|Honestly! They are THAT big!|
When it comes to sperm, size does matter, but bigger is not always better.
In fact, according to a paper published by a University of Zurich team and published in the Royal Society of London Proceedings B, the bigger the species the better it is to have small sperms while the smaller the species, the better are sperms with big tails. Better, of course, being defined as being better able to find and fertilise the egg.
There are couple of reasons for this, one of which is that females of smaller mammals tend to have more mates so each individual male's sperms often have to compete with with the sperm from several other males. Competition between sperms from different males is thus one of the main drivers in sperm evolution. There are two basic strategies open to sperms in this situation: they can be faster and so win the race to the egg, or they can be more numerous, so improving the chance of being first. The obvious strategy would appear to be, on the face of it, to make bigger, faster and more numerous sperms. So why didn't this happen?
The problem is that a single testicle has finite resources so it can either concentrate them on making fewer, bigger sperms, or on making a lot of them. The clincher seems to be the distance sperms need to swim and the space they have to swim around in. In this respect, speed (conveyed by a longer tail) is only important if, as in mice, the distance is only a few centimetres; in an elephant however, the sperms are deposited in a cavernous reproductive tract and need to expend considerable resource in actually finding their way. In this case, it's not so much speed which is important but improving the chance of any sperm actually finding the egg, and this is relative to the number produced.
Postcopulatory sexual selection is widely accepted to underlie the extraordinary diversification of sperm morphology. However, why does it favour longer sperm in some taxa but shorter in others? Two recent hypotheses addressing this discrepancy offered contradictory explanations. Under the sperm dilution hypothesis, selection via sperm density in the female reproductive tract favours more but smaller sperm in large, but the reverse in small, species. Conversely, the metabolic constraint hypothesis maintains that ejaculates respond positively to selection in small endothermic animals with high metabolic rates, whereas low metabolic rates constrain their evolution in large species. Here, we resolve this debate by capitalizing on the substantial variation in mammalian body size and reproductive physiology. Evolutionary responses shifted from sperm length to number with increasing mammalian body size, thus supporting the sperm dilution hypothesis. Our findings demonstrate that body-size-mediated trade-offs between sperm size and number can explain the extreme diversification in sperm phenotypes.*
Sperm number trumps sperm size in mammalian ejaculate evolution
Stefan Lüpold, John L. Fitzpatrick
Proc. R. Soc. B 2015 282 20152122; DOI: 10.1098/rspb.2015.2122. Published 18 November 2015
*Copyright © 2015, The Royal Society, reproduced under terms of Creative Commons licence CC-BY 4.0
Now, all this is very interesting and of course easily understandable in terms of genetic evolution. It isn't rocket science to work out that males which produce sperms which are better at getting to the female egg first will produce more descendants on average than those males whose sperms are not so good at it. Since this is inherited, it's not rocket science either to work out that this will gradually, over time, produce a population with sperm good at getting to the females eggs in competition with other sperms of the same species in the special environment of the females' reproductive tract, and therefore that the sperms of an order like mammals will diversify morphologically between species.
But this is completely incomprehensible as the product of an intelligent designer, and an intelligent designer moreover who personally decides which individuals are going to be produced as a result of their parents mating. For such a designer, there will be no competition and the chosen sperm will be guided unerringly to the egg. In fact, for such a designer, there only need be a single sperm even if it can't think of a simpler way to produce new individuals.
The entire idea of multiple sperm production, let alone multiple sexual partners in the normal reproductive process, is entirely inconsistent with intelligent design. The idea of a competition between sperms with lots of different strategies for success, and the idea of sperms being larger or smaller depending on size in order to win this competition renders the idea of an intelligent designer behind the whole thing laughably absurd.
It results in such absurdities as the sperm of a small fruit fly, for example, being many times larger than the sperm of a blue whale and a mouse sperm being several times larger than that of an elephant. Only an unintelligent idiot would design so many solutions to the same problem and complicate something which could be so simple.
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