A Jack Russell Terrier is a working terrier, and the most important physical attribute of a working terrier is not coat color, gait or expression, but chest size. If a terrier is too large to enter and work in burrows, effectively it is virtually worthless as a working dog.
The red fox is the traditional quarry of the Jack Russell Terrier, and the quarry pursued by the Reverend John Russell himself. Red fox may den in a wide variety of locations from old badger settes and drain pipes to building crawl spaces, old rabbit holes and groundhog dens, but in all cases the working Jack Russell must be small enough to get up to its quarry, which is to say a Jack Russell's chest should be no larger than that of the animal it is pursuing. Red foxes vary in size, but across the world they average about 14 pounds in weight and have a chest size, on average of 12-14 inches in circumference when measured at the widest part of the chest. As Barry Jones, the founding Chairman of the National Working Terrier Federation noted in comments directed to those in the UK who were intent on pulling the Jack Russell Terrier into The Kennel Club as a Parson Russell Terrier.
The chest is, without doubt, the determining factor as to whether a terrier may follow its intended quarry underground. Too large and he/she is of little use for underground work, for no matter how determined the terrier may be, this physical setback will not be overcome in the nearly-tight situations it will encounter in working foxes. It may be thought the fox is a large animal - to the casual observer it would appear so. However, the bone structure of the fox is finer than that of a terrier, plus it has a loose-fitting, profuse pelt which lends itself to flexibility. I have not encountered a fox which could not be spanned at 14 inches circumference - this within a weight range of 10 lbs to 24 lbs, on average 300 foxes spanned a year. You may not wish to work your terrier. However, there is a Standard to be attained, and spannability is a must in the Parson Russell Terrier.
Jack Russell Terriers are predominantly white (more than 51%) with black, tan, or tricolour markings commonly found on the face and at the base of the tail. Jack Russell Terriers have small V-shaped ears that should fold downward, and strong teeth with a scissor bite.
Jack Russell Terriers come in three coat types: smooth, broken, and rough. In all cases, the coat should be dense and not soft, feathery or linty. A smooth coated dog should be smooth coated all over, with a dense topcoat that is approximately 1cm long. A rough-coated dog should have a double coat with fur as much as 10cm long, and should be rough-coated over its entire body. A broken-coated dog is any dog with a topcoat of intermediate length, or a dog that is largely or partially smooth with longer hair on some parts of its body.
Jack Russell Terriers come in three coat types: smooth, broken, and rough. In all cases, the coat should be dense and not soft, feathery or linty. A smooth coated dog should be smooth coated all over, with a dense topcoat that is approximately 1cm long. A rough-coated dog should have a double coat with fur as much as 10cm long, and should be rough-coated over its entire body. A broken-coated dog is any dog with a topcoat of intermediate length, or a dog that is largely or partially smooth with longer hair on some parts of its body.
Jack Russell Terrier tails are straight, held high and upright. Traditionally, tails are docked to around five inches -- the length of a hand grip. It is not a serious fault to leave a tail a little long, but too short a tail creates a less useful dog in the field and a dog that looks poorly balanced.
Many dogs commonly referred to as "Jack Russells" have crooked or “benched” legs, resembling Queen Anne furniture. This is often a sign of Achondroplasia, and is a fault according to the breed standard.[1] A Jack Russell's forelegs should be strong and straight boned with joints in correct alignment, elbows hanging perpendicular to the body and working free of the sides. Hindquarters should be strong and muscular, well put together with good angulation and bend of stifle, giving plenty of drive and propulsion. Looking from behind, the hocks must be straight. Generally, a JRT should have a square appearance, with the body length in proportion to the height.
The Jack Russell is a working terrier. Terrier work requires a dog that will bark at prey so that the dog can be located underground and be dug out if necessary. As a result, Jack Russell Terriers are most definitely vocal dogs.
Jack Russell Terriers are also very intelligent, high-energy dogs – requirements of a working dog which must problem-solve in the field and work tirelessly against often formidable quarry. Due to their compact size, friendly and inquisitive nature, and intelligence, Jack Russells are popular as pets. Prospective buyers should be aware, however, that while these dogs may enjoy sitting in a lap, they are not “lap dogs” – they are dogs that require training and regular and consistent exercise to maintain their temperament and to occupy their minds.
Jack Russells that are not trained on a consistent basis, or are not exercised regularly, may exhibit unmanageable behaviour, including excessive barking, escaping from the yard, or digging in unwanted places inside and outside the house. In America, several Jack Russell rescue networks have to work constantly to find temporary and permanent homes for Jack Russell Terriers whose owners could not meet these requirements for keeping these dogs as house pets. Prospective Jack Russell Terrier owners are advised to do their homework.
Most Jack Russell Terriers easily mingle with children, though they do not tolerate even unintentional abuse. Most are outgoing, and very friendly towards other dogs, but a good number show same-sex aggression issues. Some JRT's exhibit a Napoleon complex regarding larger canines that can get them into dangerous situations. Their fearlessness can scare off a larger animal, but their apparent unawareness of their small size can lead to a lopsided fight with larger dogs if not kept in check.
Most Jack Russell Terriers easily mingle with children, though they do not tolerate even unintentional abuse. Most are outgoing, and very friendly towards other dogs, but a good number show same-sex aggression issues. Some JRT's exhibit a Napoleon complex regarding larger canines that can get them into dangerous situations. Their fearlessness can scare off a larger animal, but their apparent unawareness of their small size can lead to a lopsided fight with larger dogs if not kept in check.
It is not uncommon for a Jack Russell terrier to be cat-aggressive, and homes with other small fur-bearing animals in them (pet hamsters, rabbits, guinea pigs, ferrets etc.) would do well to think through the ramifications of bringing a working terrier into the house.
Health
Jack Russell terriers are known for good longevity and health due to a healthy gene pool and lack of in-breeding when compared to some Kennel Club 'show dog' breeds. A well-cared-for Jack Russell can live between 14 and 21 years. Health concerns with the breed include hereditary cataracts, primary lens luxation, congenital deafness, medial patellar luxation, cerebellar ataxia, Legg-Calve-Perthes Disease, myasthenia gravis, atopy, and von Willebrand's disease. Responsible breeders will have their puppies BAER tested for hearing before sale (this test is good for the life of the dog). Prospective dams and sires should be CERF tested and OFA inspected before breeding in order to reduce the chance of passing on congenital eye or joint problems. Prospective puppy buyers are encouraged to avoid dogs sired or whelped by dogs under two years of age as congenital problems in the sire or dam may not yet have expressed themselves.