Image: Battle of the Sexes / Ben Clifford 
In most species the arms race between the sexes has resulted in variations on a consistent theme: the “ardent” male vs. the “coy” female. Males, who often bear little responsibility for investing time or resources in their offspring, seek to mate with as many females as conditions allow. In contrast, females are generally much more selective about whom they choose to share their genes with in the next generation. This usual (and grossly oversimplified) dichotomy has remained virtually unchanged since Darwin first proposed it in On the Origin of Species.
However, there are dramatic exceptions to this trend. On one end, females of many species have evolved a strategy of facultative polyandry. This adaptation has been selected for under multiple environmental or social conditions: to diversify the female’s genetic portfolio through mixed paternity in their litter or clutch, to elicit benefits from multiple fathers, or to disguise paternity in an effort to avoid male infanticide. On the other side, males of some species have adapted coercive or violent mating strategies in order to prevent females from making an informed choice, especially when that choice isn’t them. In both of these extremes neither sex has “won” the war between the sexes; it has been a conflict constantly being waged with each new generation, sometimes favoring males and other times females, but was thought to never reach a final conclusion.
In a study released today by Chang S. Han and Piotr G. Jablonski in the journal PLoS ONE, a war that was never supposed to end has now found a dramatic example of supplication before the winning side.
In one species of water strider, the violent coercive mating strategy employed by males has effectively been countered by females, forcing the loser in this battle of the sexes to plead for concessions. In water striders of the genus Gerris and Aquarius the conflict between the sexes typically went as follows:
A male mounts a female without any apparent courtship behavior, grasps the female’s thorax, overcomes female resistance, and then inserts his genitalia into the female genitalia through the vulvar opening between the gonocoxae.
However, females of the species
Gerris gracilicornis have evolved a unique counter-strategy to resist this kind of unwelcome mating. In most water striders, the pre-genital segment is concave (see the figure below) leaving the female genitalia mostly exposed. However, in
G. gracilicornis, females have evolved a lengthened pre-genital segment that covers 78% of the gonocoxae. As a result, males are no longer able to insert their genitalia forcefully. But females are able to expose their genitalia if they choose by lifting their gonocoxae to allow a male to have access, but only if he can provide what she’s looking for. Han and Jablonski believe they know what that is.
Because females have employed their own form of “chastity belt” G. gracilicornis males have responded by adding a new behavior to their normal mating practice. Now, after they’ve mounted a female and attempted intromission without success, they gently tap the surface of the water with their middle legs in a distinct pattern so as to produce “pulses,” small ripples, on the surface of the water. Only if females are satisfied with the males dedication to this courtship ritual will they raise their pre-genital segment and mate with their male suitor. As the authors conclude:
It is clear that if the female genitalia remain hidden males G. gracilicornis can not achieve successful intromission. Hence, it is the female who decides when, after an apparently coercive mounting and genitalia attachment, copulation (intromission) actually begins, or whether it takes place at all.
Their results illustrate how a seeming “victory” by one sex through evolving morphological changes to their genitalia, has triggered the evolution of a unique behavior in the other. Of course, it’s important to point out that this isn’t a victory in the Darwinian sense (i.e. increased reproductive success) but it is an effective checkmate on males who now have no choice but to plead for sexual access.
But the story doesn’t end there. The courtship display that males have evolved may still have a coercive element: alerting predators that water striders are nearby.
We hypothesize that, the males use signals to coerce the female by calling attention of predators to the mating. . . male courtship signals induces females to protrude their genitalia in order to decrease the risk of attracting predators that cue on the ripple signals.
In the strider equivalent of “playing chicken” males may be willing to risk it all in order to force females to give in before it’s too late. What may at first appear to be a courtship ritual could in fact be a threat, turning the tables on females so that they must tread carefully or end up winning the battle but losing the war itself.
Reference:
Han, C., & Jablonski, P. (2009). Female Genitalia Concealment Promotes Intimate Male Courtship in a Water Strider PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005793
Last updated:
Thursday, 11 Jun
2009 - 07:05 UTC
I’m sorry, but I really disagree with your use of the term rape here. Rape implies consent—and frankly, we will never know if insects (or most other animals) are capable of a conscious decision. The word “rape” has inescapably moral connotations, and I think most of us would agree that non-human organisms cannot be moral agents. Rape is about power, not procreation.
Although I agree with your comment, Gwen, I do not think that rape implies “consent” – the opposite is surely true. Perhaps you meant to write “rape implies motivation” (on part of the rapist)?
I agree that it is not useful to use the emotive and particular term “rape” in the context of non-human animal behaviour.
How about “rape implies an ability to give or withhold consent”
Which is what I meant to say, although clearly my fingers did not cooperate :)
@Gwen @Maxine: This is a difficult issue and I’m glad you called me on it. I’m not entirely certain of my own position, so perhaps you and other readers could help me understand it better. The difficulty I have is that if the terms “violent coercive mating,” as Han and Jablonski state, or “forced copulation,” as it’s usually put in the avian and primate literature, were applied to humans it would be categorized as rape. The Oxford American dictionary I use defines rape as “forcing another person to have sexual intercourse with him without their consent and against their will.” I fully admit that the dictionary definition could represent a biased perspective, but what I get from this is that rape is defined as a male forcing intercourse without the consent of their victim. There’s nothing about the rapists motivations beyond the act itself. It could be about power (as is often the case when used as a tactic in warfare, thankfully now classified as a war crime) but it could also be about sexual desire.
