Within-litter variance in rat maternal behaviour

As a first step in determining the influence of maternal behaviour on sibling behavioural variance, we tested whether rat mothers differentially interact with neonates within the same litter. We also tested whether fading of an ink-mark on individual pups could provide an index of within-litter variance in maternal licking in laboratory rats. In Study 1, during the first postnatal week we distinguished individual Sprague–Dawley rat pups across 4 litters by placing an ink-mark on the skin and quantified variance in maternal licking frequency toward each pup and compared fading of individual pup marks to the frequency of maternal licks received and to four pup characteristics that could influence mark-fading. In Study 2, neonate mark-fading (a proxy for maternal licking) was compared to adolescent and adult offspring behaviour across 8 litters. Results indicated that: (1) there are substantial and consistent differences in how much rat mothers lick same-sex siblings within a litter, (2) differential licking rates can be documented with a non-observational method (ink-mark-fading), and (3) within-litter variance in maternal behaviour may relate to sibling behavioural variance. The findings indicate a viable research model for future experimental studies on causes and consequences of differential maternal investment within families.