Cross-fostering is a technique used in

animal husbandry Animal husbandry is the branch of agriculture concerned with animals that are raised for meat, animal fiber, fibre, milk, or other products. It includes day-to-day care, management, production, nutrition, selective breeding, and the raising ...

animal science Animal science is described as "studying the biology of animals that are under the control of humankind". It can also be described as the production and management of farm animals. Historically, the degree was called animal husbandry and the ...

, genetic and

nature versus nurture Nature versus nurture is a long-standing debate in biology and society about the relative influence on human beings of their genetics, genetic inheritance (nature) and the environmental conditions of their development (nurture). The alliterative ex ...

studies, and conservation, whereby

offspring In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...

are removed from their biological

parent A parent is either the progenitor of a child or, in humans, it can refer to a caregiver or legal guardian, generally called an adoptive parent or step-parent. Parents who are progenitors are First-degree relative, first-degree relatives and have ...

s at birth and raised by surrogates, typically of a different species, hence 'cross.' This can also occasionally occur in nature.

Animal husbandry

Cross-fostering young animals is usually done to equalize litter size. Individual animals born in large litters are faced with much more competition for resources, such as breast milk, food and space, than individuals born in smaller litters. Herd managers will typically move some individuals from a large litter to a smaller litter where they will be raised by a non-biological parent. This is typically done in

pig farming Pig farming, pork farming, pig production or hog farming is the raising and breeding of domestic pigs as livestock, and is a branch of animal husbandry. Pigs are farmed principally for food (e.g. pork: bacon, ham, gammon (meat), gammon) and l ...

because litters with up to 15 piglets are common. A sow with a large litter may have difficulty producing enough milk for all piglets, or the sow may not have enough functional teats to feed all piglets simultaneously. When this occurs, smaller or weaker piglets are at risk of starving to death. Herd managers will often transfer some piglets from a large litter to another lactating sow which either has a smaller litter or has had her own biological piglets recently weaned. Herd managers will typically try to equalize litters by number and also weight of individuals. When done successfully, cross-fostering reduces piglet mortality.

In research

Cross-fostering can be used to study the impact of postnatal environment on genetic-linked diseases as well as on behavioural pattern. In behavioral studies, if cross-fostered offspring show a behavioral trait similar to their biological parents and dissimilar from their foster parents, a behavior can be shown to have a genetic basis. Similarly, if the offspring develops traits dissimilar to their biological parents and similar to their foster parents environmental factors are shown to be dominant. In many cases there is a blend of the two, which shows both

genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

and environment play a part. In animal studies, genetically hypertensive offspring reared by normotensive dams have been shown to have lower blood pressure compared to the controls. This shows that hypertensive genotype could be modified by the changes of the postnatal environment. Besides this, hyperkinetic animals reared by a normal dam have been shown to have lower locomotor activity compared to its controls. In one experiment,

siblicide Siblicide (attributed by behavioural ecologist Doug Mock to Barbara M. Braun) is the killing of an infant individual by its close relatives (full or half siblings). It may occur directly between siblings or be mediated by the parents, and is dr ...

was shown to be somehow related to parental care. When non-obligate siblicidal blue-footed boobies were swapped with obligate siblicidal masked booby chicks, it was found that the blue-footed chicks exhibited more siblicidal behavior. In a small study, cross-fostering tammar wallabies at the average age of 67 days postpartum to mothers that had 100 days old pouch young resulted in significantly heavier fostered young at 100 days old. Analysis of maternal milk composition found higher concentrations of

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

after pouch young is transferred into their host mother. In selective livestock breeding cross-fostering can be used to combine desirable genetic qualities such as weight, fat distribution or appearance with environmentally influenced ones such as temperament. In humans, studies of children in

foster care Foster care is a system in which a minor has been placed into a ward, group home ( residential child care community or treatment centre), or private home of a state- certified caregiver, referred to as a "foster parent", or with a family mem ...

have shown that

alcoholism Alcoholism is the continued drinking of alcohol despite it causing problems. Some definitions require evidence of dependence and withdrawal. Problematic use of alcohol has been mentioned in the earliest historical records. The World He ...

is both genetic and environmental: early onset alcoholism can be linked to biological parentage, whereas adult onset alcoholism is often influenced by the alcohol abuse by foster parents.

