Bateman’s
research with fruit flies, published in 1948, reported that males did indeed show more variation in reproductive success, and that there was a stronger link between promiscuity and reproductive success in males than in females. Bateman’s principles are the foundation of the familiar idea that, because dispensing sperm is cheap, but harbouring and nurturing an egg is costly, females tend to be sexually choosy and inclined to reserve their favours for the best male on offer. Meanwhile, males – who, unlike females, have much to gain from winning multiple mates – are ardent and competitive by near-universal evolutionary design.
But consider an unexpected observation made in the 1970s by the primatologist Sarah Blaffer Hrdy: female langur monkeys in South Asia
tended to have multiple sexual partners. This flew in the face of received wisdom; theoretically, as Hrdy drolly observed, female promiscuity ‘should not have existed’. But by paying attention to females, she produced unexpected data that challenged existing scientific models.
Hrdy’s observations with langurs also resonate with more recent critical scrutiny of Bateman’s work with fruit flies. As the evolutionary biologists Zuleyma Tang-Martinez and Brandt Ryder
note, Bateman’s conclusion that only males benefit from promiscuity applied to just two of his six series of experiments. The other four series showed the same beneficial pattern for females, albeit to a weaker extent. Yet Bateman focused on the results that fit the polarised notion of competitive males and choosy females. This selective emphasis was then perpetuated by others, meaning that the unexpected benefits to females of multiple mates gained no traction in the literature. Analysing the last two series separately was a
decision, part of the construction of a scientific finding, made for reasons that remain unclear. A subsequent re-analysis of all of Bateman’s data, pooled together, led to a finding of sex
similarity in the effects of multiple mates on reproductive success.
