Biologist With A Twist: Dr. Carin Bondar

Party of One…


Posted on June 24th, by Carin in Carin's Books, Carin's Paper Pick 'o the Week. 6 comments

We humans are really fascinated with our own genitalia, aren’t we? From a very early age, humans are well aware of, and curious about, the special properties that exist in the machinery of our nether regions. Perhaps it’s an unconscious appreciation for the parts of our anatomy that will ultimately catapult our genetic code into future generations, but whether we admit to it or not, everyone engages in solo sex.

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 behavior 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.

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, called the spermatheca, during copulation in many invertebrate species. However, not all ejaculates are equal. 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.

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

So 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 absurdly 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. Let’s call it a day on the science and just enjoy its non-biological value.

*The previous is an excerpt from Dr. Bondar’s first book: ‘The Nature of Human Nature’. Available soon!

ResearchBlogging.org

Reinhardt K, & Siva-Jothy MT (2005). An advantage for young sperm in the house cricket Acheta domesticus. The American naturalist, 165 (6), 718-23 PMID: 15937751

Wikelski, M., & Baurle, S. (1996). Pre-Copulatory Ejaculation Solves Time Constraints during Copulations in Marine Iguanas Proceedings of the Royal Society B: Biological Sciences, 263 (1369), 439-444 DOI: 10.1098/rspb.1996.0066





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