There’s also another question I have. A common term that was originally reserved only for humans, but has now become widespread throughout the biological literature (including the present paper) is that of “courtship”. If the term courtship is flexible enough to include water sliders tapping their legs to create ripples on the water as well as elaborate shows of affection in humans (often consisting of ritualized behavior and community involvement), is there a distinct difference between this and referring to “forced copulation” more generally as rape? Certainly both involve higher-level reasoning and morality where humans are concerned, but why is one term useful for non-humans while the other isn’t? I’m asking because I’m not certain.
But perhaps I’m missing the point of your objection. When dealing with such a sensitive subject perhaps it’s better not to make any association to rape as “natural” since that could imply a belief that rape is an adaptive strategy in humans. I recall the justifiable outrage by many scholars when the evolutionary psychologists Thornhill and Palmer published A Natural History of Rape (for an excellent critique of this work see Evolution, Gender and Rape which one of my advisors at Duke, Christine Drea, submitted a chapter to). I certainly don’t want to inadvertently justify the unjustifiable, but I would appreciate some clarity on the fundamental problem.
[rape] could also be about sexual desire.
It could be, but it’s most often about power. I don’t think there’s any evidence that forced copulation in the animal kingdom is about power, but I could be wrong.
@Richard: Yes, but my point is that I agree with the definition that rape is defined as sex without consent. But why does that apply only to humans?
@Eric
I would guess that the main reason it applies only to humans is that we have entire schools in hundreds of universities, hundreds of books, and thousands of lectures on what rape “is” in the human context, and I would guess none on the subject of what rape is to water striders. People are educated to say knee-jerk things, like “rape is about power” without thinking. Now, I don’t disagree that rape is about power, sometimes, but it is also a mating strategy in humans, other apes, water striders and many many many other lifeforms. It doesn’t have to be a loaded word when you use it for non-human life, but people tend to take it that way.
I guess, Eric because words take on more than their dictionary meanings.
Rape is not a mating strategy among humans. It is violence, put simply.
I dislike use of anthropomorphic terms to describe non-human behaviour, because of the creep towards ascribing of motive or other conscious “thought” way beyond any data. You only have to look at the prevalent general attitudes to non-human life forms among very many people – where to start with all the assumptions made by animal-lovers? (I hasten to add there is nothing wrong with being an animal lover, I am even one myself sometimes;-) But there is so much projection going on.)
John – there is nothing knee-jerk about it. Richard is correct. Rape is about power and violence. It might, or might not, be about other things too, but it is always about power – the rapist is stronger than the rapee.
Eric – all I can say in response to your direct question is that your dictionary uses the term “person” in its definition of rape. I appreciate that language evolves, but in our society, today, “rape” has this definition and meaning. I would not apply it outside the human context, personally – and in fact though it is up to you what you headline your blog posts I would have even more doubts, if it were possible, about using the word in a headline because terms in a headline have yet one more aspect compared with terms in body text, that is, to attract potential readers to the article. I express this view not to sound censorious, which is not my intention, but because I was asked via email to provide it.
Yes, to all of the comments above. My objections stem from most of what’s been said here:
“Rape is not a mating strategy among humans. It is violence”
“it’s better not to make any association to rape as “natural” since that could imply a belief that rape is an adaptive strategy in humans”
Rape is not a reproductive strategy. It is violence, pure and simple. There is a vast literature on the consequences of human rape, and it is currently being used as a tool of war.
http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1000021
To use the term rape in this non-human context normalizes it and trivializes it. And it should never be a normal, or status-quo condition.
BTW, Thornhill’s work has been effectively criticized elsewhere: http://www.susanbrownmiller.com/susanbrownmiller/html/review-thornhill.html
To use the term rape in this non-human context normalizes it and trivializes it. And it should never be a normal, or status-quo condition.
I think this is an excellent objection and gets to the heart of the matter. What’s important to emphasize when discussing issues of sexuality in the context of evolutionary theory is that biology is not destiny. But neither is it unimportant. I think we do ourselves a disservice by isolating forced copulation into a biological category for non-humans and into a sociological category when we’re talking about ourselves. I’ve come to this view after three years with my feminist scholar partner where we’ve discussed these issues at great length. However, I understand that this isn’t universally shared (nor should it be).
I encourage this kind of discussion and I think what’s most important is reducing the prevalence of rape and sexual assault in human societies. To properly address the issue I believe we need to understand its source in both biology and the culture of patriarchy that sustains it by continuing to reinforce unequal power relationships. To counter violent coercive mating, water striders must evolve morphological adaptations. But humans are unique in that we have the ability to reduce the prevalence of rape in our society through cultural change, thus moving towards a time where it is not “a normal, or status-quo condition.”