Dog Canis lupus familiaris

The dog is the domesticated form of the grey wolf and the most widely kept companion carnivore, also worked for herding, guarding, assistance and detection. Clinically the most useful generalisation about it is that it is not one animal but a range: adult size varies more than in any other domestic species, and longevity tracks that size inversely. A study of more than 40,000 purebred dogs across 118 breeds found mean breed lifespan falling by roughly 25.6 days for every additional kilogram of body mass, with body size alone explaining about 58 percent of the variance between breeds and a predicted 4.5-year spread from the smallest breeds to the largest. Clinical records from over 13 million dogs in United States primary care give the same gradient in absolute terms: life expectancy at birth of 13.53 years for small breeds against 9.51 years for giant breeds, and 12.69 years across all dogs. The same dataset shows obesity costing about a year and a half of life. Gestation is tightly fixed at roughly 63 days from ovulation although it appears to range from 57 to 72 days when counted from mating, which is why breeding work is timed on ovulation rather than on service.