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{{Short description|Behavior in animals of taking care of offspring}}File:Hirundo rustica (Linnaeus, 1758).jpg|thumb|upright=1.3|Swallow adult feeding begging young in the nestnestParental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal kingdom.Kokko, H. & Jennions, M.D. (2008) Parental investment, sexual selection and sex ratios. Journal of Evolutionary Biology, 21, pp.919–948. {{doi|10.1111/j.1420-9101.2008.01540.x}} There is great variation in different animal groups in terms of how parents care for offspring, and the amount of resources invested by parents. For example, there may be considerable variation in the amount of care invested by each sex, where females may invest more in some species, males invest more in others, or investment may be shared equally. Numerous hypotheses have been proposed to describe this variation and patterns in parental care that exist between the sexes, as well as among species.Gonzalez-Voyer, A. and Kolm, N. (2010). Parental Care and Investment. Encyclopedia of Life Sciences. {{doi|10.1002/9780470015902.a0021907}}Parental care is any behaviour that contributes to offspring survival, such as building a nest, provisioning offspring with food, or defending offspring from predators. Reptiles may produce self-sufficient young needing no parental care, while some hatchling birds may be helpless at birth, relying on their parents for survival. Parental care is beneficial if it increases the parent's inclusive fitness, such as by improving offspring survival, quality, or reproductive success. Since parental care is costly and often affects the parent's own future survival and reproductive success, parents ensure that any investment is well-spent. Parental care thus only evolves where it is adaptive.Types of parental care include maternal or paternal care, biparental care and alloparental care. Sexual conflict is known to occur over mating, and further familial conflicts may continue after mating when there is parental care of the eggs or young. For example, conflict may arise between male and female parents over how much care each should provide, conflict may arise between siblings over how much care each should demand, and conflicts may arise between parents and offspring over the supply and demand of care.Parker, G.A., Royle, N.J. & Hartley, I.R. (2002) Intrafamilial conflict and parental investment: a synthesis. Philosophical Transactions of the Royal Society of London. Series B, 357, pp.295–307. {{doi|10.1098/rstb.2001.0950}}Although parental care increases the evolutionary fitness of the offspring receiving the care, it produces a cost for the parent organism as energy is expended on caring for the offspring, and mating opportunities may be lost.Bednekoff, P. A. (2010). “Life Histories and Predation Risk”. Encyclopedia of Animal Behavior. Academic Press. pp. 285-286. {{ISBN|978-0-123-72581-3}}.Fox, R. J.; Head, M. L.; Barber, I. (31 July 2018). “Good Parenting May Not Increase Reproductive Success Under Environmental Extremes”. Journal of Evolutionary Biology. {{doi|10.1111/jeb.13358}} As this is costly, it only evolves from a when the costs are outweighed by the benefits.Klug, H.; Bonsall, M. B. (12 May 2014). “What are the benefits of parental care? The importance of parental effects on developmental rate”. Ecology & Evolution. 4 (12): 2330–2351. {{doi|10.1002/ece3.1083}}Parental care is seen in many insects, notably the social insects such as ants, bees and wasps; in certain fishes, such as the mouthbrooders; widely in birds; in amphibians; rarely in reptiles and especially widely in mammals, which share two major adaptations for care of the young, namely gestation (development of the embryo inside the mother's body) and production of milk.

