Biological and environmental factors influencing reproductive performance in ICR mice, Mus musculus.

Background: Since strain names and breeding facilities of ICR mice used in 37 reproductive toxicity studies have changed from 1990 to 2022 in our laboratory, biological and environmental factors that affect reproductive parameters were investigated in control mice to examine the validity of the background data.

Methods: Litter size and sex ratio were measured at birth [postnatal day (PND) 0], while offspring body weight was measured on PND 0 and 21 during the lactation. The relationships between biological and environmental factors and reproductive parameters were assessed with multiple regression analysis using stepwise regression as an explanatory variable selection strategy. The biological factors of litter size at birth, secondary sex ratio (male%), body weight (g) at birth and strain name, and environmental factors of facilities (room), temperature/humidity, and bedding materials were used as explanatory variables, and reproductive parameters of litter size at birth, secondary sex ratio (male%), body weight (g) at birth, and survival index (%) of offspring at PND 21 were used as response variables.

Results: No significant effects were indicated in litter size and sex ratio (male %) with any biological and environmental factors. Male and female offspring weights were significantly affected by strain names. No significant effects were indicated in the survival index (%) at PND 21 in both sexes with any biological and environmental factors.

Conclusions: Litter size and sex ratio in this report are sufficient as background data throughout the period because no significant variables of biological and environmental factors affected litter size and gender composition.