Within-litter variation in maternal care is a key contributor to individual differences in offspring behavior and monoamine neurochemistry in female Long–Evans rats

The care that a mother rat provides is essential for the ability of her pups to survive and thrive. Maternal care naturally varies between litters, including among animals with close genetic relatedness. There are also significant differences in behavior even among offspring reared together. Our lab and others have documented stable, naturally occurring individual differences in maternal care received by individual pups within the litter that persist throughout at least the first ten days of postnatal life. In this study, we hypothesized that within-litter variation in maternal care received constitutes a significant source of variation in offspring behavior and neurochemistry in Long-Evans rats. We analyzed measures related to maternal care behavior, offspring anxiety-like and social behaviors, and neurotransmitter levels in specific brain regions after the offspring became mothers themselves. For statistical modeling, we used the coefficient of variation (CV) to standardize and directly compare between- and within-litter variation across a range of behavioral and neurophysiological outcomes. Several variables analyzed showed greater within-litter CVs than between-litter CVs, especially for offspring behavior and levels of the monoamines dopamine, serotonin, and their primary metabolites DOPAC (3,4-dihydroxyphenylacetic acid) and 5-HIAA (5-hydroxyindoleacetic acid) in the nucleus accumbens, ventral tegmental area, medial preoptic area, hippocampus, and prefrontal cortex. Our findings suggest that within-litter variation in maternal care plays a prominent role in behavioral and physiological outcomes. This study provides a methodological advance by demonstrating that within-litter variability often exceeds between-litter variability across maternal, behavioral, and neurochemical domains, challenging a key assumption in experimental designs using littermate controls.