The magnitudes of litter size and sex effects on brain growth of BALB/c mice

In a sample of 67 litters of genetically uniform BALB/c mice, litter size before weaning, which ranged from 2 to 11 animals, had a strong negative and approximately linear effect on body and brain sizes at 100 days after birth. For both males and females, the difference between litters of 11 and 2 was about 3.7 g body weight and 42 mg brain weight. The difference in brain weights was similar to effects produced by severe protein-calorie undernutrition in the postnatal suckling period. The relationship between body weight and brain weight was approximately linear and the fit was not improved significantly by including a nonlinear term or using the allometric equation. The allometric exponent was approximately .35, which is close to values commonly observed for populations of mice having large genetic variability. Although the slopes of the equations relating brain size to body size were similar for males and females, females had substantially larger brains than their male littermates. When males and females were equated statistically for body weight, the brains of females averaged about 32 mg heavier. Statistical considerations in making these estimates are discussed.