Selective pup retrieving by mother rats: sex and early development characteristics as discrimination factors.

Discrimination between own-litter pups by mother rats was studied over 14 litters, in a standardized situation eliciting maternal pup-retrieving activity. Results showed some consistency in the order in which pups of a litter were retrieved by the mother in the 4-day and 9-day tests and that this order was related to certain characteristics of the pups: 1) on Day 4 and 9, the best-developed pups of the litter (in terms of body weight and neuromotor behaviors) were first retrieved; 2) on Day 9, sex of pups became an additional discrimination factor, as males were retrieved before their female littermates. Variables included in these global discrimination factors and possible consequences of such differential mother-pups interactions are discussed.