Helpers at the nest
Encyclopedia
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 brood
s or litter
s, instead of dispersing and beginning to reproduce themselves. This phenomenon was first studied in bird
s where it occurs most frequently, but it is also known in animals from many different groups including mammal
s and insect
s. It is a simple form of co-operative breeding.
It occurs in less than five percent of bird species
worldwide, but is more common in Australia
. Bird species in which this behaviour is found include the Common Moorhen
, the Acorn Woodpecker
, and the Apostlebird
. The Sugar glider
and human
s are examples of mammals that exhibit this behaviour. It is also seen in a number of species of bee such as Carpenter bee
s (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.
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; it is necessary to demonstrate that the reproductive success of their parents is increased by their presence (in the Percula Clownfish
, for example, it is not). 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.
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
and sexually mature adolescents of either one or both sexes, remain in association with their parents and help them raise subsequent brood
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
s or litter
Litter
Litter consists of waste products such as containers, papers, wrappers or faeces which have been disposed of without consent. Litter can also be used as a verb...
s, instead of dispersing and beginning to reproduce themselves. This phenomenon was first studied in bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s where it occurs most frequently, but it is also known in animals from many different groups including mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s and insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s. It is a simple form of co-operative breeding.
It occurs in less than five percent of bird species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
worldwide, but is more common in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Bird species in which this behaviour is found include the Common Moorhen
Common Moorhen
The Common Moorhen is a bird in the Rallidae family with an almost worldwide distribution. The North and South American Committees of the AOU and the IOC have voted on or before July 2011 to split the American forms into a new species Common Gallinule, however, no other committee has voted to...
, the Acorn Woodpecker
Acorn Woodpecker
The Acorn woodpecker is a medium-sized woodpecker, 21 cm long with an average weight of 85 g.-Description:...
, and the Apostlebird
Apostlebird
The Apostlebird , also known as the Grey Jumper, is a quick-moving, gray or black bird about 13 inches long. It is a native to Australia where it roams woodlands, eating insects and seeds at, or near, ground level...
. The Sugar glider
Sugar Glider
The sugar glider is a small gliding possum originating from the marsupial family.The sugar glider is native to eastern and northern mainland Australia and is also native to New Guinea and the Bismarck Archipelago.- Habitat :Sugar gliders can be found all throughout the northern and eastern parts of...
and human
Human
Humans are the only living species in the Homo genus...
s are examples of mammals that exhibit this behaviour. It is also seen in a number of species of bee such as Carpenter bee
Carpenter bee
Carpenter bees are large, hairy bees distributed worldwide. There are some 500 species of carpenter bee in 31 subgenera...
s (note this is distinct from the behaviour of the European honey bee
European honey bee
The Western honey bee or European honey bee is a species of honey bee. The genus Apis is Latin for "bee", and mellifera comes from Latin melli- "honey" and ferre "to bear"—hence the scientific name means "honey-bearing bee"...
, where the worker bees are sterile and incapable of reproducing).
The term "helper" was coined by Alexander Skutch
Alexander Skutch
Doctor Alexander Frank Skutch was a naturalist and writer. He published numerous scientific papers and books about birds and several books on philosophy. He is best remembered ornithologically for his pioneering work on helpers at the nest.-Biography:Alexander Skutch was born in Baltimore, Maryland...
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 selectionKin selectionKin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/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' territoryTerritory (animal)In ethology the term territory refers to any sociographical area that an animal of a particular species consistently defends against conspecifics...
; 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; it is necessary to demonstrate that the reproductive success of their parents is increased by their presence (in the Percula Clownfish
Percula Clownfish
The Orange Clownfish is widely known as a popular aquarium fish. Like other clownfish , it often lives in association with sea anemones. A...
, for example, it is not). 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.