In conservation

Cross fostering has been used in

conservation biology Conservation biology is the study of the conservation of nature and of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. It is an i ...

such as the rearing of

black robin The black robin or Chatham Island robin (Moriori language, Moriori: , ; ''Petroica traversi'') is an endangered bird from the Chatham Islands off the east coast of New Zealand. It is closely related to the South Island robin (''P. australis'') ...

chicks by other species. In this instance the species was so close to

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

, with literally a handful of surviving individuals and a single mother, there was little chance of raising many offspring. In this case a related species were used to raise the eggs, with their own eggs being replaced by conservation workers with those of the robin. In this case imprinting is one of the concerns, as species raised in a different environment may not be able to recognize their own

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

. In a world first, the Adelaide Zoo successfully cross-fostered a baby tree kangaroo, whose mother was killed when it was five weeks old, with a surrogate rock-wallaby mother. The Zoo had a successful cross-fostering program between wallaby species, but this is the first time it was tried with a tree kangaroo and wallaby.

In nature

Cross fostering may occasionally occur in natural situations. In

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, the closely related species from the

cockatoo A cockatoo is any of the 21 species of parrots belonging to the family Cacatuidae, the only family in the superfamily Cacatuoidea. Along with the Psittacoidea ( true parrots) and the Strigopoidea (large New Zealand parrots), they make up t ...

family, '' Eolophus roseicapilla'' (the

galah The galah (; ''Eolophus roseicapilla''), less commonly known as the pink and grey cockatoo or rose-breasted cockatoo, is an Australian species of cockatoo and the only member of the genus ''Eolophus''. The galah is adapted to a wide variety of m ...

) and ''Cacatua leadbeateri'' (the pink cockatoo), have overlapping ranges, and

compete Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, individ ...

for

nest A nest is a structure built for certain animals to hold Egg (biology), eggs or young. Although nests are most closely associated with birds, members of all classes of vertebrates and some invertebrates construct nests. They may be composed of ...

ing holes. However, two pairs of birds may share the same nest for a time, as they do not become aggressive until several eggs have been laid and incubation begins. When they do, the pink cockatoos are always the victors, evicting the galahs in what is termed interference competition. They are not consciously aware that some of the eggs in the nest were laid by the other bird however, and thus raise offspring of both species. These natural

experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...

s have been used by Australian

ornithologist Ornithology, from Ancient Greek ὄρνις (''órnis''), meaning "bird", and -logy from λόγος (''lógos''), meaning "study", is a branch of zoology dedicated to the study of birds. Several aspects of ornithology differ from related discip ...

s Graeme Chapman and Ian Rowley to investigate the relative importance of genes and environment. For example, they discovered that the galah chicks gave normal begging calls and

alarm calls In animal communication, an alarm signal is an antipredator adaptation in the form of signals emitted by social animals in response to danger. Many primates and birds have elaborate alarm calls for warning conspecifics of approaching predators ...

, but their

contact calls Contact calls are seemingly haphazard sounds made by many social animals (such as a chicken's cluck). Contact calls are unlike other calls (such as alarm calls) in that they are not usually widely used, conspicuous calls, but rather short exclamatio ...

(used to maintain

social cohesion Group cohesiveness, also called group cohesion, social harmony or social cohesion, is the degree or strength of bonds linking members of a social group to one another and to the group as a whole. Although cohesion is a multi-faceted process, it ...

) were more like those of the pink cockatoos with which they lived. Such natural instances of cross fostering can also lead to hybridization between species that would not normally breed. A case of this is offered by the Galapagos finches. Two species of the genus ''

Geospiza ''Geospiza'' is a genus of bird in the tanager family Thraupidae. All species in the genus are endemic to the Galápagos Islands. Together with related genera, they are collectively known as Darwin's finches. Although in the past, they were class ...

'', the medium ground-finch (''Geospiza fortis'') and the common cactus-finch (''Geospiza scandens'') occasionally hybridize. The birds'

songs A song is a musical composition performed by the human voice. The voice often carries the melody (a series of distinct and fixed pitches) using patterns of sound and silence. Songs have a structure, such as the common ABA form, and are usuall ...

are a barrier to interbreeding, but sometimes young birds will not learn their own species song, e.g. if their father dies and they are nesting near another species. Another situation where birds can imprint on the wrong song is when one species takes over the nest of another but fails to remove all of its eggs. Cross fostered young can then hybridize with their foster parents' species, allowing

gene flow In population genetics, gene flow (also known as migration and allele flow) is the transfer of genetic variation, genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent ...

between the two populations. Hybrids experience reduced fitness, however, so the two species can remain separate.

References

Primary sources