Paternal care
In biology, paternal care is parental investment provided by a male to his own offspring. It is a complex social behavior in vertebrates associated with animal mating systems, life history traits, and ecology. Paternal care may be provided in concert with the mother or, more rarely, by the male alone.
The provision of care, by either males or females, is presumed to increase growth rates, quality, and/or survival of the young, ultimately increasing the inclusive fitness of the parents. In a variety of vertebrate species, both males and females invest heavily in their offspring. Many of these bi-parental species are socially monogamous, so individuals remain with their mate for at least one breeding season.
Exclusive paternal care has evolved multiple times in a variety of organisms, including invertebrates, fishes, and amphibians.
Mammals
Male mammals employ different behaviors to enhance their reproductive success. However, the benefits of paternal care have rarely been studied in mammals, largely because only 5-10% of mammals exhibit such care. For species in which males provide extensive care for their offspring, indirect evidence suggests that the costs can be substantial. For example, mammalian fathers that care for their young may undergo changes in body mass and an increase in the production of various costly hormones. Nonetheless, there is evidence suggesting that across all mammals, when males carry and groom their offspring, their female partner's fecundity increases. Furthermore, if males provide for the females, their litter size tends to be larger.Rodents
Several species of rodents have been studied as models of paternal care, including prairie voles, Campbell's dwarf hamster, the Mongolian gerbil, and the African striped mouse. The California mouse is a monogamous rodent that exhibits extensive and essential paternal care, and therefore has been studied as a model organism for this phenomenon. One study of this species found that fathers had larger hindlimb muscles than those of non-breeding males. Quantitative genetic analysis has identified several genomic regions that affect paternal care.Humans
Human cultures and societies vary widely in the expression of paternal care. Some cultures recognize paternal care via celebration of Father's Day. Human paternal care is a derived characteristic and one of the defining characteristics of Homo sapiens. Different aspects of human paternal care may have evolved at different points in our history, and together they form a unique suite of behaviors, as compared to the great apes. One study of humans has found evidence suggesting a possible evolutionary trade-off between mating success and parenting involvement; specifically, fathers with smaller testes tend to be more involved in the care of their children.Research on the effects of paternal care on human happiness has yielded conflicting results. However, one study concluded that fathers generally report higher levels of happiness, positive emotion, and meaning in life, as compared with non-fathers.
According to the United States Census Bureau, approximately one-third of children in the U.S. grow up without their biological father in their home. Numerous studies have documented negative consequences of being raised in a home that lacks a father, including increased likelihood of living in poverty, behavioral problems, committing crimes, spending time in prison, abusing drugs or alcohol, becoming obese, and dropping out of school.
Non-human primates
In non-human primates, paternal investment often depends on the species' mating system. Mating systems influence paternity certainty and the likelihood that a male is providing care towards his own biological offspring.Paternal certainty is high in monogamous pair-bonded species, and males are less likely to accidentally care for unrelated offspring. In contrast, the offspring paternity of polygamous primate societies may be uncertain. Males are therefore at higher risk of providing care for unrelated offspring and missing opportunities to mate, compromising their own fitness.
Male non-human primates motivated by biological paternity rely on past mating history and phenotypic matching to recognize their biological offspring.
Comparing male care efforts exhibited by the same species can provide insight on the significant relationship between a male's paternity certainty and the amount of paternal care exhibited by the male. For example, siamangs practice both polyandrous and monogamous mating systems. It has been found that monogamous males are more likely to carry infants and contribute to parental care compared to male siamangs in promiscuous mating systems.
Strepsirrhines
is a suborder of primates which includes lemurs, lorises, and bush babies. In this suborder, males exhibit the lowest levels of paternal care for infants among primates. Examples of observed male care in this group include playing, grooming, and occasionally transporting infants. Males have also been observed interacting with infants while mothers temporarily "park" them to feed. When female strepsirrhines park or nest their infants in nearby trees, males frequently use this as an opportunity to play with the unattended infants. In this suborder, male care and affection is directed toward multiple infants, including non-biological offspring, and young strepsirrhines can be found interacting with various males. Paternal care does not influence infant growth rates or shorten inter-birth intervals of mothers as happens in haplorhines.Strepsirrhines are constrained by their life history traits, and reproductive rates are not flexible within this group. Instead, these primates tend to give birth when food is abundant, resulting in strict seasonal breeding periods. Shortening inter-birth intervals, which is theorized to be a possible outcome of increased male care, is not beneficial for strepsirrhine mothers and can decrease infant survival. Studies also show that paternity can be highly skewed in strepsirrhines, with only one or a few male members being the only biological fathers within a single group. Instead of relying on a singular paternal figure, female mothers in this group rely on alloparenting from other group members. Infant parking and strict reproductive schedules are more beneficial for successful infant development in strepsirrhines.
