Paternal care is rare among mammals and absent among our closest great ape relatives. So why do human fathers provide for their children? With their co-authors, IAST’s Ingela Alger and Jonathan Stieglitz use evolutionary game theory to show that doting dads emerged when ecological change increased the incentives for males to cooperate with others, and to invest in their children’s development.
Why does human fatherhood pose a puzzle for evolutionary scientists?
Paternal care is widespread in modern hunter-gatherer societies, with males helping in various ways, but particularly to feed children for up to two decades. The evolution of such prolonged paternal care seems remarkable. A would-be “dad”, who provides food for his mate and children, risks being outcompeted in terms of biological fitness by a “cad”, who focuses only on promiscuous mating instead of investing in offspring. If cads abound, their competitive advantage creates a formidable barrier for the emergence of dads.
It has been suggested that this barrier was overcome when ancestral females started mating preferentially with males who provided them with food. Yet behavioral observations of chimpanzees, which provide indirect insights into ancestral hominin sociality, are not consistent with this explanation. Despite reports of chimpanzees exchanging of meat for sex, prevailing evidence points to rank and aggression as key determinants of male reproductive success. Even if buying sexual access with food is an incipient reproductive strategy, there is still the “cad vs dad” tradeoff to overcome. This scenario also requires simultaneous evolutionary changes in both sexes.
How did prehistoric climate change impact the balance of power?
Around 5-8 million years ago, the African savanna began to dry out, increasing the value of nutritious, diverse, dispersed, and hard-to-obtain plant and animal foods. Ancestral hominins adapted to this in various ways, embracing bipedal locomotion, dietary flexibility, and tools to thrive in diverse environments. Climate-induced changes in the profitability of different ecological strategies likely selected for increased brain size, greater time devoted to learning and cultural innovation, and lengthening of the juvenile period.
In these conditions, cooperation significantly increased per-capita benefits. Synergies, or “complementarities”, between males and females resulted from specialization in hunting, foraging and childcare. Protein and fat acquired by males paired well with carbohydrates acquired by females. Complementarities also arose between males due to higher returns from hunting in groups, and from sharing food to lower starvation risk.These ecological and cooperative strategies favored the evolution of caring fathers, as the impact of food provision on their children’s survival substantially increased.
How do you test the impact of ecological change?
We use evolutionary game theory to show how cooperation between women and men, and among men, can select for male provisioning. We identify a tipping point where gains from provisioning overcome costs from paternity uncertainty and the dad strategy becomes viable.
Our model reconstructs the behavior of two male types: “cads,” who do not provision offspring and mate with multiple females; and “dads,” who provision offspring and mate with only one female. In a population predominated by cads, a dad paired with a cad would have lower reproductive success. But when ecological change increases the benefits of cooperation, our model reveals that dads can gain a fitness advantage over cads. If sons inherit their biological father’s traits, then over time dads will increase in number. Our simulations suggest that, with sufficient complementarities, dads can emerge even in the face of high paternity uncertainty. Female sexual infidelity, it seems, is not an insurmountable obstacle to the evolution of paternal care, in contrast to some traditional evolutionary explanations of human pair bonding. Stable polymorphic states are also possible, meaning that dads need not necessarily eliminate cads.
What are the avenues for future research?
Our research focuses attention on critical missing pieces of paleo-climatic, archaeological, and genomic evidence about the emergence of paternal care. Increasingly sophisticated methods, such as radioisotope analysis, can provide information about diets of ancestral hominins. Since hunting requires high levels of strength and skill, and is regularly performed by men cross-culturally, evidence of animal products in the diet of juveniles could suggest provisioning, perhaps by ancestral dads. There is also growing evidence of joint neural, hormonal, and behavioral responses underlying the transition to fatherhood in modern humans, including lower testosterone production and sexual drive, higher oxytocin production to facilitate bonding, and increased activation in brain regions important for face emotion processing. Paleogenomic data may shed light on the timing and nature of these changes.
Our approach could be extended in several directions. We might investigate, for example, complementarities in tool production and domestic tasks. We could model the active selection of mates by females, or the fitness benefits of female infidelity. Would our results change if males interacted in larger groups? How would the presence of grandparents affect tradeoffs? The model may also be applicable to modern trends, such as the matrifocal family with limited paternal investment.
Extract from IAST Magazine #17 summer 2021
Photo by Steven Van Loy on Unsplash