Oh! What a Cue You Have! (To Ovulation)
The most recent issue of Current Directions in Psychological Science had three papers relevant to evolutionary psychology. (Doug Kenrick discussed this fact in his blog.) This post is about one of them, a paper by Martie Haselton and Kelly Gildersleeve called, “Can Men Detect Ovulation?” The very nice article reviews evidence that women’s behavior changes depending on their point in their ovulatory cycle and that, reciprocally, men’s behavior does as well.
Specifically, they review evidence that compared to when women are at the low fertility phase of their cycle, women at the high fertility phase speak in a higher pitched voice, are more likely to agree to dance in a club, dress more attractively, and, if they happen to be lap dancers, earn more money. Men, in turn, might be more vigilant and jealous when their romantic partners are at peak fertility.
Haselton and Gildersleeve end on a note I want to pursue here, the distinction between cues and signals.
This is, for some reason, a confusing topic. And I say that not because I think people are confused, but because the people who write about it do. Take, for instance, the book by Maynard-Smith and Harper (2003). In Animal Signals, they explain that the whole reason they wrote the book was that “there is widespread and often unrecognized confusion about the kinds of signals that exist, the mechanisms responsible for their evolution, and the terms used to describe them” (p. 1). Similarly, in a paper on quorum sensing (which is very cool, by the way), Diggle et al. (2007) echo this remark, suggesting that “considerable confusion has arisen through different researchers using the same term to mean different things or different terms to mean the same thing” in the context of communication.
What is the problem? What is so confusing about animal communication? Why is it, apparently, so difficult to get the signal/cue distinction right?
Let’s turn to the substance of the matter. Maynard-Smith and Harper (2003) define a signal as “any act or structure that alters the behaviour of other organisms, which evolved owing to that effect, and which is effective because the receiver’s response has also evolved” (p. 3).
Their first example is distinguishing two ways a stag might make another stag retreat: push him or roar at him. Pushing, they argue, isn’t a signal – it does alter the behavior or the other organism, but the response, moving backwards, didn’t evolve as a response to pushing – it’s simply a physical consequence. In contrast, retreating in response to a roar, they argue, makes the roar a signal. Roaring evolved because retreating from roars is an evolved response, the argument goes.
Note how casually Maynard-Smith and Harper make this strong claim. Labeling the stag’s roar a signal is an adaptationist claim, that the behavior in question has a function, in this case, conveying information – signaling – to a rival which, in turn, is useful in the context of intra-sexual conflict. I find it worthwhile to reiterate, at the risk of undue repetition, that this illustrates how biologists routinely make adaptationist claims based on observed patterns of behavior, rather than measuring fitness consequences, heritability, and the like.
(Note that a cue is simply a feature of the world used by an organism to guide action. There are any number of examples. You exhale carbon dioxide, which mosquitoes use as a cue to your location, but this is obviously not a signal, since exhaling CO2 didn’t evolve because of its effect on mosquitoes.)
Now, back to ovulation. Among non-human primates, especially old world primates, as is well known, genital swelling coincides with ovulation. While there seems to be healthy debate about the precise function of this swelling (e.g., Reichert et al., 2002), the consensus, from my (admittedly limited) reading is that whatever the specific function of swelling is, it is a signal of some sort, rather than simply a cue. This seems right to me. Swelling does not seem to be a necessary byproduct of ovulation – as illustrated by, for instance, the human case – and does appear to be conveying information to males, whether this is information about fertility, or some more subtle piece of information. The benefit of the swelling to the females is because of male behavioral responses, though, again, there does not seem to be consensus about the precise details.
Haselton and Gildersleeve end with this issue, discussing whether the changes women show during ovulation are signals, or only cues. Recall that the signal hypothesis, a claim of function, is the stronger hypothesis, with the associated burden of evidence. The data about women’s clothing choices during peak fertility, for instance, are suggestive, to me anyway, of a signaling interpretation, but they quite reasonably leave this as an open issue.
None of this, of course, explains why so much confusion surrounds the signal/cue distinction. My guess, for what it’s worth, is that the Maynard-Smith and Harper definition of signal requires thinking like an adaptationist, because the criteria for inclusion turn on inferring why a trait or behavior evolved, i.e., its function. The first part of the definition requires that one know that the trait evolved because of its effect on another organism. In the case of the stag’s roar, there are clues about what this trait is for. (Roars are good signals because they can be detected by the sensory apparatus of the other organism, are loud, facilitating the signal being received, etc…) In other cases, the function might be more difficult to back out. If one isn’t trained how to infer function from form, well, navigating the signal/cue distinction could be tricky…
References
Diggle, S. P., Gardner, A., West, S. A. & Griffin, A. S. (2007) Evolutionary theory of bacterial quorum sensing: When is a signal not a signal? Philosophical Transactions of the Royal Society B, 362, 1241-1249.
Maynard Smith, J. & Harper, D. (2003). Animal signals. New York, NY: Oxford University Press.
Reichert, K.E., Heistermann, M., Hodges, J.K., Boesh, C., Hohmann, G. (2002). What females tell males about their reproductive status: are morphological cues reliable signals of ovulation in bonobos (Pan paniscus)? Ethology, 108, 583–600.
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