Graded-Signal Sexual Swellings as Self-Deception?

baboon sexual swelling

This is probably THEE baboon sexual swelling photo on the internet (from Wikipedia Commons), but it’s still a good one, so there it is.

The graded signal hypothesis suggests that sexual swellings in primates represent the probability of ovulation.  Based on this model, in male philopatric species, dominant males find it most cost-effective to guard females at the height of ovulation based on the appearance of the swelling.  Other males get access outside this period.  However, sometimes ovulation & maximum swelling may not always match up, giving lower status males access to ovulating females & passing on asynchronized variation.  So what this seems to represent is female exaggeration & possibly self-deception.  Do females “think” they’re ovulating?  I doubt it, so maybe we should stick with exaggeration.  What about in other species?  Do signaling non-mammals exaggerate behavior?  And why?  One explanation might be to loosen up the hold of the philopatric males & further confuse paternity.Charles Nunn figure 2

Several features listed above qualify exaggerated swellings
as probabilistic, or graded, signals: swellings increase
in size gradually, and ovulation tends to occur at peak
swelling; however, ovulation occurs with some error relative
to peak swelling. [Nunn 1999:239)]

Some aspects regarding the distribution of exaggerated
swellings remain to be explained. This variation may
relate to female control over mating decisions. When
females are capable of freely choosing their mates, they
can effectively bias paternity while also confusing it (i.e.
they can solve the female dilemma without elaborate
sexual signals (van Schaik et al., in press). However, when
female choice is restricted, females are expected to counteract
these restrictions, and sexual behaviour, achieved
through sexual signals, is a likely female counterstrategy
(e.g. Gowaty 1997). [Nunn 1999:240]

Charles Nunn. 1999. The evolution of exaggerated sexual swellings in primates and the
graded-signal hypothesis. Animal Behavior, 58:229-246.


Christopher Lynn

About Christopher Lynn

Christopher Dana Lynn is an associate professor of anthropology at the University of Alabama, where he directs the Evolutionary Studies program.  Chris teaches undergraduate and graduate courses in biological anthropology, human sexuality, evolution, biocultural medical anthropology, and neuroanthropology.  He received his Ph.D. in Biological Anthropology in 2009 from the University at Albany, SUNY, where his doctoral focus was on the influence of speaking in tongues on stress response among Pentecostals.  Chris runs a human behavioral ecology research group where the objectives include studying fun gimmicky things like trance, religious behavior, tattooing, and sex as a way of introducing students to the rigors of evolutionary science.  In all his “free” time, he breaks up fights among his triplet sons, enjoys marriage to the other Loretta Lynn, strokes his mustache, and has learned to be passionate about Alabama football (Roll Tide!).  Follow Chris on Twitter: @Chris_Ly
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