The Origin of Paternal Investment Among Primates

This entry was written by Shaniah Elliott as part of a project done in BIAN 6133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.”


The reproductive success of any organism is only classed as successful if the offspring survives to further reproduce. This survival is dependent on parental investment, whether it is by maternal, paternal or a combination of the two. In most primates, parental investment is primarily maternal with the mother contributing to feeding, carrying and protecting the infant (Smut & Gubernick, 2017). Paternal investment however is variable amongst primates, with contributions ranging from non-existent to primary care. If any paternal investment does occur, it is categorised as direct or indirect investment. Direct investment is any action that is physically expressed to the offspring such as carrying, feeding and grooming, while indirect investment includes actions that benefit the offspring without physical interaction, such as the acquisition of survival resources, and protection from predators (Schacht, Davis & Kramer 2018).

This entry will examine the rise of paternal investment among primates by evaluating hypotheses, which include: the paternal care hypothesis, the mating effort hypothesis, and the maternal relief hypothesis. These three theorisations will be used to analyse how humans conform and differ in paternal investment when compared to non-human primates.

Main Text

The Paternal Care Hypothesis

The goal of engaging in sexual activity amongst nearly all primates, except humans and bonobos, is to increase fitness via reproduction. In most primates, maternal investment is obligate and paternal investment is either obligate or facultative (Schacht, Davis & Kramer 2018).

Obligate paternal investment occurs in primates when male care is necessary for the offspring’s survival. This type of care is associated with obligate biparental care, where both parents must invest in the offspring because the demands of raising the offspring exceed the female’s ability to provide adequate care (Smut & Gubernick, 2017). This obligate biparental care is observed in callitrichids where the females most often produce twins; the adult callitrichids are small in size, so caring for two infants alone would not be successful, and thus male involvement is necessary (Smut & Gubernick, 2017). In biparental care, the parental care is not always evenly split; in most cases, the mother demonstrates higher investment. However, the titi monkey contradicts this investment, as the male directly cares for the infant for 90% of its life and the female only intervenes for nursing bouts. Research has shown that this high paternal investment is associated with a strong pituitary-adrenal stress response in titi monkey offspring when separated from their father (Fernandez-Duque, Valeggia & Mendoza 2009).

Facultative paternal investment is the most common in primates, and it occurs when the offspring survival is not vitally dependent on the male’s care. The male can choose to either abandon the offspring or invest, however the extent of the investment is highly variable. The mating system of the species is usually indicative of the type of paternal investment observed. Pair-bonding primates demonstrate higher paternal investment whether it is necessary for the child or not. This is because paternity certainty is high, and the male knows that any effort expended, is directly benefiting his fitness, therefore making him more likely to stay and provide (Storey & Ziegler 2016). Polygamy mating groups however are influenced by more socioecological factors than monogamy groups. According to Geary (2000), males in polygamy groups are favoured to abandon their offspring if there is no direct effect on the offspring’s survival, low paternity certainty, or if the costs of paternal investment outweighs mating opportunities.

Paternity certainty is a major influencer, as it is costly to the male to invest in something that does not increase his fitness. However, if the male has high paternity certainty but his investment is not necessary, he has two options. He can either leave to find more mating opportunities or stay to indirectly invest in his offspring (Geary 2000). This indirect investment is most commonly in the form of protection from other males in the group, especially from infanticide. It is theorised (Storey & Ziegler 2016) that the risk of infanticide has led to an increase in monogamy and bi-parental care among primates as fathers are more present. As a result, facultative paternal investment is a tug of war between increasing fitness through new offspring or through already existent offspring.

The Mating Effort and Maternal Relief Hypothesis

The cost of raising offspring is often high, which is why males need to weigh the cost of paternal investment against offspring abandonment. The cost of this abandonment is variable amongst mating systems. In tight polygyny groups, like gorillas, the male has no issues with offspring abandonment as his role is facultative and mating opportunities are plenty (Geary 2005). In primates however, it is theorised that if a male finds an earnest female to which he believes mating again will increase his fitness, the male will provide infant care (Geary 2005). The same belief goes if the male has little mating opportunities due to female dispersity. The male provides infant care not only to benefit the offspring’s survival but also to increase his own chance of reproducing again. This reproduction tactic is socially and biologically beneficial for both parents.

