Natal patterns among registered dogs in the United States.

Breed, geographic, and seasonal distributions of canine births registered with the American Kennel Club for the years 1971 to 1973, accompanied by litter size and sex ratio data, were studied. A few breeds accounted for a large portion of the births. The breed distribution of births changed as popularity of various canine types waxed and waned. The geographic distribution of births generally paralleled the pattern of human population and was relatively stable. A distinct, repetitive seasonal distribution of births was evident. Human intervention probably molded much of this pattern, but genetic factors and environmental conditions also were involved. Litter size and sex ratio varied primarily with breed rather than with region or season and hence appeared genetically based.