Two papers published this week have proposed explanations regarding the evolution of social monogamy among mammals and especially primates.  Of three competing hypothesis, one proposes that a driving force in establishing social monogamy was the protection of offspring by preventing male infanticide, while the other proposes that social monogamy is the result of female intolerance towards each other and a low population density that simply prevents males from adequately "guarding" females from other males (1).

Both papers agree that the idea of paternal care is a consequence of social monogamy rather than a cause.

However, I would argue that the issue is actually more complex than this, and that neither of these proposed explanations is sufficient to account for social monogamy. Clearly both conditions can and do exist in various species, so they undoubtedly exert an influence, but they are both insufficient [on their own] to explain the results.

I would argue that the first problem that needs to be addressed is whether or not a male presence is sufficient to "guard" the female from other males.  In considering polygyandrous and monogamous societies, the clear answer is no, since males are often killed in such competitions, or at least ousted from their dominant roles.  As a result, there is no reason to believe that a male "guarding" an individual female has a greater chance of success than a male "guarding" a harem.

In addition, the role of infanticide is directly linked to this result since the removal of the "protecting" male simply provides the means for an interloper to kill any offspring that haven't been weaned and accelerate the female's return to oestrus.  Of note here, is that infanticide is a direct evolutionary result of requiring the female be in oestrus and that such a state can be specifically detected.

Both of these scenarios fail to address the fundamental difficulty in that they often arise in spite of the presence of a male, rather than because of a male absence.

An additional difficulty arises because both scenarios fail to account for female choice.  What makes this relevant is that the idea of intolerant females towards one another and a low female density, suggests that the problem of male "guardianship" is the result of a fundamental lack of female cooperation.  In other words, the female will mate with an available male while the supposed "guardian" isn't around.  

Such a condition renders the male strategy for reproduction much more risky, since it introduces the element of paternal uncertainty, since such a male can clearly not arbitrarily engage in infanticide to protect his own genetic lineage, since he has no way of knowing whether any offspring are truly his own versus those of an interloper.

As a result, it would appear that there are two strategies available to a male under such conditions.  The first being that he simply seeks opportunities for mating without regard for offspring, offering no parental investment at all.  In these cases, he is simply a marauding male that will mate at every opportunity.  Without further exploration, I would argue that such a strategy introduces more risks than benefits, and would not likely result in any increase in the number of offspring (2). The second choice would be in agreement with one of the proposed hypotheses in confining himself to a smaller selection of females [or one].  Yet this is also somewhat problematic because without a cooperative female, the male may well have restricted his reproductive opportunities with no greater assurance of paternity.  In the case of an "unfaithful" female, the male will have even fewer assurances than simply pursuing the first strategy.

This strongly suggests that social monogamy is the result of female choice, while the results proposed by both hypotheses are certainly involved, as with other explanations, they are not the cause as much as they are a consequence.

One explanation that has been offered is that social monogamy is also a strategy that could be employed by those that would normally be considered the "losers" in male competition by affording them an opportunity to focus on a single female to the detriment of a dominant male seeking to control a harem.  Such a result was suggested in an earlier paper describing how the transition from promiscuity to pair-bonding might originate.  In this scenario, the male assists in providing resources to the female in exchange for sexual access, thereby influencing her choice.

Yet, there is another factor that must be considered, since a pregnant female is clearly incapable of being fertilized again, and when interbirth intervals may last several years, then clearly there must be something that compels a male to remain (3).

In addition to female choice, the other factor to consider is that in socially monogamous societies, the signaling of female fertility is reduced or eliminated.  This prevents any inference regarding a female's ability to become pregnant and introduces an uncertainty that requires persistent and repeated sexual encounters.  However, it is equally clear that none of these creatures is cognitively aware of the relationship between sex and reproduction, therefore such a shift must serve a different evolutionary purpose.  It is my argument that the additional impetus to enforce social monogamy was the evolution of sexual pleasure (4).

In this case, even the female is generally unaware of her own specific fertility status, and sexual pleasure becomes the mechanism that produces the monogamous result.  Since such creatures will even indulge in self-gratification, it is clear that this is a powerful motivator in which access to sex is significantly more important than merely seeking to maximize reproductive results.

In fact, this is such a standard interpretation of sex, that it is often overlooked that in most sexual species, there is little indication that pleasure is a necessary ingredient.  However, in mammals one can clearly see the differences when sex is the result of hormonal stimulation, such that even inexperienced males can be aroused by a female's broadcasting of her fertility status.  In the case of the socially monogamous animals, where such broadcasting doesn't occur, the pleasurable dominance of sex must occur cognitively.  In other words, such creatures must consider sex as an end unto itself without regard for fertility.

As a result, such an evolutionary transition reduces, if not eliminates, the need to bring the female into oestrus since that is no longer the motivating factor in mating.  This eliminates or reduces the risks of infanticide and ensures a greater likelihood of a persist male presence with a cooperative female.

In conclusion, it is my hypothesis that sex was selected to be pleasurable precisely because direct signaling of fertility introduced far more problems than it solved in some species, whereas an indeterminate fertility status and the elevation of sex to be pleasurable as an end result, removed most of the factors that resulted in the original difficulties being considered.  As a result, while there may be numerous reproductive scenarios wherein a particular male may be capable of dominating a group of females and enjoy great reproductive success, the role of female choice must be considered.  From this the "lesser" strategy of committing to a single female over a longer period of time may well have produced a more predictable and persistent strategy that could eventually displace the "roving" male (5).

(1)  Note that "guarding" in this case is not about protecting the female but rather of ensuring a male presence to prevent other males from having reproductive access to the female.

(2)  In this scenario it would seem that the risks associated with predation, infanticide, and the problems of finding a fertile female would ultimately render this a losing strategy.  In addition, it is also clear that such a strategy is ultimately an "elitist" strategy in that the majority of males would fail to achieve such a status.

(3)  The attendant risk in this scenario is infanticide by a new male.  Yet, this is also self-limiting since any male that would routinely abandon a female with unweaned young risks losing his entire reproductive investment, such a strategy clearly may well be the worst of the choices considered so far.

(4)  Note that sexual pleasure is being presented as quite distinct from the question of sexual reproduction.  In the same way that we may obtain pleasure from eating certain foods, then this creates an incentive or motivation on the part of the animal to seek out such foods.  If fertility is not signalled and is uncertain, then the evolution of pleasure in sex becomes the motivation to engage in and persist in the sexual relationship regardless of any knowledge regarding the factors resulting in pregnancy.

(5)  For those that still believe that males evolved to seek multiple partners, the confusion invariably results from considering sex as being synonymous with reproduction.  While men may well seek out multiple partners for the pleasure of sex, it would be rare to consider those same males considering multiple partners for the sake of reproduction.