Types of parental care

Paternal care

Care of offspring by males may evolve when natural selection favouring parental care is stronger than sexual selection against paternal care.Alonzo, S. (2011). Sexual selection favours male parental care, when females can choose. Proceedings of the Royal Society B: Biological Sciences, 279(1734), pp.1784-1790. {{doi|10.1098/rspb.2011.2237}} In approximately 1% of bird species, males exclusively provide care after eggs are laid. Male-only care is prevalent in a variety of organisms, including fish and amphibians.{{citation needed|date=March 2022}} The occurrence of paternal care is mostly associated with biparental care in socially monogamous mating systems.{{citation needed|date=March 2022}} The rise of paternal care in primates may be explained by the Mating Effort and Maternal Relief hypotheses. The Mating Effort hypothesis suggests that males may provide care for offspring in an attempt to increase their own mating opportunities and thus enhance their future reproductive success.Kerhoas, D., Kulik, L., Perwitasari-Farajallah, D., Agil, M., Engelhardt, A. & Widdig, A. (2016). Mother-male bond, but not paternity, influences male-infant affiliation in wild crested macaques. Behavioral Ecology and Sociobiology, 70(8), pp.1117-1130. {{doi|10.1007/s00265-016-2116-0}}Minge, C., Berghänel, A., Schülke, O. & Ostner, J. (2016). Patterns and Consequences of Male–Infant Relationships in Wild Assamese Macaques (Macaca assamensis). International Journal of Primatology, 37(3), pp.350-370. The Maternal Relief hypothesis proposes that males provide care to reduce the burdens associated with reproduction for the female, which ultimately generates shorter inter-birth intervals and produces more successful offspring.The type of mating system may influence paternity certainty, and therefore the likelihood that a male is caring for his own true offspring. Paternal certainty is relatively high in monogamous pair-bonded species. Males are less likely to be caring for unrelated offspring, therefore a greater prevalence of paternal care tends to exist in association with this mating system. By contrast, paternity certainty is reduced in polygamous species. Males are at greater risk of providing care for unrelated offspring, which therefore compromises their own fitness.Lappan, S. (2008). Male care of infants in a siamang (Symphalangus syndactylus) population including socially monogamous and polyandrous groups. Behavioral Ecology and Sociobiology, 62(8), pp.1307-1317. In polygynous species, where a single male mates with more than one female, the male's role as a caregiver therefore tends to be reduced. Conversely, males may be exclusively responsible for caring for their offspring in polyandrous species, where a single female mates with more than one male.The evolution of male parental care is particularly rare in non-monogamous species because predominantly, investing effort into mating is more evolutionarily effective for males than providing parental care.Rosenbaum, S.; Vigilant, L.; Kuzawa, C.; Stoinski, T. (15 October 2018). “Caring for infants is associated with increased reproductive success for male mountain gorillas”. Scientific Reports. 8: 15223. {{doi|10.1038/s41598-018-33380-4}}Chemnitz, J.; Bagrii, N.; Ayasse, M.; Steiger S. (01 February 2017). “Staying with the young enhances the fathers’ attractiveness in burying beetles”. Evolution. 71 (4): 985-994. {{doi|10.1111/evo.13194}} One hypothesis regarding the evolution of male parental care in non-monogamous species suggests that parental behaviour is correlated with increased siring of offspring. For instance, in mountain gorillas (Gorilla beringei), males of the upper tertile, regarding their frequency of interaction with young gorillas, regardless of the young's parentage, fathered five times more offspring than males of the lower-two affiliative tertiles. Further, male burying beetles (Nicrophorus vespilloides) attracted three times more females when given the opportunity to breed and provide parental care, compared to males that were not presented with a breeding opportunity. Species such as Gorilla beringei and Nicrophorus vespilloides indicate that selection may promote male parental care in non-monogamous species.

Maternal care

In mammalian species, female parents possess adaptations that may predispose them to care more for offspring. These adaptations include gestation and the production of milk. In invertebrates, maternal care is known to be a prerequisite for the evolution of permanent family grouping and eusociality. In spiders, permanent sociality is dependent on extended maternal care following hatching.Dumke, M. (2016). Extended maternal care and offspring interactions in the subsocial Australian crab spider, Xysticus bimaculatus. Australian Journal of Zoology, 64(5), p.344. {{doi|10.1071/zo16070}} Females of some species of reptiles may remain with their clutch to provide care, by curling around their eggs for the duration of the incubation period. The most intricate example of maternal care in this group can be seen in crocodilian species, as mothers may stay with their young for multiple months.Britannica (2019). Reptile - Embryonic development and parental care. [online] Encyclopedia Britannica. Available at:weblink [Accessed 2 Jun. 2019].The general mammalian tendency for female parents to invest more in offspring was focused on in the development of early hypotheses to describe sex differences in paternal care. It was initially suggested that different levels of investment by each sex in terms of gamete size and number may have led to the evolution of female-only care. This early hypothesis suggested that because females invest more in the production of fewer and larger gametes, compared with males who produce many, smaller gametes, maternal care would be favoured. This is because females have initially invested more, and would thus stand more to lose if they did not continue to invest in the offspring.Gonzalez-Voyer, A. & Kolm, N. (2010). Parental Care and Investment. Encyclopedia of Life Sciences. {{doi|10.1002/9780470015902.a0021907}}

Biparental care

Biparental care tends to be favoured when sexual selection is not intense, and when the adult sex ratio of males to females is not strongly skewed.Remeš, V., Freckleton, R., Tökölyi, J., Liker, A. & Székely, T. (2015). The evolution of parental cooperation in birds. Proceedings of the National Academy of Sciences, 112(44), pp.13603-13608. {{doi|10.1073/pnas.1512599112}} For two parents to cooperate in caring for young, the mates must be coordinated with each other as well as with the requirements of the developing young, and the demands of the environment.Silver, R. (1983). Symbiosis in Parent-Offspring Interactions. Boston, MA: Springer US, pp.145-171. doi:weblink The selection of biparental care as a behavioural strategy is considered to be an important factor driving the evolution of monogamy, if the value of exclusive cooperation in care for mutual offspring by two parents outweighs the potential benefits of polygamy for either sex.Tumulty, J., Morales, V. & Summers, K. (2013). The biparental care hypothesis for the evolution of monogamy: experimental evidence in an amphibian. Behavioral Ecology, 25(2), pp.262-270. {{doi|10.1093/beheco/art116}} Biparental care may increase offspring survival as well as allow parents to gain further mating opportunities with the pair mate.Bales, K. (2017). Parenting in animals. Current Opinion in Psychology, 15, pp.93-98. {{doi|10.1016/j.copsyc.2017.02.026}} There is conflicting evidence for whether offspring fare equally, better or worse when receiving care by two parents rather a single parent. On one hand, it has been suggested that due to sexual conflict, parents should withhold the amount of care they provide and shift as much of the workload as possible to their partner. In this case, offspring may be worse off. Other experimental evidence contrasts this, and suggests that when both parents care for their mutual offspring, their individual contributions may have synergistic effects on the fitness of their young. In this case, offspring would benefit from biparental care.Pilakouta, N., Hanlon, E. & Smiseth, P. (2018). Biparental care is more than the sum of its parts: experimental evidence for synergistic effects on offspring fitness. Proceedings of the Royal Society B: Biological Sciences, 285(1884), p.20180875. {{doi|10.1098/rspb.2018.0875}}Biparental care is particularly prevalent in mammals and birds. 90% of bird species are monogamous, in which biparental care patterns are predominant. In birds, this parental care system is generally attributed to the ability of male birds to engage in most parental behaviours, with the exception of egg-laying. Due to their endothermy and small size at birth, there is a huge pressure for infant birds to grow up quickly to prevent energy loss. Since both sexes are able to forage and provision offspring, it is therefore beneficial for parents to cooperate in care to meet the requirements of infant birds. Offspring survival will ultimately increase the fitness of both parents.

