Sunday, February 26, 2012

I rather be a eunuch rather than to be dead


This NUS study shows how certain male spiders self- castrate rather to be eaten by the female spider.

In “ Why is sex fun”,  Jared Diamond covers why male spider is eaten by the female spider during copulation.

As he puts is,

This cannibalism clearly involves the male's consent, because the male of these species approaches the female, makes no attempt to escape, and may even bend his head and thorax toward the female's mouth so that she may munch her way through most of his body while his abdomen remains to complete the job of injecting sperm into her.
.....

Suppose that opportunities to transmit genes arise unpre-dictably and infrequently, and that the number of offspring produced by such opportunities increases with the female's nutritional condition. That's the case for some species of spiders and mantises living at low population densities. A male is lucky to encounter a female at all, and such luck is unlikely to strike twice. (1) The male's best strategy is to produce as many offspring bearing his genes as possible out of his lucky find. (2) The larger a female's nutritional reserves, the more calories and protein she has available to transform into eggs. If the male departed after mating, he would probably not find another female and his continued survival would thus be useless. Instead, by encouraging the female to eat him, he enables her to produce more eggs bearing his genes. (3) In addition, a female spider whose mouth is distracted by munching a male's body allows copulation with the male's genitalia to proceed for a longer time, resulting in more sperm transferred and more eggs fertilized.
"
.....

In Summary,

Sexual cannibalism in spiders and mantises is favored by the ecological variables of low population densities and low encounter rates, and by the biological variables of a female's capacity to digest relatively large meals and to increase her egg output considerably when well nourished.

So what do we make out of this case when the male spider actually chooses to make an escape.  Here are some questions:
  • Does the male spider grows back his reproductive organ?  If yes, then this is simply an evolutionary development to have the best of both worlds - leave his reproductive organ to continue to transmit his sperm while growing a new one to impregnate another day.
  • If the male spider does not grow back his reproductive organ, this is where everything breaks down and we have to relook the 3 clauses that I highlighted above.

By detaching his reproductive organ and leaving , the male spider continues to enjoy clause 3 of longer copulation time as his sperm continued pumping through the apparatus lodged in their reproductive organs.

But I think the article actually explains why such behaviour evolved:
  •  “ the group found that the palps had delivered only a-third of their sperm after the females stopped copulation ...... and there is female-controlled short copulation, and ultimately the female's frequent cannibalism.
Therefore, from the male spider point of view, even if I am willing to be eaten, she copulates for such a short time that perhaps only a third of my sperm is used.

In addition, 

the detached palp can prevent other males from mating with the female, thus ensuring the paternity of the self-castrating spider. The eunuch also becomes more aggressive and guards the female from competitors while the sperm is being transferred.

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