In 1948, English geneticist Angus John Bateman published what became an incredibly influential paper in the biological community. In that paper, he reported on his experiments with fruit flies. Using those experiments and referencing other scientists’ observations, he concluded that, in general, males are promiscuous in their mating habits, while females are more choosy about their mates. This rule, he said, should be applicable to both animals and plants.1
What’s the reason for this supposed trend? It’s because sperm are small and easy to produce, while eggs are large and more difficult to produce. Notice the picture at the top of this post. It shows a sperm about to fertilize an egg. Note how small the sperm is relative to the egg. Indeed, the egg is so large compared to the sperm that only a portion of the egg can be shown in the picture. A male, then, invests little in producing his sperm, so it is most advantageous for him to mate with as many females as possible. The female, however, invests a lot in the production of an egg cell, so she must be choosy as to the males with which she mates.
In the intervening 60 years, Bateman’s principle has been considered unquestionable truth in the evolutionary community. I was taught it as “scientific fact” when I was at university, and many scientists go so far as to call it a law. For example, in her book on behavioral mechanisms in evolution, Emory University’s Dr. Leslie Real writes:2
Behavioral ecology has few overriding general principles that have survived empirical investigation for very long. One of the more persistent claims is that females will generally be more choosy than males in their selection of mates. Male fitness will thus be limited by access to females (leading to increased competition among males), while female fitness will be limited by resources available for offspring production and development. This general claim has been elevated to the status of a law and often appears in the literature as “Bateman’s principle,” named after A. ]. Bateman (1948).
There’s only one problem. Bateman’s Principle is definitely not a general rule in nature, and more importantly, we now know that Bateman’s original study was fundamentally flawed.
For example, in the very first chapter of her 2002 book, Dr. Tatiana’s Sex Advice To All Creation, Imperial College’s Dr. Olivia Judson takes on Bateman and cites case after case in which his principle is simply wrong:3
…in species after species, rampant promiscuity is no malfunction. Rather, females benefit from it. My files bulge with examples. To pick a few at random, female rabbits and Gunnison’s prairie dogs both show higher rates of conception if they mate with several partners when they are in heat. The female sand lizard hatches more eggs the more lovers she’s had. The female slippery duck – a pale fish that lives on coral reefs – will have more of her eggs fertilized if she spawns with a gang than if she spawns with one fellow.
So there are clearly some exceptions to Bateman’s Principle, but do they tell us it is not a general rule? Most of biology’s general rules have some exceptions. Couldn’t the cases that Dr. Judson cites just be exceptions to the general rule? Most likely not, because a recent study now shows that Bateman’s original experiments with fruit flies were flawed. In other words, this principle that has been “elevated to the status of a law” was built on incorrect data!
In the study, the authors tried to reproduce Bateman’s results using modern genetic techniques. They could not. In essence, they showed that the way Bateman tried to deduce how many times each fly mated and how much effect it had on fertility was flawed. This is because he didn’t have modern analytical tools at his disposal. As a result, he had to choose flies that had mutations which led to easily visible traits (such as wrinkled wings). The problem is that such mutations are severe, and when a fly offspring inherited mutations from both the father and the mother, it was likely to die before it developed into an adult. Bateman considered only adult offspring in his experiments, so his observations were skewed, which led him to an erroneous conclusion.4
As one of the authors said in an interview:
Our team repeated Bateman’s experiment and found that what some accepted as bedrock may actually be quicksand. It is possible that Bateman’s paper should never have been published
Now is it a problem that a scientific “fact” has been demonstrated to be false? Of course not! This is the nature of science. Science makes a lot of mistakes, and in general, it is self-correcting. Thus, it’s not surprising that time and time again, things which are considered to be scientific “facts” are actually incorrect. However, I do think that this incident is a very good lesson on why bad ideas hang around for so long in the scientific community.
Consider the quote I gave from Dr. Olivia Judson’s book. That book was published in 2002, and she said back then that her files “bulge” with examples of where Bateman’s principle is wrong. Those studies that bulged her files didn’t all come out in 2002. Many of them were much older. For example, one study showing more promiscuity in female Splendid Fairy-wrens than in males was published in 1990!5 Despite the numerous studies that came out over the past two decades contradicting Bateman’s Principle, it took until now for someone to actually try to repeat Bateman’s classic experiment.
If this mystifies you, it shouldn’t. In fact, it is quite common in science. Once an idea becomes well-established in the scientific community, it is hard to get rid of. As studies start showing problems with the idea, they are often disregarded as flawed themselves or just special cases that don’t hurt the overall applicability of the idea. Because of this, it often takes a long time to get rid of a bad idea in science. In addition, the more influential the idea, the longer the process takes.
This is why evolution (in the flagellate to philosopher sense) has hung around in science as long as it has. Lots of studies have come out showing how evolution cannot be the main explanation for the origin of species. Nevertheless, these studies are dismissed as either flawed or special cases that don’t harm the overall applicability of evolution as a guiding principle in biology. Of course, many serious scientists (see lists here and here, for example) have paid attention to such studies and have realized that this kind of evolution just doesn’t work. However, it’s going to take time to convince the majority in the scientific community.
After all, it took more than 60 years to convince the entire scientific community that Bateman’s Principle is questionable, and it was only considered foundational to a small specialty in biology.
1. Bateman, A.J., “Intra-sexual selection in Drosophila”, Heredity 2(3):349–368, 1948
Return to Text
2. Leslie Real, Behavioral Mechanisms in Evolutionary Ecology, The University of Chicago Press 1994, p. 5
Return to Text
3. Olivia Judson, Dr. Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex, Henry Holt 2002, p. 13
Return to Text
4. Patricia Adair Gowaty, Yong-Kyu Kim, and Wyatt W. Anderson, “No evidence of sexual selection in a repetition of Bateman’s classic study of Drosophila melanogaster,” Proceedings of the National Academy of Sciences, June 11, 2012, doi:10.1073/pnas.120785110
Return to Text
5. Brooker, MG; Rowley, I; Adams, M; and Baverstock, PR, “Promiscuity: An inbreeding avoidance mechanism in a socially monogamous species?”, Behavioral Ecology and Sociobiology, 26(3):191–199, 1990
Return to Text