Background

Domestic dogs (Canis familiaris) are usually non-seasonal breeders with births occurring throughout the year and two oestrous cycles per year. However, two breeds, Basenjis and Tibetan Mastiffs, are seasonal breeders, with approximately 90% of births occurring in May – August and one oestrous cycle per year. Research on the impacts of reproductive seasonality on life history parameters is lacking, despite their potential impacts on selection and evolution. We hypothesised that the parents’ ages at the births of their litters and the interbirth intervals would be greater in the seasonal than the non-seasonal breeds.

Methods

We purchased Australian registration records for the two seasonal breeds and five non-seasonal breeds: the Cocker Spaniel, Staffordshire Bull Terrier, Boxer, Golden Retriever and Rottweiler. These records include the date of birth, dam and sire for each dog. We calculated the ages of the parents at the birth of each litter and interbirth intervals for each dam. Litters born from known or likely artificial insemination were excluded from the sire analysis.

Results

The parents’ ages at the births of their litters cluster around 12-month intervals for the seasonal breeds but show one peak for the non-seasonal breeds. The median age of the dams at the births of litters is higher in the seasonal than the non-seasonal breeds, but the sires show the opposite trend. When looking at the ages of the dams and sires together, there is a small but statistically significant difference (p = 0.015) with the seasonal breeds being older at the births of their litters. The seasonal and non-seasonal breeds show different peaks in their interbirth intervals.

Conclusion

Reproductive seasonality affects the shapes of the distributions of age at reproduction and interbirth intervals and results in a small but significant increase in parent age at reproduction, although effects differ between the sexes.