Exam Notes on Social Instinct & Gender Roles
Humans as Unusual Mammals
Humans are mammals, yet their social behaviors are notably distinct when compared to other mammals. These differences include:
Cooperative breeding: Unlike most mammals, humans often engage in cooperative breeding, where individuals other than the parents help raise offspring.
Extensive social networks: Human societies are characterized by complex social structures and networks that play a crucial role in child-rearing and support.
Mammal breeding females:
68% are solitary.
9% are socially monogamous.
23% live in social groups.
In over 90% of mammal species, infant care is exclusively by the female. This is in stark contrast to humans, where paternal and alloparental care are significant.
Humans are unique among primates because human fathers provide some care. This paternal involvement is a key element in human social structure and child development.
Variability in Paternal Care
Paternal care varies significantly across societies, influenced by cultural norms, economic factors, and ecological conditions. However, a mother's presence consistently shows a greater impact on child survival than a father's presence.
Sear & Mace (2008) study:
45 studies from natural fertility populations.
The mother had a positive effect on child survival in all studies, underscoring the critical role of maternal care in human societies.
In 8/15 populations, the father's death had no impact on child survival, suggesting that in these contexts, other factors or caregivers compensate for the absence of the father.
Examples of differences in paternal care in geographically close societies:
Hadza men spend time playing with and holding their children, indicating a high level of paternal involvement and bonding.
Datoga view childcare as ‘women’s work’, reflecting traditional gender roles where childcare is primarily the responsibility of women.
Paternal care is often traded off against mating efforts. This trade-off is influenced by hormonal and evolutionary factors.
Testosterone:
Linked to mating efforts in other species. Higher testosterone levels are often associated with increased mating behavior and reduced paternal care.
The extent of paternal care is associated with basal testosterone levels. Men with lower basal testosterone levels tend to exhibit higher levels of paternal care.
Why Males Typically Invest Less
Not simply because only females can become pregnant and lactate (circular argument). The reasons are more complex and rooted in evolutionary biology.
Paternity uncertainty:
Males are less certain of offspring paternity. This uncertainty can reduce the likelihood of paternal investment, as males risk investing in offspring that are not their own.
Investing in foreign offspring is an evolutionary mistake. Natural selection favors males who invest in their own offspring, leading to behaviors that increase paternity certainty.
Limits on reproduction due to anisogamy:
Anisogamy: sex cells differ in size (females produce large gametes, males produce smaller ones). This fundamental difference in gamete size has significant implications for reproductive strategies in males and females.
Females are generally more limited by resources. The larger size of female gametes means that females invest more energy in each offspring, making them more selective in their choice of mates.
Males are generally more limited by access to females. The smaller size of male gametes allows males to potentially produce more offspring, making them more competitive for access to females.
Ardent males, choosy females. These differing reproductive constraints lead to males being more competitive and females being more selective.
Seeking additional mates is often traded off against investing in paternal care. This trade-off is influenced by the adult sex ratio.
The outcome of this trade-off is linked to variation in the adult sex ratio.
When females are abundant relative to males, males prioritize mating effort and invest little in offspring. This is because males can easily find new mating opportunities.
When females are scarce relative to males, males stick with any female partner and defend her, leading to the evolution of social monogamy. In this scenario, males invest more in offspring to ensure their survival.
Paternal care can evolve as a by-product once social monogamy exists. This suggests that social monogamy paves the way for the evolution of paternal care.
Phylogenetic analyses indicate monogamy evolved first, then paternal care. This provides further evidence that social monogamy is a precursor to paternal care.
Male-biased adult sex-ratio leads to “sex-role” reversal.
Jacanas, coucals and several other shorebirds, where females are larger than males and where males do most/all of the work to raise offspring. These species provide examples of how sex-role reversal can occur when the adult sex ratio is skewed.
Biases in sex ratio have been linked to male mating strategies in humans.
Historical experiment in Australia in the 18th and 19th centuries where more men than women were sent to penal colonies. This situation led to changes in marriage patterns and labor force participation.
In more male-biased areas, men were more likely to be married, and women were less likely to work. This reflects the increased competition among men for marriage opportunities and the decreased need for women to work in male-dominated societies.
In Makushi (indigenous population in Guyana):
Some communities have biased adult sex ratios, providing a natural experiment to study the effects of sex ratio on mating behavior.
