The Evolution Of Paternal Investment In Primates: Unraveling The Factors That Drive Male Care

Male primates invest in their offspring in a variety of ways, including direct care such as infant carrying, feeding, and protection, as well as indirect care such as providing a safe environment for the female to feed. The amount of investment is often dependent on the type of mating system exhibited by the species.

In some species, such as the owl monkey and titi monkey, male primates are the primary caregivers and play a major role in infant survival. In other species, such as the baboon, male primates protect their offspring from attacks by peers but do not provide direct care.

The evolution of paternal care in primates is thought to be influenced by several factors, including social monogamy, infanticide risk, and the distribution of female groups.

Overall, male primates exhibit a range of investment strategies in their offspring, with the specific behaviours and level of investment varying across different species.

Paternal care in primates is complex social behaviour associated with mating systems, life history traits, and ecology

Paternal care in primates is a complex social behaviour that is associated with mating systems, life history traits, and ecology.

Mating systems

Paternal care in primates is influenced by the mating systems exhibited by each species. Mating systems influence paternity certainty and the likelihood that a male is providing care towards his own biological offspring. In monogamous pair-bonded species, males are less likely to be at risk of caring for unrelated offspring and not contributing to their own fitness. In contrast, polygamous primate societies create paternity uncertainty and males are more at risk of providing care for unrelated offspring and compromising their own fitness.

Life history traits

Life history traits also play a role in paternal care. For example, Strepsirrhini is a suborder of primates that includes lemurs, lorises, and bush babies. In this sub-order, males exhibit the lowest levels of paternal care for infants among primates. Strepsirrhini males exhibit the lowest intensity of care towards infants in non-human primates. This is because Strepsirrhini are constrained by their life history traits and reproductive rates are not flexible within this group of primates. This group of primates are programmed 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.

Ecology

The ecology of a species also plays a role in paternal care. For example, 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, it was found that males were more likely to engage with their own biological offspring, meaning that male care is directed by paternity in this species. In both chimpanzee and bonobo social groups, high-ranking alpha males sire approximately half of the offspring within their social group.

Paternal care in primates is influenced by mating effort, paternal effort, and maternal relief

Mating effort

Males of many species experience intense and persistent mating effort. Mating effort can include direct aggressive competition with other males, indirect forms of competition such as physical, vocal, and behavioral displays, increased vigilance, altered travel and grouping patterns, and persistent or coercive interactions with females. These activities can be predicted to drive a variety of costs, including the risk of injury, reduction in energetic condition, physiological stress, and the tradeoffs associated with diverting resources toward obtaining status and mating opportunities.

Paternal effort

Paternal care can promote the health and survival of offspring, but, like mating effort, consumes time, energy, and attention. While at least some forms of paternal effort can double as mating effort, male primates generally face tradeoffs between seeking new mating opportunities and investing in current offspring. Testosterone appears to mediate this tradeoff, increasing during times of high mating investment and decreasing during offspring care.

Maternal relief

Paternal care can reduce the mother's energy expenditure. In addition to this advantage for females, infant carrying can benefit males by decreasing inter-birth intervals.

Hormones such as testosterone, oxytocin, prolactin, and glucocorticoids play a role in paternal care in primates

Testosterone

Testosterone is generally lower in fathers than in men who are not fathers. In North American studies, testosterone concentrations are lower in pair-bonded men and fathers than in single, childless men. However, in some non-North American studies, no differences have been found in the testosterone concentrations of fathers and non-fathers. Testosterone concentrations were lower in fathers than non-fathers in the Hazda, who are highly paternal, but not different in the nearby Dagota, who spent little time with young children. This suggests that the extent of paternal contact is important rather than just fatherhood status.

Testosterone concentrations were lower in fathers than non-fathers in the highly paternal Senegalese and Filipino men. Lower testosterone is generally more closely linked to paternal responsiveness. However, testosterone increases in these contexts may simply reflect the men's discomfort during fairly novel or unpleasant interactions or may be involved in helping men counter any external threats to their infants.

Oxytocin

Oxytocin is higher in fathers than non-fathers. Intranasal oxytocin has been found to increase paternal responsiveness and to increase fathers' responsiveness in a 15-minute play-bout relative to the same fathers given a placebo. Intranasal oxytocin also increased fathers' ability to provide structure and reduced fathers' hostility during the interaction.

Prolactin

Prolactin is known for its role in promoting maternal behaviours in mammals and in promoting parental care in males and females of biparental bird species. Prolactin has been suggested as being involved in the transition from a non-paternal to a paternal state in male mammals. Prolactin has been found to be higher in fathers than in non-fathers in a sample of highly paternal men in the Philippines.

Glucocorticoids

Glucocorticoids are involved in coordinating reproductive and parental behaviour between mates. Glucocorticoid levels of male cotton-top tamarins increase at about the same time as female levels increase during pregnancy. Glucocorticoid concentrations were higher than cortisol concentrations during the mid-gestation peak in expectant male cotton-top tamarins, results that contrast with the general view that cortisol is the more important glucocorticoid in primates.

The evolution of paternal care in primates is influenced by social monogamy, infanticide risk, and mating competition

In primates, paternal care is influenced by the type of mating system exhibited by each species. Mating systems influence paternity certainty and the likelihood that a male is providing care towards his own biological offspring. Paternity certainty is high in monogamous pair-bonded species, and males are less likely to be at risk of caring for unrelated offspring and compromising their fitness. In contrast, polygamous primate societies create paternity uncertainty, and males are more at risk of providing care for unrelated offspring.

In socially monogamous species, paternal care is often provided in concert with the mother (biparental care). Biparental care allows the energetic demands to be shared between both parents, which can improve the number of surviving offspring a male produces. Paternal care of infants has been observed in many of the general classifications of non-human primates, including strepsirrhines, platyrrhines, cercopithecoids, and apes.

Infant carrying is the best-documented form of paternal care in non-human primates. Infant carrying by males reduces female energy expenditure and can benefit males by decreasing inter-birth intervals. There are species differences in when and how much primate fathers carry their young, with some fathers carrying infants almost exclusively from birth.

Mating competition also plays a role in the evolution of paternal care in primates. In some species, males provide care for infants to increase mating opportunities with females. This is known as the "mating effort hypothesis." In these cases, males are more likely to engage in affiliative behaviors with the infant in the presence of the mother as a form of male mating effort to enhance future reproductive success.

Paternal care in primates can be influenced by social experience and early parental loss

Paternal care in primates is influenced by social experience and early parental loss.

Social Experience

The social experience of fathers influences their paternal care. For example, in some primate species, fathers with more experience carry their young more frequently. In other species, fathers with more experience show a significant decrease in serum testosterone concentrations within 20 minutes of contact with an isolated scent from their own infant. In humans, fathers with more experience show a greater decrease in prolactin concentrations after interacting with their toddlers.

Early Parental Loss

Early parental loss also influences paternal care. For example, intranasal oxytocin produced differences in men's stress responses depending on whether or not they had experienced early parental loss.

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