Communal nesting and discriminative nursing by captive degus, Octodon degus

When two or more females rear their young in a common nest or burrow (communal nesting), mothers may be challenged to direct care to their own offspring. In a laboratory study on degus, Octodon degus, a communally nesting South American caviomorph rodent, we used a radionuclide (phorphorus-32) to track milk transfer from mothers to their young in nests occupied by two mothers and their litters. Co-nesting pairs consisted of either two unrelated mothers or two sisters. We analysed faecal samples from 2-week-old and 4-week-old young to learn whether mothers would nurse discriminately, favouring their own offspring over their co-nesting partner's offspring. Mothers housed in unrelated pairs nursed their own 2-week-old offspring preferentially (although not exclusively) compared with their co-nesting partner's offspring, whereas mothers housed with a sister nursed indiscriminately, delivering roughly equal amounts of milk to their own offspring and their nieces and nephews. Faecal analyses from 4-week-old young revealed that mothers may nurse co-nesting young indiscriminately, transferring similar amounts of milk to both types of young, regardless of the relatedness of their co-nesting partner. We propose that discriminative nursing as a function of relatedness between co-nesting female degus may be an adaptation to communal nesting when mothers share a burrow that contains many young of different degrees of relatedness.