On Thu, 8 May 2003 21:56:06 +0000 (UTC), "Anon."
 wrote:

>Representative Trantis wrote:
>> Two words for you Sperm Competition. It's a whole sub topic of sexual
>> selection. It's a fascinating topic. To summarise, any species which lives
>> in close communities has competition for females by males. That is why human
>> males are so larg, they have to fight with each other. However, the battle
>> doesn't end there, once they have impregnated the woman, they havn't
>> achieved anything unless their sperm fertilises the egg. With some animals
>> the sperm sticks together forming a seal, preventing other sperm from
>> getting to the egg. Other animals keep copulating for a while after
>> ejaculation, to ensure that their sperm is given a head start. With humans
>> it is simply a case of producing loads more than is needed, the old buying
>> more lottery tickets to enhance your chances of winning theory.
>> 
>Ah, but then the "other animals" wouldn't produce so many sperm.
>
>This problem was looked at a few years ago.  The problem was couched in 
>terms of provisioning the new zygote (the cell that's gormed from the 
>fusion of the egg and sperm).  You can produce one few large germ cells, 
>or many very small ones, or somewhere in between.  When they threw the 
>modelling at the problem ,they found that under reasonable assumptions 
>about how fitness is affected by size, you get a dimorphism, so that you 
>either produce a few large germ cells, or a lot of small ones.  The cost 
>of making the smaller germ cells slightly bigger is not sufficiently 
>offset by the additional provisioning, as most of that is done through 
>the larger germ cell.
>
>The classic description is in John Maynard Smith's book "Evolution and 
>the Theory of Games", from about 1982.  The theory there may, of course, 
>have been superceded - I didn't respond immediately because I was hoping 
>someone with more up to date information would.
>
>I think any suggestion which is specific to humans should be discarded 
>as probably nonsense, as it fails to explain the near-ubiquity of the 
>phenomenon - how many of the theories can be used to explain pollen?
>
>Sexual selection might have a role, but that would come into play by 
>defining the fitness function of size - being larger might help in 
>competition, a point that seems to have been forgotten by other responders.
>
>Bob

It is far too easy to make up "just-so" stories without any real
evidence behind the theories, no matter how well-sounding they seem.
Still, there are probably multiple reasons all working together.

Sperm competition is certainly a factor for most species, not simply
humans and not simply animals, either.  Many types of organisms
reproduce sexually with sperm and egg and those that don't still have
"gamete competition".  There is even competition between the sperm
produced by a single individual -- they do have a different genetic
composition even though they all derive from diploid cells with the
same genes!  There is also cooperation involved -- the large numbers
actually help them all survive and navigate the trip better.

Also, the trip to find an egg is, indeed, difficult and most sperm
never make it to the egg at all so, in this case, more is better.

Finally, putting a lot of nutrients into one gamete (which clearly
gives the zygote and embryo a distinct advantage) makes it rather
unwieldy and quite immobile. So we end up with gametic dimorphism
where one of the gametes (the sperm) has the full responsibility of
locomotion.  Making it small makes it more motile and faster, giving
it an advantage.

Finally, you can't have it both ways. You can't produce a lot of very
large gametes -- you don't have enough energy or resourses to allocate
that way.  So you tend to one of the two extremes -- produce large
numbers of very small offspring or very small numbers of very large
ones.  Which choice you make (this is very reminiscent of "r" vs
"K"
strategies in ecology) depends on a wide variety of circumstances.
There are clearly advantages either way and you find both solutions.
Virtually all organisms with sexual reproduction use both at the same
time, one for the egg and one for the sperm.
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