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File:Incubating white tern.jpg

Broodiness is the action or behavioral tendency to sit on a clutch of eggs to incubate them, often requiring the non-expression of many other behaviors including feeding and drinking.[1] Being broody has been defined as "Being in a state of readiness to brood eggs that is characterized by cessation of laying and by marked changes in behavior and physiology".[2] Broody birds often pluck feathers from their chest and abdomen, using them to cover the eggs. As a consequence of this, they develop one or several patches of bare skin on the ventral surface. These reddish, well-vascularized areas of skin are usually called brood patches, and improve heat transfer to the eggs. Broodiness is usually associated with female birds, although males of some bird species become broody and some non-avian animals also show broodiness.

In wild birds

In wild birds, egg incubation is a normal and essential phase in the process of reproduction, and in many families of birds, e.g. pigeons, the eggs are incubated by both male and female parents.[3]

Broodiness in males

In all the species of phalarops the males become broody rather than the female. The females leave the nest after finishing laying to let the males incubate the eggs and take care of the young.[4] Emu males (Dromaius novaehollandiae) become broody after their mates start laying, and begin to incubate the eggs before the laying period is complete.

Except a few exceptional cases like these, female broodiness is essential to the conservation of species.

Non-broodiness

A small number of atypical birds such as Passeriformes of the genus Molothrus do not become broody but lay their eggs in the nests of other species for incubation, known as brood parasitism. The Australian Brushturkey (Alectura lathami) also does not become broody, rather, it covers the eggs with a large mound of vegetable matter which decomposes keeping the eggs warm until hatching.[5] The Crab-Plover, (Dromas ardeola), which live on the coasts and islands of the Indian Ocean, let their eggs incubate primarily by the heat of the sun and will leave their nests unattended, occasionally for days at a time.[6]

In domestic poultry

File:A brooding scene 2.JPG

Broody hens can be recognized by their behaviour. They sit firmly over the eggs, and when people approach or try to remove the eggs, threaten the person by erecting their feathers, emitting a characteristic sound like clo-clo-clo and will peck aggressively. When broody, hens often temporarily cease eating or reduce their feed consumption.

Letting eggs accumulate in a relatively dark place near the floor often stimulates hens to become broody. Placing artificial eggs into nests also stimulates broodiness. Keeping hens in dark places with warm temperatures and in view of vocalising orphan chicks can induce broodiness, even in breeds that normally don't go broody.[7]

Some environmental conditions stimulate broodiness. In heavy breeds of chickens, warm weather tends to bring about broodiness[8]. Removing eggs each day, out of the sight of the hens, helps avoid broodiness not only in domestic poultry but also in some wild species in captivity. This continued egg laying means more eggs are laid than would occur under natural conditions.[9][10] Poultry farming in battery cages also helps to avoid broodiness[11][12][13].

In commercial egg-laying

File:Egg incubator.jpg

Because hens stop laying when they become broody, commercial poultry breeders perceive broodiness not as a normal physiological process, but as a drawback to egg and poultry meat production[8]. With domestication, it has become more profitable to incubate eggs artificially, while keeping hens in full egg production. To help achieve this, there has been intense artificial selection for non-broodiness in commercial egg laying chickens Gallus gallus domesticus and parent stock of poultry. As a result of this artificial selection, broodiness has been greatly reduced to very low levels in present breeds of commercial fowl, both among egg-laying and meat-producing breeds[14]

Physiological basis

Broodiness is due to the secretion of prolactine hormone by the front lobe of the hypophysis. Prolactine injection in hens provokes egg laying to stop within a few days, vitellum reabsorbtion, ovarium regression (hens only have a left ovarium[15]) and finally broodiness.[16][17] However, attempts to stop broodiness by the administration of several hormones have failed because this state, once evoked, requires time to revert.

