Evolution and systematics

Fossils show that dog-like animals, i.e., animals with legs for running and teeth to tackle a range of food including other animals, have evolved on several occasions in the last 50 million years. The exact anatomical conformation that corresponds to the canids of today appears for the first time in 10 million-year-old fossils from North America. By seven million years ago the fossil skulls were similar enough to modern species to be put in the genus Canis. It is believed that it was at about the same time that canids colonized Eurasia and Africa. Wolf-like members of the dog family are common through the fossil record and vary in size from small jackals at 15 lb (6.8 kg) to the dire wolf (Canis dirus), which probably weighed over 200 lb (90.7 kg). The latter was very common in western North America as recently as 10,000 years ago. Its stocky build and large teeth suggest that it might have been more proficient as a scavenger than as a hunter. The modern members of the wolf-like group include the wolves (but not the maned wolf [Chrysocyon brachyurus] of South America), coyotes (Canis latrans), jackals, dholes (Cuon alpi-nus), and the African wild dog (Lycaon pictus).

An early offshoot from the Canis stock were the foxes (genus Vulpes). These smaller animals range in size from 4 to 24 lb (1.8-11 kg). There are 14 species of fox living in Eurasia, Africa, and North America, and they represent the typical canid. Many of the species have restricted ranges usually in arid areas. In almost any desert from the Namib to the Mo-jave, a small pale fox (V. pallida) can be found foraging at night for insects and small mammals. A few foxes, notably the large red fox (V. vulpes) and the gray fox (Urocyon cinereoargenteus)

have been successful in more temperate areas and consequently have large ranges. Another unusual fox lives in the Arctic and its small ears and white coat are distinctive. However, genetic evidence suggests that it diverged quite recently from the swift fox (V. velox) that lives on the dry plains of Canada and the United States. The bat-eared fox (Otocyon megalotis) is specialized to eat insects with modified teeth and a special muscle to help it open its mouth rapidly and bite up its prey. Fossils with these special teeth show that the species diverged at least 3 million years ago (mya).

More specialized still is the raccoon dog (Nyctereutes pro-cyonoides). With its stocky build and mask on the face, some experts have considered it a member of the raccoon family. The animal lives in dense, temperate forest often along watercourses in eastern Asia and Japan (and has been introduced to parts of eastern Europe as an escapee from fur farms). Genetic data show that it is clearly a dog but one that diverged early. Two continents require special consideration. The first is South America, which had almost no placental mammals until it became connected to North America 2-3 mya. It appears that either two or three kinds of canid moved south. One of these groups was successful, radiating into niches occupied by coyotes and foxes. The zorros of the Chilean and Argentinean deserts look very similar to their vulpine cousins in the rest of the world although they are independently evolved, and the culpeo (Pseudalopex culpaeus) of the pampas could pass for a coyote. There are two specialized South American canids that may represent independent lineages. The maned wolf stands taller than all but the largest gray wolves, but despite its size it is solitary and has a large

Gray wolves (Canis lupus), the largest canids, circling musk oxen. (Illustration by Wendy Baker)

proportion of vegetable material in its diet. Its legs appear to allow it to see over the tall grass. The other peculiar South American species has the opposite morphology, looking like a barrel with short legsā€”the bush dog (Speothos venaticus) lives in thick forest where it hunts in packs, often along rivers and streams. It is an accomplished swimmer.

For the last 100 million years, there has not been a land bridge between Australia and Asia. It is therefore fairly certain that the dingos (Canis familiaris dingo) were brought with humans in their canoes and have gone feral. By now the dingos are a self-sustaining species with only their curly tails hinting at their ancestry. They even show a behavior, regurgitating water, that has not been reported from other canids. The dingos are one end of a spectrum from completely feral to completely domesticated. Other forms, such as the New Guinea singing dog, live mostly independent of humans while the village dogs of much of the Third World and the urban dogs of Western cities rely more on their owners.

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