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Why did Darwin ignore female promiscuity?

In this week’s Times Higher EducationTim Birkhead, a behavioral ecologist at the University of Sheffield in the United Kingdom, ponders why Darwin failed to recognize the importance of female promiscuity among animals and the related phenomenon of sperm competition. In his essay called “Sex and Sensibility,” Birkhead writes:

If Darwin had put two and two together, the study of sperm competition – now a major area of research – might have been launched in 1870 rather than 1970. Why did Darwin ignore the evidence and why did it take a century for others to make the connection?

Birkhead wonders if Darwin was just a Victorian prude or whether having a daughter editing his writing led to some censorship. Birkhead never provides a satisfying answer, but he does offer some thoughts on the impact of Darwin’s missed opportunity:

The upshot of all this was that Darwin steered clear of female promiscuity and plumped for female monogamy, an idea that then remained firmly fixed, in biologists’ minds at least, for a full century. The significance of female promiscuity only really became apparent in the early 1970s with the realisation that natural selection operated on individuals rather than groups or populations. …

I don’t think Darwin thought this subject through. I don’t think he ever thought carefully enough about the reproductive consequences of individual selection. As far as I can see, there isn’t a hint of this kind of thinking in his voluminous correspondence. This is curious because the idea of female choice was such an integral (albeit controversial) component of his concept of sexual selection. For Darwin, mate choice simply stopped at copulation.