Paternal investment demonstrates to the female that the male is of a higher standard as he is willing to contribute and provide resources (Smut & Gubernick, 2017). This aspect of showing off benefits the male as it may give him another chance to reproduce with that female. If the male is chosen to reproduce again with the female, his paternal investment further helps his mating effort by helping the female which is described in the maternal relief hypothesis. Research shows that when males contribute to infant carrying, it greatly reduces the female’s energy expenditure and allows for recovery (Storey & Ziegler 2016). This recovery benefits the offspring, by allowing the female to produce higher quality milk, and the male, by shortening the female’s inter-birth interval, allowing her to return to ovulation faster. It is noted that in obligate biparental care, this maternal relief occurs unintentionally as the offspring’s survival is more important to the male than reproducing again (Storey & Ziegler 2016). Nevertheless, for both theorisations to occur in facultative cases, the male must evaluate the cost of paternal investment versus offspring abandonment.

Human Paternal Investment

Most nonhuman primates have a specific degree of paternal investment that occurs mainly in that species. Humans, however, show great variability, with paternal investment being both obligate and facultative, and heavily influenced by spousal and socio-ecological factors. In the primate world, humans are known to have the longest infancy and maturation period, meaning they also have the longest parental dependency (Fernandez-Duque, Valeggia & Mendoza 2009); therefore, male abandonment cost is higher. Paternity certainty in humans is mostly high and does not greatly influence paternal investment. If the offspring is not of the investing males, it is still possible for paternal investment to occur. This is explained by the mating effort hypothesis, in which the non-paternal male will stay for female mating chance (Smut & Gubernick, 2017). When paternity is known, paternal investment is greatly influenced by spousal relationship. Research by Geary (2000) shows that when spousal conflict arises, the male is likely to withdraw paternal care, while maternal care remains generally unchanged. Whether this withdrawal impacts the offspring’s development is dependent on environmental factors.

Geary’s (2000) research also analysed how resource availability for the child impacts paternal investment. If the offspring is raised in a high resource area, they are likely to experience lower paternal care, with mainly indirect investment via resource provision occurring. High resource areas are also more likely to experience polygynous families due to the male viewing the cost of his paternal investment as low and chooses mating opportunities over paternal investment. The opposite occurs in low resource environments where paternal investment is higher, and mating is less frequent. This occurs because resources are scares and are thus focused on that one offspring’s survival. Geary concluded that in human monogamous circumstances, when social and ecological factors do not impose, males tend to focus on mating, while when these factors do impose, males focus on paternal investment for the offspring’s survival.


Primate paternal investment is a complex inquiry in which level of care is significantly variable among species. However, through the analysis of the paternal care hypothesis, the mating effort hypothesis and the maternal relief hypothesis, researchers can evaluate how paternal investment is higher in primates than other organisms. These theories demonstrate that the cost of male parenting is influenced by both social and ecological factors and that taking on these costs can potentially be more rewarding than withdrawal from the child to find a new mate. In the end, the male must decide whether his existent offspring’s survival is more important that his future offspring.

Literature Cited

Fernandez-Duque, E., Valeggia, C.R., & Mendoza, S.P. (2009). The biology of paternal care in human and nonhuman primates. Annual Review of Anthropology, 38, 115-130.

Geary, D.C. (2000). Evolution and proximate expression of human paternal investment. Psychological Bulletin, 126(1), 55-77.

Geary, D. C. (2005). Evolution of paternal investment. In D. M. Buss (Ed.), The evolutionary psychology handbook (pp. 483-505). Hoboken, NJ, John Wiley & Sons.

Schacht, R., Davis, H.E., & Kramer, K.L. (2018). Patterning of paternal investment in response to socioecological change. Frontiers in Ecology and Evolution, 6, 1-12.

Smuts, B.B., & Gubernick, D.J. (2017). Male-infant relationships in non56human primates: Paternal investment or mating effort? In B.S. Hewlett (Ed), Father-child relations: cultural and bioscience contexts 1950- (pp. 21-51). New York, Routledge.

Storey, A.E., & Ziegler, T.E. (2016). Primate paternal care: Interactions between biology and social experience. Hormones and Behavior, 77, 260-271.

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