, followed by subordinate "beta" wolves and a low ranking "omega" which bares the brunt of the pack's aggression. This terminology is based heavily on the behaviour of captive wolf packs composed of unrelated animals, which will fight and compete against each other for status. Also, as dispersal is impossible in captive situations, fights become more frequent than in natural settings. In the wild, wolf packs are little more than nuclear families whose basic social unit consists of a mated pair, followed by its offspring.[58] Northern wolf packs tend not to be as compact or unified as those of African wild dogs and spotted hyenas,[59] though they are not as unstable as those of coyotes.[60] Southern wolves are more similar in social behaviour to coyotes and dingoes, living largely alone or in pairs.[61] The average pack consists of 5–11 animals; 1–2 adults, 3–6 juveniles and 1–3 yearlings,[62] though exceptionally large packs consisting of 42 wolves are known. Wolf packs rarely adopt other wolves into their fold, and typically kill them. In the rare cases where strange wolves are adopted, the adoptee is almost invariably a young animal of 1–3 years of age, while killed wolves are mostly fully grown.[63] During times of ungulate abundance (migration, calving etc.), different wolf packs may temporarily join forces.[64] Wolves as young as five months and as old as five years have been recorded to leave their packs to start their own families, though the average age is 11–24 months. Triggers for dispersal include the onset of sexual maturity and competition within the pack for food and breeding.[65]
Reproduction


Wolf nursing her pups
In areas with low wolf densities, wolves are generally monogamous.[66] Mated pairs usually remain together for life if one of the wolves does not die. Upon the death of one mated wolf, pairs are quickly re-established. Since males often predominate in any given wolf population, unpaired females are a rarity.[67] Polygamy does occur, but primarily in captive situations. Multiple litters are rarely successful, due to infanticide by the pack's females.[68] The age of first breeding in wolves depends largely on environmental factors; when food is abundant, or when wolf populations are heavily managed, wolves can rear pups at younger ages in order to exploit the newly available resources. Captive wolves have been known to breed as soon as they reach 9–10 months, while the youngest recorded breeding wolves in the wild were 2 years old. Females are capable of producing pups every year, with one litter annually being the average. Unlike coyotes, wolves never reach reproductive senescence before they die.[69] Incest rarely occurs, though inbreeding depression has been reported to be a problem for wolves in Saskatchewan[70] and Isle Royale.[71]
Estrus typically occurs in late winter, with older, multiparous females entering estrus 2–3 weeks earlier than younger females. Before the rut ensues, wolf packs will temporarily dissolve until the end of the mating season.[67] When receptive, females will avert the base of their tails to one side, exposing the vulva. During mating, the pair is locked into a copulatory tie which may last 5–36 minutes. Because estrus in wolves only lasts a month, the males do not abandon their mates to find other females to inseminate as dogs do. During pregnancy, female wolves will remain in a den located away from periphoral zone in their territory, where violent encounters with other packs are more likely.[72] Old females usually whelp in the den of their previous litter, while younger females typically den near their birthplace. The gestation period lasts 62–75 days, with pups usually being born in the summer period.[73] The average litter consists of 5–6 pups. Litters of 14–17 occur 1% of the time.[74] Litter sizes tend to increase in areas where prey is abundant.[75] Wolves bear relatively large pups in small litters compared to other canid species.[76] Pups are born blind and deaf, and are covered in short soft greyish-brown fur. They weigh 300–500 grams at birth, and begin to see after 9–12 days. The milk canines erupt after one month. Pups first leave the den after 3 weeks. At 1.5 months of age, they are agile enough to flee from danger. Mother wolves do not leave the den for the first few weeks, relying on the fathers to provide food for them and their young. Pups begin to eat solid food at the age of 3–4 weeks. Pups have a fast growth rate during their first four months of life: during this period, the pup's weight can increase nearly 30 times.[76][77]
Territorial behaviours


Wolves scent-roll to bring scents back to the pack
Wolves are highly territorial animals, and generally establish territories far larger than they require to survive in order to assure a steady supply of prey. Territory size depends largely on the amount of prey available: in areas with an abundance of prey, the territories of resident wolf packs are smaller. Wolf packs travel constantly in search of prey, covering roughly 9% of their territory per day (average 25 km/d or 15 mi/d). The core of their territory is on average 35 km2 (14 sq mi), in which they spend 50% of their time.[78] Prey density tends to be much higher in the territory's surrounding areas. Despite this higher abundance of prey, wolves tend to avoid hunting in the fringes of their territory unless desperate, due to the possibility of fatal encounters with neighboring packs.[79] The size of their territory may increase when the pack's pups reach the age of 6 months, and thus have the same nutritional requirements as adults. The smallest territory on record was held by a pack of six wolves in northeastern Minnesota, which occupied an estimated 33 km2. The largest was held by an Alaskan pack of ten wolves encompassing a 6,272 km2 area. In some areas, wolves may shift territories during their prey's migration season.[80]
Wolves defend their territories from other packs through a combination of scent marking, direct attacks and howling (see Communication). Scent marking is used for territorial advertisement, and involves urination, defecation and ground scratching. Scent marks are generally left every 240 metres throughout the territory on regular travelways and junctions. Such markers can last for 2–3 weeks,[80] and are typically placed near rocks, boulders, trees or the skeletons of large animals.[81] When scent marking and howling fail to deter strange wolf packs from entering another's territory, violent interactions can ensue.[80] Territorial fights are among the principal causes of wolf mortality: one study on wolf mortality in Minnesota and the Denali National Park and Preserve concluded that 14–65% of wolf deaths were due to predation by other wolves.[82] In fact, 91% of wolf fatalities occur within 3.2 km (2.0 mi) of the borders between neighboring territories.[83] Because the consequences of trespassing can be fatal, such incursions are thought to be largely due to desperation or deliberate aggressiveness.[80]