Estimation of Heritability of Litter Size in Labrador Retrievers and German Shepherd Dogs

The purpose of this study was to estimate the contributions of genetic and environmental factors to litter size in Labrador retrievers and German shepherd dogs in the breeding colony at The Seeing Eye, Inc. The environmental factors with significant impact on litter size were the parity of the dam and contemporary group, a three-month period that represents seasonal effects, the passage of time in years, and the interaction between season and year. A multiple-trait maximum likelihood method was used to estimate the heritability, or proportion of variation in litter size caused by genetic effects. In Labrador retrievers, heritability estimates were 0.24 (±0.067), 0.28 (±0.061), 0.28 (±0.069), and 0.31 (±0.069) for number of pups born, born alive, alive at 14 d, and alive at 49 d, respectively. Heritabilities for the same traits in German shepherd dogs were 0.19 (±0.052), 0.21 (±0.054), 0.25 (±0.062), and 0.26 (±0.063). These moderate heritability estimates suggest a larger contribution of additive genetic factors to litter size than expected based on studies in other species.