While pair bonding is present across the animal kingdom, from insects to reptiles, and is dominant in birds the behavior is generally rarer with mammals.  Within our own mammalian primate lineage, only a few monkey species and Gibbons have developed mating systems closely resembling monogamy. Most apes are polygynous or have multi-male, multi-female mating systems. The uniqueness of this behavior among mammals calls into question why monogamy developed among the few species it did and whether humans are truly biologically monogamous.

Theories as to why monogamy developed in several mammal species vary widely. Some argue that resource scarcity would cause females to be spread over large territories making it adaptive for males to stay with females when found. Others argue that the adaptive advantage of having multi-parental care outweighs the reproductive cost of mating with predominantly one female. Perhaps the most interesting (and violent) theory deals with the propensity for sexually competitive primate species to go through waves of infanticide. When new males take over groups of females it is adaptive for them to kill off the infants of his predecessor so as to prompt ovulation in the females for his own reproduction. Monogamy would prevent this by diversifying the males who could mate as opposed to dominant male control of all reproduction.

While it is unclear which dominant adaptive trend prompted monogamy in the human lineage, it is possible to test how biologically monogamous humans are. In non-monogamous species, male genes and female genes go through molecular arms races in the womb of the growth of the fetus. Since males will likely not continually reproduce with the female it is adaptive for their genes to try and leech out as much growth from the female as possible through growth factors. Females on the other hand would rather not invest all their metabolic energy into one child but rather have multiple children so they develop suppressants for the male growth factors. This molecular arms race doesn’t exist in pair bonding species since the male will be continually reproducing with the same female and has no incentive to diminish the health of the female by creating an individual super-grown child. Scientists can look at the number of these growth factors/suppressors of growth factors present during human development and compare them to the numbers of these growth factors present in trophy and pair bonding species. When tested, humans appear to be midway between the two suggesting that either the shift to monogamy happened more recently in the human lineage or we are biologically less inclined towards monogamy than other pair bonding species.

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