Alloparental care

Alloparental care, caring for non-descendant offspring, is a seemingly altruistic and reproductively costly behaviour; it has both adaptive benefits and evident costs. It has been observed in over 120 mammal and 150 bird species.JOURNAL, Riedman, Marianne L., The Evolution of Alloparental Care and Adoption in Mammals and Birds, Quarterly Review of Biology, 57, 4, 1982, 0033-5770, 10.1086/412936, 405–435, 85378202, It is a defining feature of eusociality, which is found in insects, including various ants, bees, and termites.JOURNAL, Wilson, Edward O., E. O. Wilson, Hölldobler, Bert, Eusociality: Origin and Consequences, PNAS, 20 September 2005, 102, 38, 13367–13371, 10.1073/pnas.0505858102, 16157878, 1224642, 2005PNAS..10213367W, free, For mammalian mothers, alloparenting may be beneficial in promoting earlier weaning of infants (as long as earlier weaning does not compromise infant survival). This strategy results in shorter inter-birth intervals and increased reproductive success. Frequent alloparenting may provide mothers more opportunities to feed without their young, which may ultimately increase their net energy gains and permits them to invest more energy in milk synthesis. However, potential costs of alloparenting may include the expenditure of time and resources in caring for non-descendant offspring with no apparent direct benefits to alloparents.JOURNAL, Bădescu, Iulia, Watts, David P., Katzenberg, M. Anne, Sellen, Daniel W., Alloparenting is associated with reduced maternal lactation effort and faster weaning in wild chimpanzees, Royal Society Open Science, 3, 11, 2016, 2054-5703, 10.1098/rsos.160577, 160577, 28018647, 5180145, 2016RSOS....360577B, 9616877, The offspring that experience alloparental care may benefit from increased protection from predators and the learning of group dynamics through social interactions.JOURNAL, Stanford, Craig B., Costs and benefits of allomothering in wild capped langurs (Presbytis pileata), Behavioral Ecology and Sociobiology, 30, 1, 1992, 0340-5443, 10.1007/bf00168591, 10064314, In the eusocial insects, the evolution of a caste system has driven workers to sacrifice their own personal reproductive fitness to assist in the reproductive success of the colony. Indirect fitness benefits are gained instead through assisting related members of the colony. It may be in the best interest of a worker to forgo her own personal reproduction and participate in alloparenting, or rearing drones, so that there is an enhanced likelihood that males from her colony will ultimately mate with a queen. This would provide a greater chance for her colony's genes to be represented in the future colony.Oldroyd, B. & Wongsiri, S. (2006). Asian honey bees. 1st ed. Cambridge, Mass.: Harvard University Press, p.112. Similarly, worker ants tend to raise their sisters rather than their daughters, due to their greater relatedness. The survival of the colony is believed to be the main reward that drives the altruism of the workers.Ridley, M. (2014). Ants, altruism and self sacrifice. [online] Rationaloptimist.com. Available at:weblink [Accessed 5 Jun. 2019].

