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Episode 249 – Sexual Dimorphism

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In many animal species, females and males are vastly different. This episode, we explore the variety, the extremes, and the mysterious deep history of Sexual Dimorphism.

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Female, Male, and More

Dimorphism means “two forms.” Sexual dimorphism refers to the differences between adult male and female anatomy, typically in animals. Most animal species, and some plants, exhibit some amount of sexual dimorphism.

Left: A large male and small female northern elephant seal (and a pup). Like many mammals species, male elephant seals are significantly larger than females. Image by Mike Baird, CC BY 2.0
Right: A large female and small male banana spider. Like many arthropods species, female spiders are often significantly larger than males. Image by Sanba38, CC BY 2.5

Many famous examples of sexual dimorphism are differences in size, shape, and appearance. In many species, one sex is significantly larger than the other, or one sex is more dramatically colored or ornamented. But sexual dimorphism can also include differences in behavior, diet, aging, immunity, and much more.

In most bird species, males exhibit bright, complex, or large feathers, while females are more plainly colored and more simply adorned, although there are examples of the opposite.
Top: A male (left) and female (right) Mandarin duck. Image by Francis C. Franklin, CC BY-SA 3.0
Bottom: A male (left) and female (right) Eclectus parrot. Image by Doug Janson, CC BY-SA 3.0

Most of these differences relate to the distinct roles of male and female individuals. In many species of birds, males are brightly colored to assist in attracting mates, while females are more drab colors to help them camouflage while guarding their nest. In the most extreme cases of sexual dimorphism, one sex lives an entirely different lifestyle than the other. Because these differences typically relate to reproduction, sexual dimorphism usually doesn’t become apparent until these animals reach maturity.

On the other hand, species with less distinct sexual roles tend to have less pronounced sexual dimorphism. In many primates, including humans, male and female anatomy is overall very similar, and there is a great deal of variation and overlap in features like body size. Some species have essentially no sexual dimorphism: adult snails and slugs typically have both male and female genitalia, and therefore no consistent distinction between the sexes.

In some species, females and males are so radically different that they practically live like two different types of animal. Male vapourer moths (left) have large and brightly-colored wings, while females of the species (right) have vestigial wings and a grey-ish body color. Image by Patrick Clement, CC BY 2.0

Sexual dimorphism is often a consequence of the importance of reproductive success for evolution. If a trait helps an individual find a mate or protect offspring or avoid competing with the other sex for food, that trait is likely to be passed on. This phenomenon is called sexual selection, and it can result in the “runaway” evolution of extremely exaggerated features like moose antlers or elephant tusks, which are physically cumbersome but crucially important for attracting mates or defending territory.

Identifying sexual dimorphism in the fossil record is extremely difficult, mostly due to two factors: one, it is usually impossible to confirm the sex of a fossil skeleton; and two, in order to confidently demonstrate two “morphs” of an ancient species, we require a large sample of skeletons, which is typically not available for fossils.

Illustrations of proposed “male” (left) and “female” (right) Protoceratops skulls, as depicted in a paper that argues these are not, in fact, different sexes. Image from Maiorino et al 2015

Many studies have proposed evidence for sexual dimorphism in fossil species, especially dinosaurs, but most are heavily debated, especially since it’s difficult to rule out other potential explanations for the apparent differences: what appears to be a “female” and “male” individual might actually be a young and adult, or might have lived in very different habitats, or might even be two different species.

Fossil of the trilobite Olenoides, highlighting what appear to be “clasper” limbs, which are present in males of some living arthropods and used to hold onto a mate during mating. Image from Losso and Ortega-Hernández 2022

The most convincing cases of fossil sexual dimorphism tend to be species with very close living relatives which show the same features, or cases where reproductive structures themselves are fossilized.

Learn More

Sexual Selection (technical, open access)
Sexual dimorphism in flowering plants (technical, open access)

The Evolution of Sexual Dimorphism (technical, open access)

Sexual dimorphism in dinosaurs: a review of the evidence and approaches (technical, open access)
Separating sexual dimorphism from other morphological variation in fossil marine reptiles (technical, open access)
Claspers in the mid-Cambrian Olenoides serratus indicate horseshoe crab–like mating in trilobites (technical, open access)

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