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Cooperative breeding is a social system in which members of the social group provide parental care to the offspring of other parents. The suckling by offspring from females other than their mother is referred to as allosuckling. The provision of milk to the offspring of other mothers is referred to as allonursing. Allonursing is often believed to have evolved by kin-selection, however, other causes have been hypothesized. My thesis examines the misdirected parental care, kin-selection, reciprocity, milk evacuation, improved nutrition, compensation hypotheses, and these hypotheses can co-occur and influence each other. Using reindeer (Rangifer tarandus) as a model species, behavioural observations were collected in the field in 2012 and 2013. Allosuckling and allonursing were common in reindeer, and most mothers allonursed and most calves allosuckled. Reindeer calves stole milk, and mothers discriminated their offspring from the offspring of others. Mothers exchanged allonursing at the group level and at the dyadic level, which supported the reciprocity hypothesis. Mothers did not allonurse while their offspring was still attempting to allosuckle, which did not support the milk evacuation hypothesis. Percentage of mass gain and mass at the end of the study increased as the number of allosuckling bouts increased, and allosuckling was not influenced by low birth mass or measures of insufficient maternal milk supply, which supported the improved nutrition hypothesis but not the compensation hypothesis. Mothers in the closely related group allonursed more often than mothers in the distantly related group, which supported the kin-selection hypothesis at the extremes of genetic relatedness. Given that we initially did not find an effect of genetic relatedness, we suggest that kin-selection alone is not sufficient to explain alloparental care. My thesis provides evidence that allonursing contributions detected can depend upon the research design, and we suggest that the indirect fitness benefits of alloparental care may have been overestimated. The evolution of allonursing in reindeer may have originated from inclusive fitness, but the results of my thesis demonstrate that the direct fitness benefits of milk-theft, reciprocal allonursing and improved nutrition and mass gain maintain allonursing in reindeer.