Say what you will about Greg Cochran, but one thing is for certain: he’s certainly not one to present his opinions meekly. Recently, Greg posted his feelings about the relevance of sperm competition in humans, and he is of the mind that sperm competition is not terribly important for humans. For those of you not in the know, sperm competition refers to conditions under which a female has mated with more than one male at a time during which she might conceive. As the name might suggest, the sperm from different males can be thought of as “competing” to fertilize the egg in question (or eggs, depending on the species). The question of interest, for our present purposes, then, is whether or not such conditions (a) might have existed in ancestral human populations and (b) have been important enough to potentially drive male adaptations for solving the problem and winning the competition.
Greg begins his post by making, what I feel, is an poor point, writing: “The non-paternity rate is an upper limit to the rate of sperm competition”. Unless I’m misunderstanding his meaning here, the rate of non-paternity (when a child is being unknowingly raised by a male that is not their father) would not be not the upper limit for sperm competition unless every single instance of sperm competition resulted in non-paternity. Presumably, there are many instances of sperm competition that the in-pair male would win, meaning that the non-paternity rate would be an underestimate of how much sperm competition there might be. The upper limit of sperm competition should, it seems, be the infidelity rate (how often men are being cheated on by their partners) or the number of times when report having sex with more than one male within the period of a few days. Depending on what numbers one wants to use there, the potential amount of sperm competition that might exist can go up dramatically.
On the other hand, however, not every instance of non-paternity results from sperm competition: sometimes a female might cheat on her partner while also not sleeping with him, allowing the rival’s sperm a competition-free environment. In such cases, non-paternity rates would overestimate the amount of sperm competition that exists. As to which set of issues are more common, I can’t say. There’s also the matter of how contraceptives and abortion might affect the issue, but I won’t consider them further. So while the non-paternity data is certainly informative in some sense, it’s far from what we might consider a complete picture when it comes to sperm competition. In any case, I’ll use Greg’s numbers of approximately 2% non-paternity on the whole across human populations and across time, just for the sake of argument.
On top of “how common is sperm competition”, then, another question we want to consider to get a full picture of the issue is, “how important is non-paternity?” As Greg notes, humans aren’t built for sperm competition in the way that, say, a chimpanzee male is. However, the corollary question he fails to consider concerns whether or not the consequences of non-paternity are identical between humans chimps. For a chimp, male paternal investment tends to reach the heights of not killing infants; for human males, investment might involve decades of protecting and provisioning. On that front, while pair-bonded human males might be far more assured of their paternity than your average chimp, the consequences of being mistaken that respect are also far larger for humans than chimps. In humans, a little bit of sperm competition goes a long way, so to speak. In such cases, a simple comparison between how much sperm competition exists will miss the bigger picture.
So we should expect the non-paternity rate to be low, in some absolute sense, as the consequences to non-paternity are so high; if non-paternity was too high, pair-bonding strategies would be unlikely to evolve in the first place, or persist once they had. Now, as Greg also mentions, there are likely adaptations humans possess to deal with the non-paternity issue that do not deal with sperm competition. As Greg so eloquently put it:
If, for example, your old lady knows that you will knock her block off if she strays, that deters all kinds of paternal uncertainty, not just those involving sperm competition. Your jealousy might also deter other guys from trying – adaptations for sperm competition don’t do deterrence.
This is certainly true enough: if a female partner knows that her straying will lead to physical aggression or the withdrawal of investment, she might be pressured into not having that affair; the same goes for rival males. However, aggression is not always the smartest strategy, as aggression carries costs. That rival male you seek to deter might well be bigger and stronger than you, and on top of being cheated on, you might also end up with an ass-kicking if you tried anything. Women may also have friends and family that prefer you didn’t hurt her, thank you very much. So while sperm competition doesn’t serve a deterrent function, it also avoids costs associated with the aggressive deterrent function. Further, if deterrence fails, for whatever reason, sperm competition might also be able to serve as a secondary buffer against the non-paternity outcome.
This also raises the follow question: is non-paternity low, at least in part, because opportunities for sperm competition are rare or are they low because counter strategies by pair-bonded males are relatively effective? Perhaps the non-paternity rate might be substantially higher if in-pair males had no way of effectively mitigating the risks of sperm competition from rival males. After all, given that any particular sexual act is unlikely to result in conception, 2% non-paternity implies there were likely many more opportunities for non-paternity that were not realized. Now, to be clear, I’m not saying that we should expect human males to be absolutely chock-full of adaptations for sperm competition, nor am I suggesting that we show evidence of one particular strategy or another. I just want to point out some of the nuances of the argument for or against it that I think Greg gets wrong, or at least fails to discuss.
Adaptations for sperm competition might be more subtle than larger testicles, for instance. Perhaps the frequency of sex – or at least the frequency and intensity of sexual interest – correlates with infidelity cues; perhaps the number of sperm per ejaculate could be varied facultatively as a function of sperm competition risk. As Shackelford, Pound, & Goetz (2005) put it, humans might not show some hallmarks of adaptations designed for persistently-high levels of sperm competition (as such contexts were not persistent themselves, as they might be in chimps, for instance), but might show evidence of adaptations designed for contexts in which sperm competition risk is temporarily elevated. These adaptations might not be readily detected, but to write off their existence entirely on the basis of low-non-paternity rates itself would be premature.
The question in my mind is not one of whether sperm competition matters or not for human populations, but rather to what degree it has. Dichotomizing such variables (“sperm competition was/was not important, depending on what precisely important means”) is unlikely to help us get a full picture of sperm competition in humans specifically or other species more generally. After all, chimps, humans, and gorillas were all descendent from a common ancestral species, and that species was unlikely to display the full range of mating behaviors evidence by all it’s future relatives. Some small initial degree of sperm competition must have been sufficient to get the ball rolling on later adaptations for such conditions. Now perhaps humans have been facing less sperm competition over time, and what we see are the degraded remnants of previous adaptations designed to deal with more of it. Then again, perhaps our species has gone the other way, or perhaps we have some adaptations designed for some modest levels of sperm competition. To be sure, some hypothetical adaptations for dealing with sperm competition will be wrong, and people’s estimates of how common it is or isn’t might be way off. I just wouldn’t close the door on the matter (or claim, as Greg does, that such adaptations “don’t exist”) because of that.
References: Shackelford, T., Pound, N., & Goetz, A. (2005). Psychological and physiological adaptations to sperm competition in humans. Review of General Psychology, 9, 228-248.