Direwolves

"There are no direwolves south of the Wall."

- Robb Stark

Direwolves are an unusually large and intelligent species of wolf. A grey direwolf on a white field is the sigil of House Stark.

Range
Direwolves are held to be near-mythical in most of the south of Westeros. Inhabitants of the North acknowledge that they are a real animal, but they are very rarely encountered there. There are some rumors of them living in the Wolfswood near Winterfell, but these are unconfirmed. There may have been larger direwolf populations in the North in past millennia, but centuries of human encroachment on their territory have pushed their range back to the furthest north.

There have been confirmed sightings north of the Wall by the Night's Watch who guard it. The wildlings who live beyond the Wall claim that direwolves can be found in greater numbers in the Haunted Forest to the north of the Wall. By the reign of King Robert Baratheon no direwolf had been sighted south of the Wall, even in the Stark lands of the North, in over 200 years.

Biology
An adult direwolf is as large as a small horse, and can rip a man's arm from its socket. Even a juvenile direwolf is quite capable of killing a man by ripping out his throat. Physically they are not simply larger versions of wolves, but have slightly different proportions than their smaller cousins; their heads are larger with a leaner, more pronounced muzzle, their legs are longer in proportion to their body, and they have proportionately larger teeth that are often used to crush bones.

Direwolves can give birth to litters of at least six puppies.

Behavior
Direwolves are loyal to their masters and will attack anyone who threatens them. Arya Stark's direwolf Nymeria attacked Prince Joffrey after he threatened her with a sword, while Bran Stark's Summer killed a would-be assassin in Bran's bedroom. They will also come to the aid of those they know are loyal to their masters, since Ghost defended Samwell Tarly and Gilly, both friends of Jon Snow, from Night's Watch bullies, and later prepared to fight alongside Ser Davos Seaworth, Eddison Tollett and a handful of Jon's loyal brothers against the Night's Watch mutineers who betrayed and murdered Jon not long before.

At the Battle of Oxcross during the War of the Five Kings, Robb Stark's direwolf Grey Wind plays a vital part in Robb's victory, killing several Lannister sentries and panicking their horses. During the Battle of Castle Black, Ghost killed numerous wildlings, among them at least one Thenn.

The Stark direwolves
The six children of Lord Eddard Stark adopted six orphaned direwolf cubs found in the Wolfswood. They are as follows:

Alive

 * Ghost, adopted by Jon Snow. An albino with red eyes and the runt of the litter. Ghost followed Jon to his service with the Night's Watch at the Wall and beyond. Currently at Winterfell with Jon.
 * Nymeria, adopted by Arya Stark. Fearing for Nymeria's life, Arya chased her away after the wolf cub attacked Prince Joffrey Baratheon to defend her owner. She is now loose somewhere in the Riverlands, the first direwolf seen that far south (other than Grey Wind) in many centuries.

Deceased

 * Lady: adopted by Sansa Stark. Lady is killed by Lord Stark in place of Nymeria for biting Joffrey, under the order of Robert Baratheon at Cersei Lannister's urging. The first of the direwolf litter to have been killed.
 * Grey Wind, adopted by Robb Stark. Accompanied the main Stark army to the South, used by King Robb as a ferocious war dog. Killed at The Twins by Frey men with crossbows during the Red Wedding, after which both he and Robb were decapitated and his head sewn onto Robb's corpse. The second of the direwolf litter to have been killed.
 * Shaggydog, adopted by Rickon Stark. Easily distinguishable by black fur. Likewise fled Winterfell with Rickon. After separating from Bran's group, Rickon, Osha, and Shaggydog fled to the Last Heartb to seek refuge with the House Umber's. Shaggydog was later killed by the Umbers immediately prior to their betrayal of Rickon and alliance with Ramsay Bolton, and Ramsay apparently skinned him to use his fur as a carpet. The third of the direwolf litter to have been killed.
 * Summer, adopted by Bran Stark. Fled Winterfell with Bran following the castle's Sack of Winterfell. Accompanied Bran his companions Beyond the Wall to the Cave of the three-eyed raven, where he was later mauled by Wights buying time for Bran to escape. The fourth of the direwolf litter to have been killed.

Behind the scenes

 * In Season 1, the production team used adult dogs to stand in for the juvenile direwolf puppies. Northern Inuit Dogs from Mahlek Kennels were used. In interviews they have stated that this was done to allow the audience to grow attached to them, which would have been more difficult had the dogs been created by special effects. As the direwolves continue to grow, however, more special effects are going to be used.
 * When Season 1 wrapped up, the dogs used as direwolves were offered to the young actors who played their owners in the show. Sophie Turner (Sansa) took the dog who played Lady. Isaac Hempstead-Wright (Bran) wanted to take Summer, but his family already had two dogs at home.
 * In Season 2, real wolves were used to portray the adolescent direwolves. Using a variety of camera tricks and techniques, the shots were augmented to make the wolves appear larger, and then composited with the background and actors for the final shots. The result is that in Season 2 the direwolves are played by real-life wolves that have been super-sized in post-production.
 * In Season 5, Ghost is the only direwolf that appears. Because he is supposed to be older and larger, the production team wanted to use a full-sized trained wolf named Quigley to portray him (in "The Wars to Come" and "The Gift"). Quigley's trainer Andrew Simpson owns about 30 wolves and is based in Calgary, Canada. The production team had been reaching out to him since Season 1, but he was busy training wolves for other film projects. In earlier seasons they had hoped to have one of his wolves transported to Northern Ireland, but by Season 5 they decided it would be easier to simply fly out to Canada to film the scenes they needed with the wolf in a Calgary studio.
 * It wasn't possible to translate "dire wolf" into German, so the species was named "Schattenwolf" in the german dub, what means "shadow wolves" . This name was possibly chosen because there's also an animal species in Westeros called Shadowcats.

In real-life
The name dire wolf (Latin: ​Canis dirus​) is given to an actual species of Pleistocene megafauna that once existed on real-life Earth, like the mammoth and sabertooth cat. Dire wolves inhabited the Americas, from 1.8 million years ago to about 10,000 years ago.

Note that the name of the real-life animal is spelled as two words, "dire wolf", but that the name in the A Song of Ice and Fire novels is consistently spelled as one word, "direwolf".

The large Pleistocene mammals went extinct when the last ice age ended, roughly 10,000 years ago. Large herbivores such as mammoths died out due to climate change (and possibly hunting by prehistoric humans), and carnivores such as dire wolves died when their food sources disappeared. Dire wolves and mammoths were adapted to the cold ice age climate, their size maximizing heat retention and strength but at the cost of speed. In the warmer climate following the ice age, smaller but faster animals thrived that didn't need to make these tradeoffs for cold weather. Not only did dire wolves' old large prey such as mammoths die out, but they were being out-competed by smaller and faster grey wolves. Dire wolves, however, were not the largest wolves. Currently extant subspecies the Yukon wolf (Canis lupus pambasileus) and the Northwestern wolf (Canis lupus occidentals) were approximately the same size as the dire wolf.