Every Lady Animal Has Eyelashes: Sexual Dimorphism

Frogs with eyelashes. Birds with breasts. Balding fish.

While differences between male and female animals do exist in the real world, they rarely appear in the way Disney might have you think.

Sexual dimorphism, or “two shapes”, is when the adult male and adult female of a species have different traits, apart from their primary sex organs (ovaries, testes, uterus, etc). These traits usually fall into one of four categories: secondary sexual characteristics, coloration, behavior, and size.

Male lions have large manes, which juveniles and females lack.

Despite its fancy-sounding name, secondary sexual characteristics are the form of sexual dimorphism that you’re probably most familiar with. These are things like male lions’ manes, antlers on many types of deer (including moose), and larger tusks in male elephants. Secondary sexual characteristics are body parts that are associated with one sex, but that aren’t directly involved in making babies. These traits are often used to fight for mates, or to show that the animal is healthy and has an excess of resources. Humans have secondary sexual characteristics too -- breasts and beards are the most visible examples.

Male mallard ducks have a vibrant green head and yellow beak, while the hens are speckled brown as camouflage.

Coloration is a second type of sexual dimorphism. Birds are very common in this category -- think cardinals, goldfinches, and many hummingbirds. Most often, male birds are more brightly colored in order to attract females, while the female birds have more muted colors to blend in while they incubate the eggs. Birds aren’t the only examples though! Anything from guppies to butterflies can have color differences.

It's not entirely known why female mantises sometimes eat their mates.

A less obvious type of sexual dimorphism is behavior. Adult animals will show different behaviors between males and females. These behaviors are often linked to mating and are seen in a wide range of animals. This can range from weird mating dances in birds, to female spiders and praying mantises who eat their mates. Not all behavior is related to mating, though. In lion prides, only the females go out hunting, while in honeybees, all of the worker bees who gather honey are female.

The final type of sexual dimorphism is size, where one sex is heavier and bulkier than the other. In mammals and some birds, males tend to be larger than females in order to compete for resources and mates. On the other hand, in fish, many insects, and other birds, females tend to be larger, in order to create eggs and protect themselves and their offspring.

Male Krøyer's Anglerfish look like parasites or extra fins when attached to a female.

One of the most extreme examples of this is some species of anglerfish, where the males have evolved to be only centimeters long as they seek out females, which can reach over a meter in length.

With all of these types of differences -- secondary characteristics, coloration, behavior, and size -- it’s not too surprising that one species would show multiple types of sexual dimorphism.

Peacocks show color and behavioral differences, as well as differing secondary sexual characteristics.

Take peafowl for instance. Peacocks show an example of both color differences and a secondary sexual characteristic. Peacocks famously have large and vibrant tail feathers, as well as bright blue neck and chest feathers. Meanwhile, the females (called peahens) are mostly brown and white and do not have the same extensive tail feathers.

What does this mean for writing and worldbuilding? It means you can go wild!

From simply a design perspective, there can be differences between male and female animals not just in coloration or size, but even whole body parts.

For deeper worldbuilding, think about why the different sexes of a species look the way they do. Is there fierce competitions each year for mates? Who competes and what body parts are involved? Do males or females incubate the eggs, and do they need to hide from predators? As long as a trait helps an individual animal to reproduce more than other animals, then that trait will stick around and become more common!

This was a bit of a long read, so here are two take-aways for you:

1. Nature does have differences between sexes! They just don’t look like eyelashes and balding fish.

2. Differences are most often in color, size, behavior, or extra body parts.

Now go forth and create! There’s a good chance Nature has come up with something even stranger.