The breed's general appearance also varies greatly depending on the particular line's emphasis. As with many working breeds that are also shown in the ring, there are differences of opinion among breeders over what makes an ideal Australian Shepherd.
Size
Reflecting the great variation that exists in the breed, an Aussie can stand between 18 and 23 inches (46 to 58 cm) at the withers and weigh between 35 and 70 pounds (16 to 32 kg). For show dogs, females should fall in the lower heights and males in the higher ranges. There is a conflict, though, concerning the Miniature Australian Shepherd. Since there is no strict height or weight measurements, some say mini Aussies are the same as the original breed, simply smaller, and should not be considered a separate breed. Others stick to an opinion that minis evolved somewhat separately and should be their own breed.
Color
A 65 lb, 18 month Old, Red Merle Australian Shepherd (playing with a black Labrador mix).
The four colors of Aussies are blue merle (black and gray patchwork) red merle (liver red and beige patchwork), black, and red. Any of these colors can have white markings on the face in the form of a blaze, white collars, chests, belly and legs. They may also have copper/tan points above their eyes, on the cheeks, inside the ear, on the chest, legs and under the tail. White should not appear on the body of the dog from topmost point of the shoulder blade to the tail.
Dogs with tan and white along with the primary color are called tri-color. Dogs with white or copper only along with the primary color are called bi-color. Dogs with no white or copper are referred to as "self colored".
The merle gene which produces a mingled or patchwork combination of dark and light areas is the most common pattern associated with the breed. This merle gene (M) is dominant so that affected dogs (Mm) show the pigmentation pattern, however, when two merles are bred, there is a statistical risk that 25% of the offspring will end up with the two copies of the merle gene (Homozygous). These dogs usually have a mostly white coat, blue irises, are often deaf and/or blind. In this case the deafness and blindness is neither dominant nor recessive defective gene, but is linked to having two copies of the merle gene that disrupts pigmentation and produces these health defects.
Eyes
There is also great variety in the Aussie's eye color. An early nickname for the breed was "ghost-eye dog". Aussie eyes may be green, hazel, amber, brown, or blue; they may have two different colored eyes, or even have bicolored or "split eyes" (for example, a half-brown, half-blue eye), which appear to be linked to the merle coloration. Merled eyes occur as well, where one color is mixed in and swirled with another. Any combination of eye color is acceptable in the breed standard, so long as the eyes are healthy. In general, however, black Aussies (self, bi-color or tri-color) tend to have brown eyes, while red (self, bi-color or tri-color) Aussies tend to have amber eyes.
A hallmark of the breed is a short bobbed or docked tail in countries where docking is permitted. Some Aussies are born with naturally short bobbed tails, others with full long tails, and others with natural partial bobs, where the tail is midlength and appears stubby. Most breeders dock the tails when the puppies are born.
Temperament
The Australian Shepherd is unique with regard to its temperament. There are two distinct types of personality to look for depending on the lines, as well as many shades within these two types.
Generally the breed is an energetic dog that requires exercise and enjoys working, whether it is learning and practicing tricks, competing in dog agility, or any other physically and mentally involving activity. Many need to run, full out, regularly. It is usually a sweet and affectionate dog who is faithful to its owners and may be good with children, although its overwhelming instinct to work may subvert its ability to function as a family dog.
Dogs with strong working instinct may show more reserved, guarding behaviors along with a tendency to chase or nip at running children or strangers if not properly trained. Its protective instinct and behaviors can be frightening to children, strangers, and small animals. Those bred for a more family-oriented temperament are more friendly and affectionate with strangers and generally more reliable around children. Because the breed was developed to serve on the ranch, a job which includes being protective of its property, it sometimes can be annoying with its inclination to bark warnings about neighborhood activity, but it is not generally an obsessively barking dog.
Although most Aussies don't have tails, the wagging movement of the hind end still occurs. The Aussie is intelligent, learns quickly, and loves to play. This means that a bored, neglected, unexercised Aussie will invent its own games, activities, and jobs, which to a busy owner might appear to be hyperactivity in the house (for example, an Aussie may go from being at rest to running at top speed for several 'laps' around the house before returning to rest, all apparenty for no purpose) around fragile furnishings or involve the destruction of yard and property. Without something to amuse them, Aussies often turn destructive. Aussies also do best with plenty of human companionship: they are often called "velcro" for their strong desire to always be near their owners and for their tendency to form intense, devoted bonds with select people.
The Australian Shepherd has a reputation as a highly intelligent and versatile stock dog with a range of working styles. While improperly trained or frustrated Aussies may exhibit excessive running and barking, a good working Aussie is quick, thoughtful, and easy with its stock. The ability for the breed to adapt to the situation and think for itself makes it an excellent all-around worker. For this reason the Aussie is often chosen to work unusual livestock such as ducks, geese, and commercially raised rabbits. One interesting thing about Aussies is that they are seen 'smiling', pulling up their lips and showing their teeth. This is often mistaken as a challenge, but is actually friendly.