A case of co-operative nursing and offspring care by mother and daughter feral horses

Among mammals, non-offspring nursing is the most extreme form of communal parenting. This is because lactation is the most energetically costly part of parental investment (Clutton-Brock, 1991; Packer, Lewis & Pusey, 1992). Non-offspring nursing is most common in species characterized by large litters and small kin groups (Packer et al., 1992; e.g. lions Panthera leo: Pusey & Packer, 1994). Although non-offspring nursing has also been reported in monotocous species (e.g. water buffalo Bubalus bubalus, Murphey et al., 1995; African elephant Loxodonta africana: Dublin, 1983; Lee, 1987; Indian elephant Elaphus maximus: MacKay, 1973; Rapaport & Haight, 1987; fallow deer Cervus dama: San José & Braza, 1993) it is almost always associated with reproductive errors (Riedman, 1982) such as milk theft or exclusive adoption (Packer et al., 1992). However, simultaneous non-offspring nursing in monotocous species has been reported in some bat species (e.g. McCracken, 1984; Eales, Bullock & Slater, 1988), African elephants (Lee, 1987), and captive Indian elephants (Rapaport & Haight, 1987). Recent research, however, suggests that nutritive non-offspring nursing in African elephants is rarer than previously thought as most reported instances were probably non-lactating juveniles allowing infants to suckle (Lee & Moss, 1986; Lee, 1987, 1989).