In groups of animals

Invertebrates

File:Potter Wasp building mud nest near completion.JPG|thumb|Potter waspPotter wasp{{Further|Social insect}}Parental care is not frequently observed in invertebrate species. In Dipterans, oviposition is instead commonly observed. Adults lay their eggs before leaving them to hatch and develop into larva, then pupa, then adults. For example, Phormia regina adults lay their eggs preferentially on carrion and corpses.Byrd, Jason H.; Allen, Jon C. (August 2001). "The development of the black blow fly, Phormia regina (Meigen)". Forensic Science International. 120 (1–2): 79–88. {{doi|10.1016/S0379-0738(01)00431-5}} PMID 11457615. Though biparental and male-only care are rarely observed, female-only care does exist in some invertebrates.Tallamy, D.W. (2000) Sexual selection and the evolution of exclusive paternal care in arthropods. Animal Behavior, 60, pp.559–567. {{doi|10.1006/anbe.2000.1507}}Zeh, D.W. & Smith, R.L. (1985) Paternal investment by terrestrial arthropods. American Zoologist, 25(1587), pp.785–805. {{doi|10.1093/icb/25.3.785}}Some insects, including the Hymenoptera (ants, bees and wasps), invest substantial effort in caring for their young. The type and amount of care invested varies widely. Solitary wasps such as the potter wasps (Eumeninae) build nests for their young, provisioning them with food, often caterpillars, caught by the mother. The nests are then sealed, and the young live on the food until they leave the nest as adults.WEB, Grissell, E. E., Potter wasps of Florida,weblink University of Florida, 12 June 2015, April 2007, In contrast, social wasps and honeybees raise young in substantial colonies, with eggs laid mainly by queens (mothers), and the young cared for mainly by workers (sisters of the young).JOURNAL, Wong, Janine W. Y., Meunier, Joel, Molliker, Mathias, The evolution of parental care in insects: the roles of ecology, life history and the social environment, Ecological Entomology, 2013, 38, 2, 123–137, 10.1111/een.12000, 82267208, free, Outside the Hymenoptera, parental care is found among the burying beetles and the magnificent salt beetle.BOOK, Allport, Susan, A Natural History of Parenting: A Naturalist Looks at Parenting in the Animal World and Ours,weblink 2003, iUniverse, 978-0-595-27130-6, 19–20, Subdued forms of parental care are also seen in the Lepidoptera and various other plant feeding insects, with females laying eggs on plant species the offspring can best feed and develop on.JOURNAL, Jones, L. C., Rafter, M. A., Walter, G. H., Insects allocate eggs adaptively across their native host plants., Arthropod Plant Interactions, 2019, 13, 2, 181–191, 10.1007/s11829-019-09688-x, 92443593, Crickets have also been known to lay eggs in optimal environments for the young.Many species of Hemiptera take care of their young, for instance in the Belostomatidae genus Abedus.{{cn|date=October 2021}}Some crustaceans also show parental care. Mothers of the crab species Metopaulias depressus raises their young in water-filled bromeliads, cleaning them of debris, defending them against predators and feeding them with captured prey.JOURNAL, Diesel, Rudolf, 1989, Parental care in an unusual environment: Metopaulias depressus (Decapoda: Grapsidae), a crab that lives in epiphytic bromeliads,weblink Animal Behaviour, en, 38, 4, 561–575, 10.1016/S0003-3472(89)80001-6, In the desert isopod Hemilepistus reaumuri, juveniles share their parents' burrow for the first 10-20 days of their life, and are supplied with food by their parents.JOURNAL, K. Eduard Linsenmair, 1974, Some adaptations of the desert woodlouse Hemilepistus reaumuri (Isopoda, Oniscoidea) to desert environment,weblink Verhandlungen der Gesellschaft für Ökologie, 4, 183–185, 10.1007/978-94-017-4521-5_18, 978-90-6193-180-5, Finally, some species of Synalpheus shrimps are eusocial, living in colonies with one or a few breeders of each sex together with non-breeders that defend the colony.{{Citation |last=Duffy |first=J. Emmett |title=Ecology and Evolution of Eusociality in Sponge-Dwelling Shrimp |date=2007-08-01 |work=Evolutionary Ecology of Social and Sexual Systems |pages=387–410 |editor-last=Duffy |editor-first=J. Emmett |url=https://academic.oup.com/book/6612/chapter/150625728 |access-date=2024-02-29 |edition=1 |publisher=Oxford University PressNew York |language=en |doi=10.1093/acprof:oso/9780195179927.003.0018 |isbn=978-0-19-517992-7 |editor2-last=Thiel |editor2-first=Martin}}

Fish

File:Tehotny morsky konik.jpg|thumb|upright|Pregnant male seahorseseahorse{{Further|Mouthbrooding|Male pregnancy}}Several groups of fish have evolved parental care. The ratio of fish genera that exhibit male-only: biparental: female-only care is 9:3:1.Reynolds, J.D., Goodwin, N.B. & Freckleton, R.P. (2002) Evolutionary transitions in parental care and live bearing in vertebrates. Philosophical Transactions of the Royal Society of London. Series B, 357, pp.269–281. {{doi|10.1098/rstb.2001.0930}} Some fish such as pipefish, sea dragons and seahorses (Syngnathidae) have a form of male pregnancy, where the female takes no part in caring for the young once she has laid her eggs.JOURNAL, Wilson, A. B., Orr, J. W., The evolutionary origins of Syngnathidae: pipefishes and seahorses, Journal of Fish Biology, 78, 1603–1623, 2011, 10.1111/j.1095-8649.2011.02988.x, 6, 21651519,weblink JOURNAL, Wilson, A. B., Ahnesjo, I., Vincent, A., Meyer, A., The dynamics of male brooding, mating patterns, and sex roles in pipefishes and seahorses (family syngnathidae), Evolution (journal), Evolution, 2003, 57, 6, 1374–1386, 10.1111/j.0014-3820.2003.tb00345.x, 12894945, 16855358, free, Males in other species may take a role in guarding the eggs before they hatch.Mouthbrooding is the care given by some groups of fish (and a few other animals such as Darwin's frog) to their offspring by holding them in their mouth for extended periods of time. Mouthbrooding has evolved independently in several different families of fish including the cardinalfish, sea catfish, bagrid catfish, cichlids, snakeheads, jawfishes, gouramis, and arowanas.BOOK, Helfman, G., Collette, B, Facey, D., The Diversity of Fishes, Blackwell's, Blackwell, 1997, 978-0-86542-256-8,

