Artificial selection for increased maternal defense behavior in mice.

Maternal aggression is directed towards intruders by lactating females and is critical for defense of offspring. Within-family selection for increased maternal defense in outbred house mice (Mus domesticus; Hsd:ICR strain) was applied to one selected (S) line, using total duration of attacks in a 3-min test as the selection criterion. One control (C) line was maintained and both lines were propagated by 13 families in each generation. Prior to selection, heritability of maternal aggression was estimated to be 0.61 based on mother-offspring regression. Duration of attacks responded to selection with a mean realized heritability of 0.40 (corrected for within-family selection) after eight generations. At generation 5, the S and C line also differed significantly for litter size at birth and at mid-lactation (both lower in S), average individual pup mass at midlactation (higher in S), and pup retrieval latency (longer in S), but not for other maternal measures that we studied (e.g., dam mass). Additionally, number of entries to middle and closed plus maze compartments was significantly higher in S mice in Generation 5. This is the first study to select for high maternal defense and these mice will be made available as a tool for understanding the genetic and neural basis of maternal aggression.