Dogs That Changed the World Photo Essay: From Wolf to Dog
From Wolf to Dog
Scientists continue to debate when, where, and why domesticated dogs evolved from the ancestors of modern wolves, but they are clear on one issue: the physical alterations that mark the transformation. Turning a wolf into a dog involved a number of changes, both morphological and behavioral, many of which are linked to a phenomenon common in domesticated animals known as paedomorphosis -- or the retention of juvenile traits in the adult form. In other words, full-grown adult dogs bear many of the characteristics of young wolf pups. Although different developmental paths can lead to paedomorphosis, in dogs it occurred because of a process called neoteny, in which the physical development slows down so much that sexually mature adults still have puppy-like bodies and behaviors.
With the exception of a few modern breeds, dogs generally have a smaller overall body size than wolves.
The structure of the skull undergoes several changes in the transition to dogs. Compared to their ancestor wolves (and like young wolf pups) dogs have a wider cranium, a shorter snout, and a pronounced vertical drop, or "stop" in the front of the forehead. The bony case of their eardrum is also reduced.
Like other animals that have been domesticated, dogs have a reduced cranial capacity -- a smaller brain size -- compared to their wild counterparts. Because of their relatively smaller brains, dogs are generally less perceptive of their environment (in terms of their senses of hearing, vision, and smell, for example), and have a lower state of alertness. (They are not living in the wild and simply don't need to be so vigilant.) However, scientific studies have shown that dogs are much better than wolves -- and indeed, the best of any animals -- at interpreting human behavior.
The shortening of the snout is accomplished by a reduction in the length of the jaw, which led to first a crowding of the teeth (especially the molars and premolars) and eventually to an overall decrease in the size of the teeth. For example, the teeth of a very large dog like a Great Dane are smaller and less complex than those of wolves.
The domestication of dogs led to changes in coat color and markings. For example, dogs can exhibit piebald patterns, or large black and white patches, which are never seen in wolves. This change was observed in an experiment begun in the 1950s on wild silver foxes by Russian scientist Dimitri Belyaev. Over 40 years, Belyaev and his colleagues selectively bred foxes based on their friendliness toward humans. Among the earliest characteristics noted among the increasingly tame, floppier-eared foxes were changes in coat color: they had black and white patches. Researchers have suggested that the patchy patterns are related to developmental delays in the migration of melanocytes. The cells produce the pigment melanin, which is chemically linked to adrenaline (also known as epinephrine), the "fight or flight" hormone whose production is affected by changes in the animals' fear responses.
Dogs reach sexual maturity at a much younger age than their most common ancestor, the gray wolf (6 to 12 months vs. 2 years); they have 2 to 3 breeding periods per year, while wolves have only one. Male dogs are able to produce sperm -- and therefore are fertile -- year round, but male wolves generally can't produce sperm outside of the breeding season.