Amphibians

There is an equal prevalence of female-only and male-only care in amphibians. However, biparental care is uncommon.Beck, C.W. (1998) Mode of fertilization and parental care in anurans. Animal Behavior, 55, pp.439–449. {{doi|10.1006/anbe.1997.0619}} Provisioning in this animal group tends to be rare, and offspring guarding is more prevalent. For example, in the frog species Bibron's Toadlet, male frogs are left to care for the nest. Parental care after the laying of eggs has been observed in 5% of caecilian species, 18% of salamander species and 6% of frog species,BOOK, Crump, Martha L., Parental Care: Evolution, Mechanisms, and Adaptive Significance, 1996, Advances in the Study of Behavior, 978-0-12004-525-9, 25, 109–144, Parental care among the Amphibia, 10.1016/S0065-3454(08)60331-9, though this number is likely an underestimate due to taxonomic bias in research JOURNAL, Stahlschmidt, Zachary R., 2011, Taxonomic Chauvinism Revisited: Insight from Parental Care Research, PLOS ONE, 6, 8, e24192, 2011PLoSO...624192S, 10.1371/journal.pone.0024192, 3164163, 21904614, free, and the cryptic nature of many species.JOURNAL, Oneto, Fabrizio, Ottonello, Dario, Pastorino, Mauro Valerio, Salvidio, Sebastiano, December 2010, Posthatching Parental Care in Salamanders Revealed by Infrared Video Surveillance, Journal of Herpetology, 44, 4, 649–653, 10.1670/09-181.1, 40983661, 85714252, Six modes of parental care are recognized among the Amphibia, in different species: egg attendance, egg transport, tadpole attendance, tadpole transport, tadpole feeding, and internal gestation in the oviduct (viviparity and ovoviviparity). Many species also care for offspring (either eggs or tadpoles) in specially adapted structures of their body. For example, the male pouched frog of eastern Australia protects tadpoles in pouches on the lateral surface of their skin,BOOK, Anstis, Marion, Tadpoles and Frogs of Australia, 2013, New Holland Publishers, 9781921517167, Sydney, 526–531, the gastric-brooding frog raised tadpoles (and potentially eggs) in their stomachBOOK, Anstis, Marion, Tadpoles and Frogs of Australia, 2013, New Holland Publishers, 978-1-92151-716-7, Sydney, 668–673, and the common Suriname toad raises eggs embedded in the skin on its back.

Reptiles

File:Alligator mississippiensis 113744549.jpg|thumb|right|Unlike most reptiles, crocodilians care for their young even after they have hatched.JOURNAL, Gans, Carl, 1996, An Overview of Parental Care among the Reptilia, Advances in the Study of Behavior, 25, 153, 10.1016/s0065-3454(08)60332-0, 9780120045259, ]]Reptiles provide less parental care than other tetrapods. When it does occur, it is usually female-only or biparental care.JOURNAL, Reynolds, J. D., Goodwin, N. B., Freckleton, R. P., 29 March 2002, Balshine, S., Evolutionary transitions in parental care and live bearing in vertebrates, (Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences), 357, 1419, 269–281, 10.1098/rstb.2001.0930, 0962-8436, 1692951, 11958696, Székely, B., Kempenaers, T., Many species within this group produce offspring that are self-sufficient, and are able to regulate their body temperatures and forage for themselves immediately after birth, thereby eliminating the need for parental care. Maternal care exists in crocodilians, where the mother assists hatchlings by transporting them in her mouth from the nest to the water. She may stay with the young for up to several months.JOURNAL, Pooley, A. C., 20 August 2009, Nest opening response of the Nile crocodile Crocodylus niloticus, Journal of Zoology, Wiley, 182, 1, 17–26, 10.1111/j.1469-7998.1977.tb04137.x, 0952-8369, Parental behavior have also been observed in Cunningham's skink, a viviparous lizard that protects its offspring against predators.Observations supporting parental care by a viviparous reptile: aggressive behaviour against predators demonstrated by Cunningham’s skinks

Birds

Birds are distinctive in the way they care for their young. 90% of bird species display biparental care, including 9% of species with alloparental care, or helpers at the nest.Cockburn, A. (2006) Prevalence of different modes of parental care in birds. Proceedings of the Royal Society of London, Series B, 273, pp.1375–1383. {{doi|10.1098/rspb.2005.3458}} Biparental care may have originated in the stem reptiles (archosaurs) that gave rise to the birds, before they developed flight.JOURNAL, Wesolowski, Tomasz, The origin of parental care in birds: a reassessment, Behavioral Ecology, 2004, 15, 3, 520–523, 10.1093/beheco/arh039, free, In the remainder of bird species, female-only care is prevalent, and male-only care is rare.Lack, D. (1968) Ecological Adaptations for Breeding in Birds. Chapman and Hall, Methuen, London. Most birds, including passerines (perching birds), have their young born blind, naked and helpless (altricial), totally dependent for their survival on parental care. The young are typically raised in a nest; the parents catch food and regurgitate it for the young. Some birds such as pigeons create a "crop milk" which they similarly regurgitate.WEB, Ehrlich, Paul R., Paul R. Ehrlich, Dobkin, David S., Wheye, Darryl, Parental Care,weblink Stanford University, 15 January 2016, 1988, David Lack developed a hypothesis that clutch size has evolved in response to the costs of parental care known as Lack's principle. It has since seen modifications but is still used as a general model.