Prolactine injections inhibit the production of gonadotropin hormone, a hormone that stimulates ovarian follicles which is produced in the frontal lobe of hypophysis.[18][19]

Castrated males can go broody with baby chicks[8], showing that broodiness is not limited to females, however, castrated males do not incubate eggs.

Contrary to common opinion, the temperature of broody hens barely differs from that of laying hens[8]. Broody hens pluck feathers from their chest, using them to cover the eggs. As a consequence of this, they develop one or several patches of bare skin on the ventral surface. These reddish, well-vascularized areas of skin are usually called brood patches.[8] which improve heat transfer to the eggs.

Genetic basis

Broodiness is more common in some chicken breeds than others, indicating it an inherited character. Breeds such as Cochin, Cornish and Silkie exhibit a tendency to broodiness, including brooding eggs from other species such as quails, pheasants, turkeys and geese. In some breeds such as the White Leghorn, broodiness is extremely rare.[1]

Some studies on crosses of chicken breeds point to the hypothesis of complementary genes acting on broodiness. Other results point to the hypothesis of sex-linked genes, or, inheritance through the maternal chromosome. Although these studies have been made on different breeds of chickens, their results are not contradictory. There is common agreement that artificial selection for egg production succeeded in reducing the incidence of broody hens in chicken populations.[14][8]

Chicken breeds that commonly exhibit broodiness

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Chicken breeds that rarely exhibit broodiness

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Broodiness in non-avian animals

Lungless salamanders in the family Plethodontidae lay a small number of eggs in a cluster among damp leaf litter. The female salamander often broods the eggs and in the genus Ensatinas, she has been observed to coil around them and press her throat area against them, effectively massaging them with a mucous secretion.[20] The black mountain salamander mother broods her eggs, guarding them from predation as the larvae feed on the yolks of their eggs. They eventually break their way out of the egg capsules and disperse.[21] Some species of Gymnophiona (caecilians, with long, cylindrical, limbless bodies) brood their eggs.[22]

File:Female Python sebae brooding eggs Tropicario, FIN.jpg

Most pythons coil around their egg-clutches and remain with them until they hatch. A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even “shiver” to generate heat to incubate the eggs.

Some cichlid fish lay their eggs in the open, on rocks, leaves, or logs. Male and female parents usually engage in differing brooding roles. Most commonly, the male patrols the pair's territory and repels intruders, while females fan water over the eggs, removing the infertile and leading the fry while foraging. However, both sexes are able to perform the full range of parenting behaviours.

Mouthbrooding

Mouthbrooding, also known as oral incubation, refers to the care given by some groups of animals to fertilized eggs or their offspring by holding them in the mouth of the parent for extended periods of time. Although it has been observed in a variety of animals, most mouthbrooders are fish. The parent performing this behavior invariably feeds less often and afterwards will be underweight, requiring a period of feeding and restoring the depleted energy reserves.[23]

Others

Marsupial frogs are so-called because they possess a dorsal brood pouch. In some species the eggs are fertilized on the female's lower back, and are inserted in her pouch with the aid of the male's toes. The eggs remain in contact with the female's vascular tissue, which provides them oxygen.

Some animals have a common name that includes the word 'brood' or its derivatives, although it is arguable whether the animals show 'broodiness' per se. For example, the female gastric-brooding frog (Rheobatrachus sp.) from Australia, now probably extinct, swallows her fertilized eggs, which then develop inside her stomach. She ceases to feed and stops secreting stomach acid and the tadpoles rely on the yolks of the eggs for nourishment. After six or seven weeks the mother opens her mouth wide and regurgitates the tadpoles which hop away from her mouth.[24] The Brooding sea anemone (Epiactis prolifera) is a colonial hermaphrodite that fertilizes and incubates its eggs internally. The motile larvae, after swimming out of the mouth, migrate down to the disk and become fixed there until they become little anemones, ready to move and feed independently.

