Saturday, January 19, 2013

Four Observations About Mating

I have been too busy to do much reading recently, so here is something from my Human Nature course.  I offer four observations that lay out one of the general theories of sociobiology. 
Observation 1: Males are larger, stronger, better armed, and more aggressive, across a wide range of species. 
This is not universally true.  Spiders are a conspicuous exception.  There the reproductive machinery resists reduction in size.  Ask a black widow male what he thinks about this.  It is however true of a vast number of species including elk, gorillas, walruses, and human beings.  Why? 
Observation 2: Males compete with other males for reproductive opportunities. 
In most species that I am aware of, almost all of the females can successfully mate.  They may compete with other females for access to the most competitive males, but they get to mate with someone.  By contrast, males will often go without mating opportunities at all.  This is especially true in harem species, where males enjoy access to a considerable number of mates so long as they can defend their position against other males. 
In harem species the sexual dimorphism is rather large.  In promiscuous species, larger males may not enjoy exclusive access but they often enjoy the first opportunity to mate with a fertile female.  In some monkey species, the males compete not with size and offensive weapons like antlers, but with large testes, the better to flush out the work of the last suitor.  In yet another strategy, males compete by features that attract females, such as color or flamboyant tail features. 
In the case of harem species, female choice is limited but not extinguished.  A female may cry out to arouse the dominate male if an interloper appears.  She doesn’t want to mate with the Johnny-come-lately unless he can stand his ground.  Where females can easily flee, female choice is accordingly increased.  Hence the colorful male birds. 
In all such cases, males with weapons leave more offspring than males without or with lower caliber weapons.  Accordingly, they sire sons with the same weapons. 
Observation 3: females are choosy but males are eager. 
Across a wide range of species, males will take any opportunity to mate that they can get, whereas females will resist any suitor that they regard as unsuitable.  While female choice will be more or less constrained depending on the mating system, they will exercise as much choice as possible.  Males will mate with anything that looks vaguely female.  Mount a female turkey’s head at the appropriate level and a male turkey will move behind it and try to mate. 
Observation 4: sperm is cheap; eggs are expensive.  

 The male’s minimal investment in a single reproductive act is often very minimal.  For most males, each mating is chance to sire more offspring.  Hence eagerness to mate is selected for.  For females of many species, the minimal investment in a single reproductive act is very large.  Indeed, the universal definition of female in biology is the sex with the largest sex cell. 
We can see this spread across a number of mating systems.  Where the female lays eggs to be fertilized externally by the male, her investment is relatively small for a female.  However, it is still larger than the males.  In hermaphrodite fish, where each individual produces both eggs and sperm, the eggs are more precious.  This fact results in a social contract called parceling.  When two such fish meet, each lays a parcel of eggs for the other to fertilize and then they repeat the exchange.  A fish that managed to fertilize all the eggs of another without giving up any of its own eggs would be able to cheat many other fish before he/she finally released his/her own cache of eggs. 
Females that develop an egg within their own bodies must devote a lot of time and resources to each reproductive opportunity.  Those that allow the young to develop to live birth must devote even more. 
The result is that the minimal cost of each reproductive act for females is much larger than it is or can be for the male.  More coupling does not result in more offspring.  Accordingly, the female is inclined to make sure that each coupling counts and that means being choosy (when she can) about whom to mate with. 
Thus the four basic observations can be arranged so that each one helps to explain the previous one.  Males are better armed because they compete with other males; they compete because males are choosier than females; males are choosier because their minimal investment is less.  Given Darwinian theory, we can now understand a lot about the sexual behavior of elk and the guys down at the Elks Lodge. 

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