Mammals

File:Harbour seal breast feeding 1150144.jpg|thumb|Harbour sealHarbour sealFile:Japanese Snow Monkey (Macaque) Mother Grooms Her Young.jpg|thumb|Japanese snow monkey mother grooming her young.]]There is maternal care in all species of mammals, and while 95% of species exhibit female-only care, in only 5% biparental care is present.{{Citation needed|date=March 2022}} Thus, there are no known cases of male-only care in mammals.Clutton-Brock, T.H. (1991) The Evolution of Parental Care. Princeton University Press, Princeton, NJ. The major adaptation shared by all live-bearing mammals for care of their young after birth is lactation (the feeding of milk from the mammary glands).{{Citation needed|date=March 2022}} Further, many mammals exhibit other parental care behaviors to increase the fitness of their offspring, for example, building a den, feeding, guarding, carrying, huddling, grooming and teaching their young.BOOK, Gubernick, David J., Parental Care in Mammals,weblink 11 November 2013, Springer Science & Business Media, 978-1-4613-3150-6, JOURNAL, Woodroffe, Rosie, Vincent, Amanda, August 1994, Mother's little helpers: Patterns of male care in mammals,weblink Trends in Ecology & Evolution, en, 9, 8, 294–297, 10.1016/0169-5347(94)90033-7, 21236858, Others, consider also as a type of care when males provision the pregnant females.JOURNAL, West, Hannah E. R., Capellini, Isabella, September 2016, Male care and life history traits in mammals, Nature Communications, en, 7, 1, 11854, 10.1038/ncomms11854, 2041-1723, 4911630, 27299954, 2016NatCo...711854W, Humans(File:Father and son surf lesson in Morro Bay, CA.jpg|thumb|Human parental care extends far beyond providing food and protection. Here a father teaches his son how to surf.)Parenting or child rearing in humans is the process of promoting and supporting the physical, emotional, social, financial, and intellectual development of a child from infancy to adulthood. This goes far beyond anything found in other animals, including not only the provision of food, shelter, and protection from threats such as predators, but a prolonged period of support during which the child learns whatever is needed to live successfully in human society.BOOK, Davies, Martin, The Blackwell encyclopedia of social work,weblink Wiley-Blackwell, 2000, 245, 978-0-631-21451-9,

In evolutionary biology

In evolutionary biology, parental investment is the expenditure of time and effort towards rearing offspring that benefits the offspring's evolutionary fitness at a cost to parents' ability to invest in other components of the species' fitness. Parental care requires resources from one or both parents that increases the fitness of their offspring and of themselves.Trivers, R.L. (1972) Parental investment and sexual selection. In: Sexual Selection and the Descent of Man (ed. B. Campbell). pp. 139– 179. Aldine, Chicago, IL. These resources thus cannot be invested in the parents own survival, growth or future reproduction. Therefore, parental care will only evolve in a species that requires care. Some animal groups produce self-sufficient young and thus no parental care is required.For species that do require care, trade-offs exist in regards to where parental investment should be directed and how much care should be provided, since resources and time are limited.Krebs, J., Davies, N. and West, S. (2012). An introduction to behavioural ecology. 4th ed. John Wiley & Sons, Ltd, pp.223-238. For example, if the strategy of parental care involves parents choosing to give each of a relatively small number of offspring an increased chance of surviving to reproduce themselves, they may accordingly have evolved to produce a small number of zygotes at a time, possibly only one.WEB, Gilbert, James, Evolution of Parental Care,weblink Oxford Bibliographies, 15 January 2016, 30 September 2013, WEB, Gilbert, James, Reproductive Allocation in Animals,weblink Oxford Bibliographies, 15 January 2016, 23 May 2012, The ideal amount of parental investment would guarantee the survival and quality of both broods. Parents need to trade off investment into current and future reproductive events, since parental care increases offspring survival at the expense of the parent's ability to invest in future broods. Nonetheless, there is some evidence suggesting that in mammals provinding male care actually leads to more fecund females, and thus caring for the offspring can lead to having more number of litters.Predation on offspring and species habitat-type are two potential proximate causes for the evolution of parental care. Generally, parental care is expected to evolve from a previous state of no care when the costs of providing care are outweighed by the benefits to a caring parent. For example, if the benefit of increased offspring survival or quality exceed the decreased chance of survival and future reproductive success of the parent, then parental care may evolve. Therefore, parental care is favoured when it is required by offspring, and the benefits of care are high.Klug, H. and Bonsall, M. (2010). Life history and the evolution of parental care. Evolution, 64(3), pp.823-835. {{doi|10.1111/j.1558-5646.2009.00854.x}}Types of parental care and the amount of resources invested by parents vary considerably across the animal kingdom. The evolution of male-only, female-only, biparental or alloparental care in different groups of animals may be driven by multiple factors. Firstly, different groups may have diverse physiological or evolutionary constraints that may predispose one sex to care more than the other. For example, mammary glands may make female mammals preadapted to exclusively provide nutritional care to young.JOURNAL, Safari, Ignas, Goymann, Wolfgang, Certainty of paternity in two coucal species with divergent sex roles: the devil takes the hindmost, BMC Evolutionary Biology, 18, 1, 13 July 2018, 1471-2148, 10.1186/s12862-018-1225-y, 110, 30005606, 6043945, free, Secondly, the costs and benefits of care by each sex may be influenced by ecological conditions and mating opportunities. Thirdly, operational and adult sex ratios may influence which sex has more mating opportunities, and thus predisposes one sex to care more. Furthermore, parenting decisions may be influenced by the confidence of either sex in being the genetic parent of the offspring, or paternity certainty.

