Adoption as a gosling strategy to obtain better parental care?Experimental evidence for gosling choice and age-dependency ofadoption in greylag geese

摘要

Adoptions of unrelated young by successful breeders are a form of alloparental care which has been observed in many species of geese. Depending on costs and benefits to the parents, adoptions might represent an inter-generational conflict or a mutually beneficial strategy. Although most studies of wild populations suggest benefits of large brood sizes, incidental observations mostly report aggressive behaviour of parents towards lone goslings. No studies have investigated mechanisms and behaviour during adoptions in order to test whether adoptions are driven by parents or goslings. To test whether goslings might use adoption as a strategy to obtain better parental care, we carried out an experiment where lone greylag goose (Anser anser) goslings could choose between a dominant and a subordinate foster family. In a second experiment we also tested whether adoption was age-dependent. Except for one case, all lone goslings (N = 16) chose the dominant family. Parents showed very little aggression towards lone goslings at three days after hatch, but aggression increased until 9 days and remained high thereafter. At the same time as aggression increased, the chance of successful adoption decreased. In the first five weeks of life, goslings which had been adopted were no further away from parents than original goslings during grazing. These results show that goslings might choose foster families according to dominance. The fact that with increasing gosling age parents are less willing to adopt could be due to improved individual recognition and reflect decreasing benefits of gaining an additional family member. More detailed studies on state-dependent costs and benefits of adoptions are required to determine whether adoptions in geese represent conflict or mutualism, and why this changes with gosling age.