Differences Between Seasons of the Year and Breeds in Mating Frequency, Gestation Length and Litter Size in 13 Breeds of Dogs

Objectives - to determine whether the season of the year influences when bitches of different breeds are mated, and whether there is a correlation between season, litter size, number of stillborn pups, or the duration of pregnancy. We also studied whether the duration of pregnancy varies with breed, and if the number of pups in the litter influences the duration of pregnancy. Breeds were of small, medium and large size.

Materials and methods - Data from all the litter registrations at the Swedish Kennel Club during 2004 for 13 breeds of dogs were analyzed. Data that was collected consisted of breed, registration number of the dog and bitch, date of mating, date of whelping, litter size, number of dead or euthanized pups, number and sex ratio of registered pups. Data from a total of 2469 litters from 13 breeds of dogs was analyzed. The year was for the statistical analyses divided into 4 seasons: December – February (winter), March – May (spring), June – August (summer) and September – November (autumn). The statistical analyses were performed by X2-analyses (variation between breeds for seasonal distribution of matings, and between seasons for all matings), Spearman rank correlation for estimating the association between gestation length and litter size, and analysis of variance to study the effect of breed and season on gestation length. P<0.05 was considered as significant.

Results - The frequency of matings varied significantly between seasons in 7 of the breeds, but in different ways. In German Shepherd dogs, Irish Soft Coated Wheaten Terriers, Labrador Retrievers and Norwegian Elkhounds most matings took place during winter, in Golden Retrievers during winter and spring, in Beagles during spring, and in Basenjis during autumn. Over all, most matings occurred during winter, with spring in second place, and with no difference between summer and autumn (Figure 1). Gestation length, litter size and pup death rates were not different between seasons. [...]