Where women > men, men tend to report being more interested in casual relationships, less interested in long-term commitment. This suggests that when women are abundant, men are more likely to pursue short-term mating strategies.
Where men > women, there is no difference in men and women’s willingness to engage in uncommitted sex. This indicates that when men are abundant, both men and women may be more open to casual relationships.
Ancestral Human Mating System
Humans are unusual among mammals in that males invest anything at all into offspring. This paternal investment is a key feature of human social behavior.
The extent of paternal care is quite variable across human societies, influenced by cultural and ecological factors.
Monogamy and paternal care are more likely when males are relatively abundant compared to females. This is because males need to invest more to secure and maintain a mate in such conditions.
The ancestral human mating system likely involved monogamous pair bonds but may have been serially monogamous. The exact nature of early human mating systems is still debated, but evidence suggests a tendency towards pair bonding.
Insights come from:
Testes size relative to body size. Testes size can provide clues about mating behavior in different species.
In species where females mate with multiple males, males tend to have bigger testicles. Larger testes size is associated with increased sperm production, which is advantageous in sperm competition.
Humans are between gorillas and chimps in terms of relative testes size, but much closer to the gorilla end of the spectrum, suggesting human females were unlikely to have been as promiscuous as chimp and bonobo females, hinting at pair-bonding. This suggests that human mating systems have likely involved some degree of pair bonding.
Sexual size dimorphism (SSD). The degree of difference in size between males and females can indicate the intensity of competition among males for mates.
Gorilla males are much larger than females, reflecting the mating system where one male defends access to several females. In highly polygynous species, males tend to be much larger and more heavily built than females.
Human males and females are much more similar in size. This reduced sexual size dimorphism suggests that competition among human males for mates may have been less intense than in species like gorillas.
There is some evidence that SSD has declined over hominin evolution. This trend may reflect changes in mating systems and social structures over time.
Our mating markets were likely a bit more colorful than lifetime monogamy. While pair bonding may have been common, humans likely engaged in a range of mating behaviors.
One possibility: largely monogamous pair-bonds, perhaps with some short-term additional mates. Serial monogamy is also a possibility. These variations in mating behavior may have been influenced by factors such as resource availability and social norms.
Contemporary human mating systems are diverse. Human societies exhibit a wide range of mating systems, from monogamy to polygamy.
Polygynous marriage is sanctioned in > 80% of societies. Despite this, most individuals are monogamously married/pair-bonded.
However, most individuals are monogamously married/pair-bonded, suggesting that monogamy remains the most common mating system in human societies.
Extra-pair paternity:
Studies with DNA testing in industrialized societies estimate extra-pair paternity to be 1.7 – 3.3 %. This indicates that a small percentage of children are not biologically related to their presumed father.
In forager and other non-industrial societies, EPP can be < 9 %. Extra-pair paternity rates tend to be lower in societies with strong social norms around fidelity and family.
Both estimates are lower than EPP rates in socially monogamous birds ( 20%). This comparison highlights the relatively low rates of extra-pair paternity in humans compared to other monogamous species.
Relatively high paternity certainty can help favor paternal investment in offspring. High paternity certainty increases the likelihood that males will invest in their offspring.
Importance of Family
Human mothers would have been embedded in vast networks and children would have been raised by many caregivers, not just the mother. Alloparental care is a crucial aspect of human child-rearing.
The Western model of the self-sufficient nuclear family which emphasizes the parental unit and the role of the mother in raising offspring, is a glaring outlier both cross-culturally and when considered in broad historical perspective. This model is not representative of most human societies throughout history.
Attachment theory
Attachment theory emphasizes the role of secure attachment to a primary caregiver (mother) in producing emotionally stable and well-adjusted children. While attachment to a primary caregiver is important, it is not the only factor in child development.
Attachment theory fails to consider our history as a profoundly social and cooperative species. Human child-rearing has always involved multiple caregivers and social support networks.
Mothers are important, but human children would have been raised by several other family members, not just the mother. Alloparental care from family members significantly contributes to child well-being.
Longitudinal data show that children who get to experience multiple social relationships (e.g. by attending daycare) either develop the same as those who are exclusively reared by mother or show fewer emotional and conduct problems. This suggests that exposure to multiple caregivers can be beneficial for child development.Some studies show that children in daycare have more developed social skills.