Helpers at the nest
Helpers at the nest is a term used in behavioural ecology and evolutionary biology to describe a social structure in which juveniles and sexually mature adolescents of either one or both sexes, remain in association with their parents and help them raise subsequent broods or litters, instead of dispersing and beginning to reproduce themselves. This phenomenon was first studied in birds where it occurs most frequently, but it is also known in animals from many different groups including mammals and insects. It is a simple form of co-operative breeding.
It occurs in between three and eight percent of bird species worldwide (estimates vary), but is much more common in Australia and Southern Africa. Bird species in which this behaviour is found include the common moorhen, the acorn woodpecker, and the apostlebird. The sugar glider and humans are examples of mammals that exhibit this behaviour. It is also seen in a number of species of bee such as carpenter bees (note this is distinct from the behaviour of the European honey bee, where the worker bees are sterile and incapable of reproducing).
The term "helper" was coined by Alexander Skutch in 1935 and defined more carefully in 1961 in the avian context as "a bird which assists in the nesting of an individual other than its mate, or feeds or otherwise attends a bird of whatever age which is neither its mate nor its dependent offspring." The term has been criticised as being anthropomorphic, but continues to remain in use.
Three explanations for the occurrence of helpers at the nest have been put forward; they are not mutually exclusive, and in any particular species an investigation of the exact benefits and costs will be needed to see what combination of these factors may have driven the evolution of helping.
- Advantage to the helpers, who may be protected from predation, or gain skills that they will need when they subsequently reproduce, as a result of staying in the parental nest.
- Kin selection: since subsequent litters or broods from the same parents will be full siblings to the helpers, they are as closely related genetically as their own offspring would be. Helping their parents is therefore as productive for the juveniles as reproducing themselves would be, and if their parents are better able to reproduce, the balance of advantage may be greater.
- Delayed advantage to the helpers, in particular because they stand to inherit their parents' territory; this explanation is particularly compelling if suitable territories are in short supply, but requires specific quantitative conditions to be met, favouring a stable queue of potential heirs.
Although it is frequently assumed that helpers are non-breeders, molecular evidence suggests that this may happen, and the term "secondary helper" is sometimes used in this case to indicate helpers that mate with or are not related offspring of the pair being assisted. The term "primary helper" being used for the commoner case of the helper being offspring of the pair and not involved in mating. Extrapair mates are chosen by the females and are these then contribute to the care of the young who may be sired by them.
Juveniles living in association with their parents cannot automatically be regarded as helpers. In a number of species, such as the logrunners and the Siberian jay, young remain in the parental territory, but never help feed nestlings. However the delayed advantage explanation for the juveniles' association with their parents can still work in the absence of effective helping, whereas the kin selection explanation cannot.
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- Rubenstein, DR (2007). "Female extrapair mate choice in a cooperative breeder: trading sex for help and increasing offspring heterozygosity". Proceedings of the Royal Society B 274 (1620): 1895–1903. doi:10.1098/rspb.2007.0424. PMC 2270931. PMID 17526455.
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