Haplorhines
, a suborder of the order Primate, includes tarsiers, New World Monkeys, Old World monkeys, apes, and humans. Haplorhini is broken into two sister groups which are commonly distinguished by the characteristic of the primate nose: Catarrhini and Platyrrhini. Paternal care is highly variable between the two sister groups and the species within them.Catarrhines
is composed of Old World Monkeys ''' and apes. These primates are geographically located in Africa, Asia, and Madagascar.Cercopithecines, the largest primate family, include primate species such as baboons, macaques, colobus, and vervet monkeys.
Apes consist of species of gibbons, siamangs, bonobos, chimpanzees, gorillas, orangutans, and humans.
Non-human catarrhines are often organized into a multi-male/multi-female primate social systems and utilize polygamous mating systems, creating paternity uncertainty. It is predicted that males in promiscuous mating systems do not engage in infant care due to the high costs of caring for an infant and missing opportunities to mate with receptive females. Male care in this group of primates is often portrayed through actions such as grooming, carrying, tolerance of the infant, as well as protection against agonistic interactions and infanticide. High ranking males can also provide access to food for developing infants. Compared to male intervention when an infant is being harassed by conspecifics, direct care, such as grooming or play, is not as common.
In Cercopithecus, male involvement in the infant's interactions with conspecifics is common in many species of baboons but, between species, paternal care is not always biased towards biological offspring. Male yellow baboons direct care towards their own biological offspring. Males in this species are more likely to intervene and protect infants from harassment against other group members when the infant is predicted to be their own. Studies have shown that male yellow baboons selectively choose to remain in closer proximity to their own offspring and engage in long-term investment beyond early infancy, when the infant is at greatest risk for infanticide. Infants receiving paternal investment in yellow baboons have shown enhanced fitness and accelerated maturation through males creating a safe zone for infants to exist in. Male baboons also direct care towards unrelated offspring based on male affiliations with female mothers. Baboon males and females within a social group often exhibit "friendships" with females which begin during birth of her infant. These friendships have been observed to end abruptly if the infant dies. Males establish associations with females which they have previously mated with, resulting in affiliative behavior and protection towards her offspring. Relationships created by male and female members are significant for infant survival in chacma baboons because the risk of infanticide in early infancy is higher in this species. Paternal care in the form of protection for the infant is therefore more beneficial than long term investment in chacma baboons and is believed to be directed towards both biological and non-biological infants in the group.
Similarly to baboons, paternal roles and the underlying causes for the evolution of paternal care vary within macaque species. In Celebes crested macaques, both male rank and the relationship to the mother predicted male care towards an infant instead of true biological paternity. In both Celebes crested and Barbary macaques, males adopted a "care-then-mate" strategy, in which care is provided to infants regardless of paternity in order for the male to increase future mating opportunities with the mother. In both species, it was observed that male macaques are more likely to initiate care towards and positively interact with the infant in the presence of the mother. In Assam macaques, biological paternity was the most significant predictor of male affiliations with infants, and therefore males biased care towards infants presumed to be their own. Observers found that Assamese males were more likely to engage and provide care for infants in the absence of their mothers, reducing the likelihood that care provided to infants will impress the mother and secure access to mating possibilities. In rhesus macaques, males providing protection and greater access to food resulted in higher weight gain for both male and female infants. This had a positive effect on infant survival and was significant in the first year of infancy, when the risk of infanticide is the highest.
Chimpanzees are organized into fission-fusion social groups and provide an example of a polygamous mating society. Male chimpanzees often engage with infants in the form of grooming, playing, and providing protection towards other group members. In both Western and Eastern chimpanzees, studies found that males were more likely to engage with their own biological offspring, suggesting that male care is influenced by paternity in these species. In both chimpanzee and bonobo social groups, high ranking alpha males sire approximately half of the offspring within their social group. More research needs to be done addressing how reproductive skew affects paternal care and infant/male relationships in non-human primates including chimpanzees and bonobos.