Go to any bar and you’ll see a lot of males standing and sitting around not mating. I’ll bet you would have guessed that the reason they are not mating is that no females will mate with them for one reason or another. But there is the distinct possibility that they are very inconspicuously resisting mating opportunities. It turns out that males can do this …. avoid mating without conspicuous resistance … more easily than females. For obvious reasons.
This could be why what has become (inappropriately) known as “reversed sexual aggression” often goes unnoticed, and a recent study of the African antelope Damaliscus lunatus (a.k.a. “topi”) explores this possibility.
Consider mammals. Mammals have internal fertilization, so there is little opportunity for males to make much of an investment in offspring. Females gestate the young and then lactate to provide additional nutrition. So, females end up making most of the investment in offspring, or at least, a lot more than males do, in many species of mammal. Also, it is physically possible for a male to inseminate a fairly large number of females all of which can theoretically have that male’s offspring, while females reproduce at a much slower rate with fewer mates.
Herein lies the basis for most of the persistent sex differences we see in mammals. However, the totally obvious nature of this relationship between mammalian males and females, and the dramatic way in which it usually plays out, has caused scientists to loose sight of the fact that males do in fact pay reproductive (especially mating) costs. The article we are looking at now speaks mainly of sperm depletion, but there are two other major and obvious (though often overlooked) costs that males may suffer. One is direct competition. Mating may seem like a good idea at a particular moment, but the other male, the one that is not you, but has a similar idea and big antlers, horns, tusks, or claws, may hurt you for even having that thought.
I am told that for every male monkey on Cayo Santiago, a major Macaque colony off Puerto Rico, there is something like 1.7 testicles. The costs of mating can be significant.
The other major cost is the risk of venereal disease. It simply is not the case, despite rumors to the contrary, that venereal disease is a curse placed on certain humans by god. It can be safely assumed that all animals that have sexual contact are host to a suit of microbes that use this contact as a means of dispersal. When the microbes (and I use that term loosely … I mean viruses, bacteria, and protists, yeasts, fungi, the whole shebang) make you sick, it is called a venereal disease. When they don’t, well, we don’t call it anything generally because we don’t even know about it.
Anyway, there are real costs, and as a result of this, it is not always case that the optimal mating rate for males is infinity (though it sometimes seems that way). Since females are, and should be, choosy about males they mate with, they may also be in competition with each other. So the pattern of demure females observing males in competitive tournaments (head butting, horn locking, pissing contests, etc.) is not the only possible pattern.
According to the author of this report…
…in promiscuous species, females might benefit from high mating rates as a result of increased conception probability with favored males, whereas favored males benefit from mating selectively because of sperm depletion. When this results in higher optimum mating rates for females than for males, there is potential for reversed sexual conflicts between persistent females and resistant males. Here I report evidence of such a reversed sexual conflict in a promiscuous antelope, the African topi. Rather than mating randomly, favored males prefer to balance mating investment equally between females as predicted by strategic sperm allocation theory. Females, however, enhance their probability of mating with favored males through aggression toward mating pairs.
If a female is likely to mate with multiple males during one reproductive bout or season, there will be sperm competion. Sperm competitive capacities are thus selected for, so it is in the interest of a female to enhance competition as much as possible. The best way to do this is to mate with more males.
From the male perspective, it may make sense to mate many times with one female (lots of sperm) but it also makes sense to avoid mating with a previously mated female and mate with a new, different female. The male is weighting the trade off between winning the Sperm War being waged within one female on one hand vs. engaging in a novel opportunity on the other.
This leaves open the possibility that the female optimum and the male optimum are in conflict in the “opposite” relationship than they usually are in mammals.
This paper is a fairly sophisticated yet understandable exposition of a model of these conflicts. The system is described as having two theoretical traits … persistence and resistance. Commonly, among mammals, one expects persistence to be favored in males and resistance (choosiness) to be favored in females, but it would be incorrect (possibly) to assume that only one trait exists in each sex. Both exist, but one is typically overwhelmingly expressed (the traits in a sense, compete within the model).
Topi lek. Yes, that is a sentence. Topi have a lek system of mating, which is where males hang out on a “lek” (a place of no consequence other than as a breeding ground) and compete for position within the lek. Position is thought to reflect quality. The animals may also display traits that also reflect quality. Females pick a male to mate with on the lek. In many leking species, all the females tend to pick one or a small number of males. It could be partly because in Topi the females come into season almost at the same time (over a few weeks) that they actually mate with a larger number of males … sperm competition comes into play. Having a system with both lekking and sperm competition, and seasonal mating to boot, is fairly uncommon.
Under these conditions, you have such intense sperm competition that resistance may be selected for in males, and persistence in females. That is indeed what seems to happen with the topi.
BRO-JORGENSEN, J. (2007): Reversed Sexual Conflict in a Promiscuous Antelope. Current Biology, , doi:10.1016/j.cub.2007.11.026 .