None of the theories we have discussed explains convincingly why marriage is the only or the best solution to a particular problem. In addition, some comparative evidence on mammals and birds casts doubt on those theories.15 How can evidence from other animals help us evaluate theories about human marriage? If we look at animals that, like humans, exhibit some stable female-male mating, as opposed to those that are completely promiscuous, we can perhaps see what factors may predict male-female bonding in the warm-blooded animal species. Female-male bonding occurs in most species of birds and some mammals, such as wolves and beavers. Among 40 mammal and bird species, neither division of labor, nor prolonged infant dependency, nor greater female sexuality correlates with male-female bonding in the direction predicted by the theories discussed earlier. The birds and mammals studied have nothing comparable to a humanlike division of labor, but they exhibit female-male unions nevertheless. The two other suggested factors—prolonged infant dependency and female sexuality—relate to male-female bonding in the opposite direction to what was expected. Mammal and bird species that have longer infant dependency periods or more female sexuality are less likely to have male-female bonding.
Does any factor predict male-female bonding among other mammals and birds and thereby also help explain human marriage? It appears that animal species in which females are able to simultaneously feed themselves and their babies after birth ( postpartum) tend not to mate stably. On the other hand, species in which postpartum mothers cannot feed themselves and their babies at the same time do typically form male-female bonds. Among the typical bird species, a mother would have difficulty feeding herself and her babies simultaneously. Because her young cannot fly for some time, the mother risks losing them to predators if she leaves the nest untended to search for food. If a male has bonded with her, as males in most bird species do, he can bring food or take his turn watching the nest. Among animal species that have no postpartum feeding problem, babies are able to travel with the mother almost immediately after birth as she moves about to eat (as do such grazers as horses), or the mother can transport the babies as she moves about to eat (as do baboons and kangaroos). We believe the human female has a postpartum feeding problem. When humans lost most of their body hair, babies could not readily travel with the mother by clinging to her fur. And when humans began to depend on certain kinds of food-getting that could be dangerous, such as hunting, mothers could not safely engage in such work with their infants along.16
Research by Frank Marlowe on the Hadza foragers of Tanzania appears to support our view of female-male bonding. Hadza women and men both forage for food, with the women providing more berries and other plant foods while the men provide more meat and honey. Generally, it appears that Hadza women could support themselves and their children without a mate. Married women forage more regularly and contribute more food (in terms of calories) to the family diet, except when they have a nursing infant. Marlowe found that the father of an infant made up for the mother’s lower food contribution while she was nursing. In fact, Hazda fathers with nursing children contributed significantly more food to the household than fathers with older children.17
Even if we assume that human mothers have a postpartum feeding problem, we must ask if marriage is the most likely solution to the problem. It may well be, because other conceivable solutions probably would not work as well. If a mother took turns babysitting with another mother, for example, neither might be able to collect enough food for both mothers and the two sets of children dependent on them. A mother and father share the same set of children, making it easier for them to feed themselves and their children adequately. Another possible solution is males and females living in a promiscuous group. We believe that a particular mother in that kind of arrangement would probably not be able to count on any male, if he didn’t think he was the father, to watch her baby when she had to go out for food or to bring her food when she had to watch her baby. The need to solve the postpartum feeding problem by itself helps to explain why some animals, including humans, have relatively stable male-female bonds.18 Can research of this kind with other animals be applied to human beings? We believe it can, though not everybody will agree.