Which sex provides care

The type of mating system may influence which sex provides care. In monogamous species that establish long-term pair-bonds, parents are likely to cooperate in caring for their offspring. In polyandrous mating systems, paternal or male-only care tends to evolve. Conversely, polygynous mating systems are associated with little or no male contribution. Males rarely provide care for offspring in promiscuous mating systems, since there is high paternity uncertainty.JOURNAL, Kappeler, P. M., Male Reproductive Strategies, Nature Education Knowledge, 2012, 2, 82, JOURNAL, Krasnec, M. O., Cook, C. N. & Breed, M. D., Mating Systems in Sexual Animals, Nature Education Knowledge, 2012, 3, 10, 72, Male care is most prevalent in species with external fertilisation, while female care is more common with internal fertilisation.JOURNAL, Gross, Mart R., Shine, Richard, Parental Care and Mode of Fertilization in Ectothermic Vertebrates, Evolution, 2408247, 35, 4, 1981, 775–793, 0014-3820, 10.2307/2408247, 28563147, Explanations include the suggestion by Trivers (1972) that this depends on paternity certainty, which may be less with internal fertilisation unless the male undertakes "mate guarding" until the female lays eggs or gives birth.JOURNAL, Queller, David C., Why do females care more than males?, Proceedings of the Royal Society of London. Series B: Biological Sciences, 264, 1388, 22 November 1997, 0962-8452, 10.1098/rspb.1997.0216, 1555–1557, 1688720, A second explanation is Richard Dawkins and T. R. Carlisle's (1976) theory that the order of gamete release, and therefore the opportunity for each parent to desert, may influence which sex provides care.JOURNAL, Dawkins, Richard, Richard Dawkins, Carlisle, T. R., Parental investment, mate desertion and a fallacy, Nature, 262, 5564, 1976, 0028-0836, 10.1038/262131a0, 131–133, 1976Natur.262..131D, 4278681, Internal fertilisation may provide the male parent with an opportunity to desert first, as is seen in some bird and mammal species; the roles may be reversed with external fertilisation. In fish, males often wait until a female lays her eggs before he can fertilise them, to prevent his small gametes from floating away. This allows the female to desert first, and leave male parents to care for the eggs.Thirdly, George C. Williams's (1975) hypothesis indicates that an association with the embryos may predispose one sex to care for the offspring. With internal fertilisation occurring in the mother, the female parent is most closely associated with the embryo, and may be preadapted to care for the young. With external fertilisation, eggs are often laid by the female in a male's territory.BOOK, Williams, G. C., George C. Williams (biologist), 2020, Sex and Evolution (MPB-8), 8, Princeton University Press, {{pn|date=October 2021}} Male territoriality is particularly common with external fertilisation. Therefore, the male is most closely associated with the embryos. Males may defend their territories and thereby incidentally defend their eggs and young. This may preadapt males to provide care. Male care consequently involves less opportunity costs in this case, since males can still attract mates while simultaneously guarding territory and eggs. Females may even be more attracted to, and preferentially select to mate with, males that already have eggs in their nest.JOURNAL, Hale, Rebecca E., St Mary, Colette M., Nest tending increases reproductive success, sometimes: environmental effects on paternal care and mate choice in flagfish, Animal Behaviour, 74, 3, 2007, 0003-3472, 10.1016/j.anbehav.2006.12.013, 577–588, 53193765, Male territoriality with internal fertilisation exists in some bird species. Nest size and nest building behaviour are two sexually selected traits that may attract a female to a male's territory for mating. Since the female lays her eggs in the nest within the male's territory, paternal care may evolve, even though fertilisation is internal.JOURNAL, Szentirmai, István, Komdeur, Jan, Székely, Tamás, What makes a nest-building male successful? Male behavior and female care in penduline tits, Behavioral Ecology, 16, 6, 17 August 2005, 1465-7279, 10.1093/beheco/ari080, 994–1000, free,

