Abstract
Adoption is rare in animals and is usually attributed to kin selection. In a 6-year study of eastern grey kangaroos (Macropus giganteus), 11 of 326 juveniles were adopted. We detected eight adoptions by observing behavioural associations and nursing between marked mothers and young and three more by analysing the relatedness of mothers and young using microsatellite DNA. Four adoptions involved reciprocal switches and three were by mothers whose own pouch young were known to subsequently disappear. Adoptive mothers were not closely related to each other or to adoptees but adoptive mothers and young associated as closely as did biological pairs, as measured by half-weight indices. Switch mothers did not associate closely. Maternal age and body condition did not influence the likelihood of adoption but females were more likely to adopt in years with high densities of females with large pouch young. Adoption did not improve juvenile survival. We conclude that adoptions in this wild population were potentially costly and likely caused by misdirected care, suggesting that eastern grey kangaroos may have poorly developed mother-offspring recognition mechanisms.
Highlights
Adoption, the exclusive care of conspecific non-offspring, is rare in animals [1]
Most adoptive mothers were prime-aged, multiparous and in above-average body condition when captured with a large pouch young, the time when adoptions occurred (Table 2), but the only significant parameter retained in the logistic regression model of the likelihood of adoption was density of females with large pouch young (Table 3; see Fig 2)
Survival to 21 months increased with maternal body condition and density of females with large pouch
Summary
The exclusive care of conspecific non-offspring, is rare in animals [1]. Instances of alloparenting, including nursing, food provisioning, guarding, carrying and grooming, have been reported in at least 120 mammalian species across most major orders, including marsupials (in captivity) [1]. Alloparenting has been hypothesized to result from (1) kin selection [2], (2) reciprocal altruism [3], (3) improved parental experience and/or (4) misdirected care [1,4], but allonursing in particular appears to evolve where costs are low [5]. Kin selection involves related females sharing parental duties, including the nursing of each other’s young, as often occurs in communally breeding rodents [6] and carnivores [7] but it has been invoked for adoption in an asocial sciurid [8]. Reciprocal altruism involves unrelated females sharing parental duties: young could gain immunological benefits such as antibodies from allosuckling, potentially increasing their survival [9], but there are no published cases of reciprocal.
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