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.