Amount of care

(File:Mother-child in the woods.jpg|thumb|Mother and child in the woods)Increasing parental investment in any one young benefits that particular offspring, but decreases resources for other offspring, possibly decreasing parental fitness. Hence, a trade-off exists between offspring quantity and quality within a brood. If a parent disperses its limited resources thinly among too many offspring, then few will survive. Alternatively, if the parent uses its resources too generously among one small brood, this reduces the ability of the parent to invest in future broods.Williams, G. C. (1966). Natural selection, the costs of reproduction, and a refinement of Lack's principle. American Naturalist, 100, pp.687–690. {{doi|10.1086/282461}} Therefore, there is a theoretical optimal brood size that maximises productivity for each brood.In groups with biparental care, there is sexual conflict over how much care should be provided. If either parent is temporarily removed, the other parent may increase its work rate.Krebs, J., Davies, N. & West, S. (2012). An introduction to behavioural ecology. 4th ed. John Wiley & Sons, pp.223-238. This demonstrates that both parents have the capacity to work harder and provide greater levels of care. One parent may be tempted to cheat, relying on the other parent. In biparental care, the key theoretical prediction is that parents should respond to reduced partner effort with incomplete compensation. A parent who does not put in their fair share of work then suffers reduced fitness, because their offspring receive less resources from both parents. This has been experimentally demonstrated with birds.Harrison, F., Barta, Z., Cuthill, I. & Szekely, T. (2009). How is sexual conflict over parental care resolved? A meta-analysis. Journal of Evolutionary Biology, 22, pp.1800–1812.

Flexibility of parental care

Parental care may vary in relation to costs and benefits

When one parent is not sufficient, both parents may need to care for offspring. Each parent would like to minimise the level of care they must invest at the expense of the other parent. If one parent were to die or cease providing care, then the remaining partner may be obliged to desert the eggs or young.The extent of parental care provided to a current brood may also be influenced by prospects of future reproduction. Field experiments on a passerine bird species indicated that in areas where broods were fed with extra carotenoids, their mouths became redder. This consequently enhanced their begging displays and led parents to increase their provisioning. This was likely because the redder mouths indicated that offspring were healthier, and thus worth investing in. In other territories, the adults were also provided carotenoid-rich sugar diets, which increased the likelihood of them having a second brood in that season. Since parents that had second broods did not respond to the increased begging signals of their current brood, this indicates that parents strategically vary their sensitivity to their current broods demands in relation to their future prospects of reproducing in that season.Thorogood, R., Ewen, J.G. & Kilner, R.M.  (2011). Sense and sensitivity: responsiveness to offspring signals varies with the parents’ potential to breed again. Proceedings of the Royal Society of London, Series B, 278, pp.2638–2645.The act of eating one's own offspring, or filial cannibalism, may be an adaptive behaviour for a parent to use as an extra source of food. Parents may eat part of a brood to enhance the parental care of the current brood. Alternatively, parents may eat the whole brood to cut their losses and improve their future reproductive success.Rohwer, S. (1978). Parent cannibalism of offspring and egg raiding as a courtship strategy. American Naturalist, 112, pp.429–440.

Parental care may vary in relation to mate attractiveness

In theory, a parent should invest more when paired with a mate of a high phenotypic or genetic quality. This is explained by the differential allocation hypothesis.Sheldon, B.C. (2000). Differential allocation: tests, mechanisms and implications. Trends in Ecology & Evolution, 15, pp.397–402. This was shown through experimentation on zebra finches. Males were made more attractive to females by experimentally giving them red leg bands. Females increased their provisioning and raised more young when paired with these attractive males compared to when they were paired with less attractive males that had blue or green leg bands.Burley, N. (1988). The differential allocation hypothesis: an experimental test. American Naturalist, 132, pp.611–628. Further experimentation on mallard ducks has displayed that females lay larger eggs and increase their provisioning when paired with more attractive males.Cunningham, E.J.A. & Russell, A.F. (2000). Egg investment is influenced by male attractiveness in the mallard. Nature, 404, pp.74–77. Female peacocks have also been shown to lay more eggs after mating with males that possess more elaborate tails.Petrie, M. & Williams, A. (1993). Peahens lay more eggs for peacocks with larger trains. Proceedings of the Royal Society of London, Series B, 251, pp.127–131. Furthermore, female birds are generally more likely to care for the offspring of males that spend more time nest building, and build more elaborate nests. As a consequence, the reproductive success of males tends to increase with nest size and building behaviour.Szentirmai, I., Komdeur, J. & Székely, T. (2005). What makes a nest-building male successful? Male behavior and female care in penduline tits. Behavioral Ecology, 16(6), pp.994-1000. {{doi|10.1093/beheco/ari080}}Therefore, differential allocation is expected because the offspring of these pairings would likely inherit the quality of the attractive parent, if attractiveness signifies genetic quality. Differential allocation may also work the other way around, where parents may invest less in their offspring if paired with unattractive mates. By reducing the amount of care invested in these offspring, individuals may save resources for future reproductive attempts with a more attractive mate.Ratikainen, I., Haaland, T. and Wright, J. (2018). Differential allocation of parental investment and the trade-off between size and number of offspring. Proceedings of the Royal Society B: Biological Sciences, 285(1884), p.20181074. {{doi|10.1098/rspb.2018.1074}}Differential allocation is mostly expected from females, since in many animal groups females are more choosy when assessing potential mates. However, in many bird species, males are known to be involved in caring for young, which may lead to differential allocation by males as well as females.

References

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