Helpers at the nest is a term used in
behavioural ecology
Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for animal behavior due to ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined four questions to address wh ...
and
evolutionary biology
Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
to describe a social structure in which
juveniles and
sexually mature
Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definiti ...
adolescents of either one or both sexes remain in association with their parents and help them raise subsequent
brood
Brood may refer to:
Nature
* Brood, a collective term for offspring
* Brooding, the incubation of bird eggs by their parents
* Bee brood, the young of a beehive
* Individual broods of North American Periodical Cicadas:
** Brood X, the largest b ...
s or
litters, instead of dispersing and beginning to reproduce themselves.
This phenomenon was first studied in
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s where it occurs most frequently, but it is also known in animals from many different groups including
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s and
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
s. It is a simple form of
co-operative breeding
Cooperative breeding is a social system characterized by alloparental care: offspring receive care not only from their parents, but also from additional group members, often called helpers. Cooperative breeding encompasses a wide variety of group s ...
. The effects of helpers usually amount to a net benefit, however, benefits are not uniformly distributed by all helpers nor across all species that exhibit this behaviour.
There are multiple proposed explanations for the behaviour, but its variability and broad taxonomic occurrences result in simultaneously plausible theories.
[Dickinson, J. L.; Hatchwell, B. J. (2004) "Fitness consequences of helping" in ''Ecology and evolution of cooperative breeding in birds'' by Walter D. Koenig, Janis L. Dickinson. Cambridge University Press. ]
The term "helper" was coined by
Alexander Skutch
Alexander Frank Skutch (May 20, 1904 – May 12, 2004) 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 o ...
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
Anthropomorphism is the attribution of human traits, emotions, or intentions to non-human entities. It is considered to be an innate tendency of human psychology.
Personification is the related attribution of human form and characteristics t ...
, but it remains in use. Other terms used especially in mammals, depending on the specific contexts, are non-maternal (care by other than the mother), alloparental (care by other than the parents), cooperative (care by non-breeding helpers) and communal (care by other breeding females) care.
Examples
It occurs in between three and eight percent of bird
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
worldwide (estimates vary), but is much more common in
Australia
Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
and
Southern Africa
Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number o ...
. Bird species in which this behaviour is found include the
common moorhen
The common moorhen (''Gallinula chloropus''), also known as the waterhen or swamp chicken, is a bird species in the rail (bird), rail family (Rallidae). It is distributed across many parts of the Old World.
The common moorhen lives around well-v ...
, the
house sparrow
The house sparrow (''Passer domesticus'') is a bird of the Old World sparrow, sparrow family Passeridae, found in most parts of the world. It is a small bird that has a typical length of and a mass of . Females and young birds are coloured pale ...
, the
acorn woodpecker, and the
apostlebird.
Damarland mole rats,
meerkats and
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
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 bees are species in the genus ''Xylocopa'' of the subfamily Xylocopinae. The genus includes some 500 bees in 31 subgenera. The common name "carpenter bee" derives from their nesting behavior; nearly all species burrow into hard plant m ...
s (note this is distinct from the behaviour of the
European honey bee, where the worker bees are sterile and incapable of reproducing).
Evolution
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
Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
: 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
A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal.
In international politics, a territory is usually either the total area from which a state may extract power resources or a ...
; 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.
Misconceptions
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.
Associated effects
Reproductive success
Effects on reproductive success can be but are not always positive, and the strength of that positive correlation varies by species. Sometimes, the energy benefits to breeders, instead of being invested in improving reproductive performance, are used to offset reductions in reproductive performance.
However, in many species, nestling survival rates are increased from helper investments.
Further reproductive success benefits include better quality offspring,
and nestling
immune response
An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which coul ...
.
Parental investment
Helpers at the nest can provide advantageous plasticity in the amount of
investment
Investment is the dedication of money to purchase of an asset to attain an increase in value over a period of time. Investment requires a sacrifice of some present asset, such as time, money, or effort.
In finance, the purpose of investing is ...
parents need to give to their eggs and chicks. The presence of helpers can be associated with lower investments as the helpers can be depended on to provide certain elements of care and raising.
For example, in the
superb fairy-wren
The superb fairywren (''Malurus cyaneus'') is a passerine bird in the Australasian wren family, Maluridae, and is common and familiar across south-eastern Australia. It is a sedentary and territorial species, also exhibiting a high degree of se ...
, nests with helpers present exhibit different average egg sizes than those without helpers.
This plasticity can be seen in reference to change in climate conditions. In unfavourable conditions there is a need for female parents to maintain high egg investment, but in favourable conditions, egg investment by mothers declines as helpers can assist with some of the care-load.
Superb fairy-wrens with helpers at the nest see average egg sizes increase in dry and hot conditions and decrease in colder conditions compared to nests without helpers.
Helper variability
Generally, helpers provide a net benefit to parents raising offspring, but this benefit is not necessarily distributed uniformly. There can be an uneven distribution of benefits between species, or even between sexes within a species.
Benefits provided by one sex may more heavily fluctuate according to various factors such as food availability. In
acorn woodpeckers, while helpers of both sexes provide relatively the same benefit to reproductive success, male helper behaviour fluctuates. This fluctuation is dependent upon the size of recent acorn crops. Large acorn crops translate to increased behavioural effects from male helpers such as higher rates of granary tending and time spent in the home territory.
Benefits and consequences for helpers
Helpers often also benefit from joining a breeding couple. After unsuccessfully breeding, juveniles may join breeding couples to become helpers. By opting to become helpers while still juveniles and having poor breeding prospects, helpers can benefit from increased survival rates.
A trade-off of this increased survival over non-helper juveniles is that their successful breeding rate after becoming helpers decreases compared to non-helpers.
See also
*
Alloparenting
*
Altruistic behaviour
*
Ethology
Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objecti ...
References
{{reflist
Behavioral ecology
Ethology
Birds