According to Darwinian sexual selection, males are typically eager to mate (sometimes aggressively clashing with other males for the opportunity) while females are choosy about who they copulate with. This is a result of different selection pressures on males and females: Sperm are smaller and cheaper than the relatively bigger, costlier eggs.

And this century-and-a-half-old theory of sex differences still holds up, according to new work published in Science Advances this week. Researchers studying 66 species offer comprehensive evidence that across the animal kingdom, these conventional sex roles are still present. 

In the last few decades, researchers have found evidence that both sexes can experience similar levels of sexual selection and that sex roles can be reversed – leading some to conclude that Darwin’s sexual selection theory may be flawed. Now, a team led by Tim Janicke of Université de Montpellier measured the strength of sexual selection in males and females using 72 previously published studies from between 1900 to 2014 on invertebrates, fish, amphibians, reptiles, birds, and mammals.

Based on their statistical analyses, males benefit more from mating than females overall. That is, sexual selection is stronger on males. The differences between sexes are rooted in their differently sized gametes – what’s called anisogamy. These findings support the male eagerness to mate as well as their elaborate ornamentation and weaponry, ranging from colorful peacock feathers to enormous stag antlers. It also helps explain why dads tend to provide less parental care than moms. 

Of course, while the “sex role syndrome” appears to be generally valid for the animal kingdom, there are many exceptions. Some females benefit from multiple mating, while some males are the ones who take care of the offspring. According to the authors, “These exceptions to the rule highlight the importance of incorporating environmental conditions when interpreting animal mating systems.” But environmental conditions alone can’t explain the variation in sex roles across the animal kingdom.

^{A roaring male red deer (Cervus elaphus) with multiple females. Oliver Krüger}