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Animal Biology Is Not Binary
Both human and nonhuman animals can have diverse anatomical sex characteristics.
Posted April 4, 2022 Reviewed by Tyler Woods
Key points
- Like humans, some nonhuman animals are intersex.
- Some nonhuman animals can be functionally both male and female, either simultaneously or sequentially.
- Some nonhuman mammals have unexpected anatomical sex characteristics, such as female hyenas with a penis.
As much as 6 percent of people are intersex, having chromosomes, gonads, and/or genitals that do not fit neatly into a male or female category. For this and other reasons, human biology is not binary.
Intersex Animals
Intersex conditions are not specific to humans. They occur in a wide variety of animals. In some cases, at least, they may confer advantages and thus be maintained in populations through natural selection. For example, there is a crayfish species in which 1-8 percent of the population is intersex. Some of these intersex crayfish have typically female chromosome combinations, but typically male sexual structures and behaviors. These individuals mate with typical females to produce offspring, of which 75 percent are typical females (determined by their chromosomes). Having more female offspring means having more offspring that can produce eggs and thus more “grandchildren.” This can be a big advantage if a local population is depleted, because the intersex crayfish can trigger fast population growth.
This is the tip of the iceberg when it comes to the diversity of biological sex characteristics of nonhuman species, which have been detailed by Joan Roughgarden of Stanford University in Evolution’s Rainbow.
Asexual Reproduction
Many single-celled organisms reproduce asexually by splitting in two, producing clones. Asexual reproduction has many advantages, including speed and efficiency. Some animals, including vertebrates, also engage in asexual reproduction. For example, in many fish species, every fish is female and produces eggs containing their full set of chromosomes (instead of half the chromosomes, as we typically do when we make sperm or egg cells). Some amphibians and reptiles can also reproduce asexually. Other fish species require sperm from a different species to start the egg dividing to form an embryo, but they don’t actually use the DNA from the sperm, so their offspring are still clones of the mother.
Some species of aphids—a kind of plant-eating insect—reproduce by cloning for much of the year but then produce a few male offspring and switch to sexual reproduction as winter approaches. Sexual reproduction generates much more genetic diversity than asexual reproduction, as it creates almost limitless combinations of chromosomes from the two parents. This genetic diversity can be crucial if the environment changes in an important way and only certain genetic combinations allow individuals to thrive. By engaging in both asexual and sexual reproduction, aphids may have the best of both worlds.
Even for species that exclusively use sexual reproduction, the sexes are not necessarily male and female. For example, some fungi have thousands of sexes and typically can mate with any sex other than their own.
Male and/or Female
What does it even mean to be male or female? In evolutionary biology, a male makes very small germ cells—called sperm cells in animals—that provide nothing but chromosomes, while a female makes much larger germ cells—called eggs in animals–that contain all the nutrients and cellular structures needed for survival and growth, in addition to chromosomes. No other characteristics are necessarily associated with being male or female.
In mammals, having a Y chromosome usually makes you male and the absence of a Y chromosome usually makes you female, but in other vertebrates, sex is determined by other chromosome combinations, or by the temperature of the egg in some fish, amphibians, and reptiles.
Some fish species maintain both female and male sexual organs and can produce both eggs and sperm at the same time—they are simultaneous hermaphrodites. Some simultaneous hermaphrodite species exhibit a kind of social equality. For example, after choosing a mate, fish called hamlets take turns laying eggs and releasing sperm onto them.
Other simultaneous hermaphrodites compete to adopt the male role, presumably to produce more offspring using less metabolic energy. Some flatworm species, for example, are simultaneous hermaphrodites that engage in “penis fencing” to determine who will stab the other to inject sperm.
Still other species switch from male to female or female to male, depending on their social situation–they are sequential hermaphrodites–which allows them to become the sex that can reproduce most successfully in each situation. For example, clown fish, famous as Nemo and Marlin in Finding Nemo, typically switch from male to female once they grow large enough to produce many eggs. In contrast, cichlids typically switch from female to male if and when they are large enough to defend a territory.
Anglerfish (also featured in Finding Nemo) live deep in the ocean where it is hard to find a mate. In some species, a male is a fraction of the size of a female. If a male encounters a female, he latches onto her permanently and her circulation grows into his body. He becomes essentially an organ of the female–her testes–which she can use to produce sperm as needed to fertilize her eggs.
There are also some mammals with hermaphroditic characteristics, though they are rarely if ever true hermaphrodites that can reproduce as either male or female. In one species of moles, for example, each female has ovotestes—gonads that are intermediate between an ovary and a testis.
In spotted hyenas, females have a penis—an elongated genital that is almost as long as a typical male’s penis—through which they urinate, mate, and give birth. In the wild, many mothers and even more infants die in the process of giving birth. Female spotted hyenas may have maintained a penis through evolution because it plays a critical role in social signaling in their matriarchal colonies: females signal subordinate status to a dominant female using their erect penises.
It should be clear from these examples that anatomical sex characteristics are diverse, not only in people but also in nonhuman animals. There are combinations of typically male and typically female body parts and there are body parts that are intermediate between typically male and typically female.
So biology is not binary, not in people and not in nonhuman animals.
Next time, I’ll discuss how brains are also not binary. From genitals to brains, biology refuses to be forced into just two categories.
A version of this post has appeared on Medium.
