Wednesday, December 2, 2009

The Circumvention of Compulsory Sex

Males are sexually coercive. During initial encounters with the opposite sex, they will go to great lengths to impress and entice; they’ll offer gifts, sing, dance, and do whatever else is necessary to win a female’s affections. However, should these preliminary advances be rejected, the males from a multitude of different animal species will unhesitantly apply the tactics of deceit, intimidation and even brute force to satisfy their reproductive drives.

Through forced couplings males enhance the spread and flow of their genes within the larger gene pool. By mating with both those females that accept his flirtations and those that don’t, the total number of a male’s sired offspring in the population is increased. This strategy provides the male with a reproductive leg-up over his rivals - the other males in the population. Energetically speaking, the tactic of repeated mating comes at a bargain price, because unlike females, males don’t usually incur the taxing reproductive expenses associated with pregnancy, tending to offspring, or for that matter even production of female reproductive cells – the ova.

In most vertebrates, female and male gametes differ in size and structure; this phenomenon is referred to as anisogamy. The female gametes (ova) are larger in size than that of the male gametes (sperm). Ova are larger because, in addition to containing the female’s DNA, they provide all of the nutrition and protection required for the resulting embryo’s development. Spermatozoa by contrast are merely vehicles for delivering the male genetic compliment and are far less costly to manufacture and send. In other words, sperm are cheap, eggs are expensive.

Although cheap sperm and compulsory sex are effective tools for propelling the male’s genes into the bodies of future generations, they do so with little regard for the reproductive goals of the female’s genome. Recall from yesterday’s discussion, that the female gender of a species is typically better positioned to be the ‘chooser’ when selecting potential sexual partners; this is in large part due to the above described costs associated with eggs, pregnancy and child rearing. Through selecting only the most fit, well-matched mates, females are able to reap the benefits of their own inclusive fecundity and improve their own reproductive fitness. In opposition, sexual coercion eliminates the female’s choice and adversely impacts her reproductive fitness.

Reproduction drives the process of natural selection. As a consequence of this biological tenet, a decline in a group’s reproductive fitness is countered in one of two ways; extinction or adaptation. Females challenged with deteriorating fitness often undergo morphological and behavioral adaptation to mitigate for the coercive sex practices of males. Just as selection favors the advancing genes of the male, it also favors those traits that enforce the female’s fitness. As weapons in the back-and-forth battle for reproductive victory, the female may brandish many weapons; she may practice avoidance, she might form coalitions with other females or a dominant protective male, she could incite male-male combat, or her physiology could adapt to delay egg deposition or to interfere with the transfer of the coercive male’s sperm. The behavioral and physiological possibilities are nearly endless.

As a case in point, the female ‘agile frog’ (Rana dalmatina) shares a habitat with two-different species of male frogs; the males of her own kind, and those called ‘European common brown frogs’ (Rana temporaria). Both species of male frogs utilize coercive sex as a reproductive tactic, and both will force themselves on the female agile frog. However, if she is subjected to forced amplexus (frog sex) by the male ‘common brown,’ the resultant discharge of expensive eggs will not be profitably fertilized. Because the common brown is a different species, the fusion of his sperm with the agile frog’s ova will not result in the development of viable frog embryos. In this instance, coercive sex results in the female wasting an entire clutch of eggs that could have potentially carried her genes into the future. To mitigate for the loss of eggs by forced sex, the female agile frog will ‘intentionally’ delay egg-laying in hopes of discouraging the male’s incessant reproductive efforts. If after time the male doesn’t seem to loose interest, the female will release a reduced quantity of eggs. Thinking his mission a success, as a response the male will then release his sperm onto the eggs and part ways in pursuit of the next female. By using the somewhat sneaky, though affective, reduced-eggs scheme, the female exerts ‘cryptic choice’ as a method of insuring her reproductive investment and safeguarding her fitness. This case highlights one of many examples in which a female has manipulated a compulsory sex encounter to circumvent a potential decrease in her fitness.

Hettyey, A., Baksay, S., Vági, B., & Hoi, H. (2009). Counterstrategies by female frogs to sexual coercion by heterospecifics Animal Behaviour, 78 (6), 1365-1372 DOI: 10.1016/j.anbehav.2009.09.006

Rana dalmatina Photo by Fabio Turel


  1. That's fascinating. Anyway - next spring, when walking around there, I'll go back to the pond and check the other frogs - indeed, I remember there were two different kinds of them.

  2. Fabio,

    I appreciate you stopping by, and many thanks for the photo!

    Let me know if you get any updated pics in the spring, maybe we can post them here?