3 Domesticated dogs
3.1 The origins of domesticated dogs
Archaeologists and biologists agree that dogs (Canis familiaris) were the first species to become domesticated. Francis Galton, Darwin's younger cousin, suggested at the end of the 19th century that domestication began when humans captured and raised wolf puppies. The resulting adults ate scraps of human food, assisted in hunting and acted as guard dogs around camps. Among the evidence in support of this hypothesis is the observation that tribal people all over the world take wild animals as pets. As well as wolf puppies, animals as diverse as goldfish, bear cubs and parrots are fed and tended as companions or objects of curiosity, not as food. Some ‘pets’ acquired a sacred dimension, becoming personal or tribal totems that are often represented as models or images. More recent research suggests that pet-keeping by itself cannot explain domestication of dogs (see below).
At the end of 2005, the first sequencing of the dog genome was published, based on material from a female boxer, chosen because she appeared to have very few heterozygous loci. The second dog genome to be sequenced was that of a standard poodle. Single nucleotide polymorphisms (SNPs pronounced ‘snips’) identified in the boxer were compared with those of 11 other breeds of dogs and samples taken from wolves, jackals and coyotes collected in various parts of the world. The best match was with wolves (Figures 5b and 11a), particularly those native to northern China, suggesting that the Far East was a major site of dog domestication and grey wolves (Canis lupus) were the principal ancestors. However, there was also clear evidence of genes derived from jackals (Figure 11b), coyotes (Canis latrans) and Ethiopian wolves (Canis simensis) in domestic dogs. Recent sequencing of exons and introns of small fractions of the genomes of 31 (of 34) living species of Canidae (Lindblad-Toh et al., 2005) suggests that golden jackals diverged from coyotes, wolves and dogs about 3–4 My ago. Nonetheless, all these species of the genus Canis can interbreed under artificial conditions. Evidently, these interspecific hybrids were fertile because their descendants are our working dogs and pets. Modern genetic studies reveal a small amount of natural interbreeding between several wild canid species.
The grey wolf (Figures 5b and 11a) used to be among the most widespread of all mammals; at the end of the Pleistocene (about 10 000 years ago), the species occurred throughout the Northern Hemisphere except in arid deserts, and was common in many places. Small isolated populations survive today only in areas of Scandinavia, Russia, Mongolia, northwestern China, Canada and Alaska that are too barren to support domesticated livestock or agriculture. There are some regional differences in adult size and coat colour, but the patterns were always camouflaged and graded (as in Figures 5b and 11a), never piebald.
Wolves are the most social of all living canids. They live and hunt in packs led by one or a few dominant animals. Usually only the senior pairs breed, but subordinate, non-reproducing pack members catch and bring food to the pups. Pack hierarchy is maintained and the wolves’ activities are coordinated by varied and elaborate communications involving gestures and postures of the ears, eyes, muzzle, fur, tail and body (Figure 11a), and a wide range of sounds and scents.
Domesticated dogs use most of these behaviours to communicate with each other. Observe some local dogs and list some calls, postures, gestures and facial expressions. Do dogs recognise and respond to human sounds and gestures?
Dogs communicate with each other and with people by tail-wagging, licking, erecting hair on the back, ears laid back, growling, snarling, whining, submissive crouching and many more such behaviours. Dogs respond to human commands and signals and to many unintentional communications, e.g. they recognise people's fear.
Hunting in packs enables wolves to kill animals much larger than themselves. They mainly prey on bovids, cervids and wild horses, but when food is scarce, they take smaller species and sometimes scavenge.
Jackals and their New World equivalent, coyotes, are smaller than wolves and are scavengers as well as predators of rodents, rabbits, ground-nesting birds and other small species. Several jackal species are monogamous and forage in pairs rather than in packs, and even where larger family groups do form, their social behaviour is not as elaborate or as well coordinated as that of wolves. Coyotes and several species of jackals are still widespread and, in places, fairly common.
Modern studies of ancient dog bones reveal numerous cut marks, indicating that humans' first use for dogs was as food. In parts of the Far East and Polynesia, dogs are still bred and raised for the table, alongside domesticated artiodactyls (Figures 1, 2, 7 and 9) and poultry (Figure 10). However, in most of the rest of the world (including modern China), dogs are used for managing other livestock, for various roles in hunting including following scent trails, pointing and retrieving, as guards and as companions. In these roles, temperament, intelligence, learning ability and other aspects of behaviour are as important as physique.
The central role of behaviour in domesticated dogs was recognised by scientists more than two centuries ago. The 18th-century surgeon and anatomist John Hunter, like many of his contemporaries, was very interested in the biology of domestication, which he believed illuminated the mechanisms of natural evolution. He and his collaborators made several attempts to hybridise dogs and wolves and for a while in the 1770s, he owned a wolf-dog hybrid. In 1786, he obtained a puppy born from the mating of a male spaniel with a female jackal (probably Canis aureus, see Figure 11b) originating from India. He sent one of this animal's offspring to his former pupil and lifelong friend Edward Jenner who kept it at his home in Berkeley, Gloucestershire. This astute observer of animal and human behaviour reported in a letter to Hunter: ‘The little jackal bitch you gave me is grown a fine, handsome animal but she certainly does not possess the understanding of common dogs. She is easily lost when I take her out and is quite inattentive to a whistle.’ In other words, this hybrid was anatomically and physiologically sound, but lacked the mental ability to interact with humans as well as domesticated dogs can.
(Dr Edward Jenner FRS (1749–1823) is chiefly remembered as a pioneer of vaccination against smallpox, the basis for the only completely successful extermination of a human pathogen. During his lifetime, he was known for fundamental research on the anatomy and habits of cuckoos and the relationship between angina, heart attacks and fatty deposits in the arteries.)
Many breeds of dogs originated from a very few ancestors. The small ancestral gene pool combined with much inbreeding favour genes becoming common through genetic drift. Such effects have undoubtedly occurred, and many purebred dogs are unusually prone to certain diseases including hereditary deafness and some forms of cancer.
Would the action of natural selection eliminate (a) deafness and (b) cancer?
(a) Yes, because deaf animals would be more vulnerable to predators and other hazards, including traffic.
(b) Only if the cancer directly affected fertility or became disabling early enough in life to reduce lifetime fecundity and nurturing the offspring. Natural selection cannot eliminate diseases of old age.
With the recent rise in the keeping of specialised breeds of dogs (e.g. terriers, dachshunds, retrievers, bulldogs) as pets rather than as working dogs, hybrids between breeds have become very common. Some have become feral: although descended from domesticated ancestors, they live and breed as wild animals. All modern breeds of dogs, with their variation in size and shape, habits, coat colour and texture, are mutually fertile and share certain species-specific physiological characteristics; for example, the length of the gestation period is the same for all breeds regardless of body size. In spite of the huge range of phenotypes, dogs have not formed discrete species.
List some reasons why the intensive study of many different aspects of dog biology provides insight into the process of evolution under domestication.
Relevant topics include:
Dogs are among the most ancient domesticated animals.
The dog genome has now been sequenced.
Dogs are highly social mammals that communicate using sounds, facial expressions and postures; these behaviours are readily interpreted by humans and dogs respond to human-generated sounds and to human gestures.
Dog behaviour has been thoroughly studied.
Many breeds that differ in habits and behaviour also differ in structure and appearance.
Many dog breeds are highly inbred but mongrels are also common and viable.