Effect of litter size at birth on lactation in mice.

The relative influence of litter size at birth and dam body weight at parturition on lactational performance was determined with five among-line crossfostering experiments involving 17 populations of mice. Included were lines selected for postweaning growth and maternal performance, and unselected controls. Twelve-day litter weight of a crossfostered litter standardized to six or eight pups was used as the criterion of lactational performance. Litter size at birth had a small positive correlation with 12-day litter weight when pooled within lines in each crossfostering experiment (.06 to .30, with mean of .17). The correlation between dam body weight at parturition and her crossfostered 12-day litter weight was consistently higher (.13 to .51, with mean of .35). Litter size was positively correlated with dam body weight at parturition (.33 to .56, with mean of .47). The partial correlation between litter size and 12-day litter weight, with dam body weight at parturition held constant, was reduced (-.07 to .18, with mean of .02) compared to the simple correlation. Multiple regression analyses indicated that dam body weight at parturition consistently accounted for more of the variation in 12-day litter weight than litter size. The data provide no evidence to support the hypothesis that increased fetal number or mass results in a greater lactational output by the dam.