Modern dog breeds show more variation

Every year, between 6 and 8 million dogs and cats enter US animal shelters.[107] The Humane Society of the United States (HSUS) estimates that approximately 3 to 4 million of those dogs and cats are euthanized yearly in the United States.[108] However, the percentage of dogs in US animal shelters that are eventually adopted and removed from the shelters by their new legal owners has increased since the mid-1990s from around 25% to a 2012 average of 40% among reporting shelters[109] (and many shelters reporting 60–75%).[110]
Biology
Lateral view of a dog's bone structure.
Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes.[3] Modern dog breeds show more variation in size, appearance, and behavior than any other domestic animal. Nevertheless, their morphology is based on that of their wild ancestors, gray wolves.[3] Dogs are predators and scavengers, and like many other predatory mammals, the dog has powerful muscles, fused wrist bones, a cardiovascular system that supports both sprinting and endurance, and teeth for catching and tearing.
Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier, that stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.7 in) in length along the head-and-body, and weighed only 113 grams (4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lb) and was 250 cm (98 in) from the snout to the tail.[111] The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder

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