Parental 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. 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.
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.
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. .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. 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.
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
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. 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. The occurrence of paternal care is mostly associated with biparental care in socially monogamous mating systems. 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. 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. 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. 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. 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: https://www.britannica.com/animal/reptile/Embryonic-development-and-parental-care [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.
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. 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: https://doi.org/10.1007/978-1-4684-4565-7_7. 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. 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. 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.