In Darwin's Frog (Rhinoderma darwinii), the female lays about 30 eggs and then the male guards them for about two weeks, until they hatch. The male then takes all the survivors and carries around the developing young in his vocal pouch. When the tiny tadpoles have developed they hop out and swim away. In this animal, the parents hold the hatched young rather than eggs in their mouths, so is arguably not showing 'broodiness'.

See also

References

  1. 1.0 1.1 Homedes Ranquini, J. y Haro-García, F. Zoogenética. 1ra. edición, 1958, (La Habana, 1967 Ed. Revolucionaria)
  2. Merriam-Webster definition. URL accessed on 18 September 2012.
  3. Garrido, O. H. Las Palomas. Ministerio de Cultura. Editorial Científico-Técnica, 1986
  4. es:Phalaropus fuliarius
  5. Clements, J. F. 2007. The Clements Checklist of Birds of the World, 6th Edition. Cornell University Press. Downloadable from Cornell Lab of Ornithology
  6. De Marchi, G., Chiozzi, G., Fasola, M. (2008). "Solar incubation cuts down parental care in a burrow nesting tropical shorebird, the crab plover Dromas ardeola". Journal of Avian Biology 39 (5): 484–486
  7. Burrows, W.H. and Byerly, T. C. The effect of certain groups of environmental factors upon the expression of broodiness. Poultry Science 77, 324-330 (1938)
  8. 8.0 8.1 8.2 8.3 8.4 8.5 Hutt, F.B. Genética Avícola. Salvat Editores,S.A. 1ra.ed. España, 1958
  9. Pearl, R. The mode of inheritance of fecundity in the domestic fowl. J. Exptl. Zool., 12, 99-132 (1912)
  10. Coleman, W. B. Method of breeding quail at White Oak Quail Farm, Richmond, Virginia. Circular mimeográfica del mes de abril, 1930 (1930)
  11. Orozco Piñán, O. y J. A. Castelló, 1963. Alojamiento y manejo de las aves. Edicion Revolucionaria. La Habana, pp. 447 - 449.
  12. Cornoldi, J. 1964. Avicultura Moderna, Edit, Sintes, Barcelona, pp. 291 - 298.
  13. Pampín, M. y Ruíz, Cristina 1998 Caracterización de aves semirrústicas. Cloquez. Rev. Cubana de Ciencia. Avícola,1998, 22: 69 - 71. [1]
  14. 14.0 14.1 Poultry farming in the United States, Poultry farming in the United States.
  15. Ede, D. A. Anatomía de las aves. Agricultural Research Council, Poultry Research Centre, Edimburgh. Tomado de la 2da. edición, 1965 (Ed. Ciencia y Técnica, Instituto del Libro, La Habana, 1970)
  16. Bates, R. W. y cols. (1935) The gross action of prolactine and follicle-stimulating hormone on the mature ovary and sex accesories of fowl. Amer. J. Physiol. 111, 361-368 (1935)
  17. Riddle, O. y cols. (1935) Prolactine induces broodiness in fowl. Amer. J. Physiol. 111, 352-360 (1935)
  18. Bates, R. W. y cols. The mechanism of the anti-gonad action of prolactin in adult pigeons. Amer. J. Physiol. 119, 610-614 (1937)
  19. Nalbandov, A. V. A study of the effect of prolactin on broodiness and on cock testes Endocrinology 36, 251-258 (1945)
  20. Stebbins, Robert C.; Cohen, Nathan W. (1995). A Natural History of Amphibians, Princeton University Press.
  21. Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology, Saunders College Publishing.
  22. Caecilia 1911 Encyclopædia Britannica. Retrieved 2012-08-16.
  23. Loiselle, P.: The Cichlid Aquarium, Tetra Press, 1985. ISBN 3-923880-20-0
  24. Semeyn, E. (2002). Rheobatrachus silus. Animal Diversity Web. University of Michigan Museum of Zoology. URL accessed on 2012-08-05.
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