The Biological Use for Masturbation

Masturbation, we’re told by health professionals, is a natural and necessary process that is enjoyed by males and females, young and old.

Although at first glance one might think that masturbation in males is wasteful because genetic blueprints are being released without a potential egg-target, there may be some biological use for this behaviour after all.

Sperm expulsion by males is a common occurrence in several vertebrate and invertebrate species, and in some cases it’s a strategic maneuver that results in greater reproductive success.

House Crickets

It has been observed in house crickets (Acheta domesticus) that males routinely expel spermatophores at times other than during sex. Spermatophores are packages of sperm that are transferred to the female sperm storage organ, the spermatheca, during copulation in many invertebrate species.

However, not all ejaculates are equal.

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Female house crickets are more likely to store young sperm in their spermathecas. A polyandrous sexual system in this species means that multiple males can mate with a female in order to fill up her sperm storage organ.)

Young sperm can be superior to older sperm for a number of reasons, including a greater fertilization ability and a higher survival rate in resultant embryos.

Due to the fact that younger sperm is dominant in the spermathecas of female crickets, a male’s chances at reproductive success are greatly improved if he provides a relatively new package.

How does he preferentially deposit young sperm?

By discarding the old sperm, of course. Autonomous sperm ejection (cricket masturbation) is the mechanism by which a male cricket gives himself a younger sperm content and increases his chances of successfully fertilizing his partner’s eggs.

Marine Iguanas

Not all masturbatory emissions are so carelessly discarded. Male marine iguanas (Amblyrhynchus cristatus) in the Galapagos archipelago have developed a strategy for improving their reproductive success via storage of their own ejaculates.

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There are large size differences between sexually mature marine iguana males. The largest males are the most aggressive, and they hold territories in which the female iguanas nest and feed. Smaller males must somehow sneak into the territories and copulate with females when the larger males are otherwise occupied.

Since female iguanas only copulate once per season, smaller males have developed elaborate strategies to get the most bang for their buck (yes, pun intended) during these rare opportunities.

It takes approximately 2.8 to 3.1 minutes of intercourse before a male iguana initiates ejaculation. For the large dominant males, this time interval is easily achieved, and such males have a high rate of successful copulations (95 per cent). 

The story is not as positive for the smaller males, who generally realize a much lower copulatory success rate and are forcefully separated from their female partners by territory-holding males before ejaculation 29 per cent of the time.

However, the crafty underdogs are able to increase their fertilization success by masturbating before having sex and storing the prepared viable ejaculate in small pouches near the penis.

Storing this extra sperm that can be transferred to the female almost immediately upon mounting her increases the overall fertilization success of the small male iguanas by a respectable 41 per cent. A stored ejaculate strategy may be common in species having a size-dominance hierarchy and sexual interference. Where there’s a will, there’s a way.

Why Do Humans Masturbate? 

The aged sperm scenario seems like a good reason in some cases; controlling for sperm age is a standard procedure in human in vitro fertilizations.

However, this doesn’t explain the high frequency of masturbation in males, nor its existence at all in females, also observed in other primate species including bonobos and macaques.

Although the clitoris is found in all female mammals, scientists have yet to come up with a suitable biological explanation for it, or for the existence of the female orgasm. The long and short of it may be that it’s simply recreational.


This post is an adapted excerpt from my book “The Nature of Human Nature

 

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