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“Biological Sex” in the Biological World

In light of recent political proposals in the UK, Alex Powell explores how the concepts of sex and gender are represented in nature, comparing this to the restrictive binary humans have created for ourselves.

A dog photographed at Malmö Pride 2016. Image Credit: Susanne Nilsson on Wikimedia Commons.


“It’s just not natural.”

A statement many LGBT+ people have to face time and time again upon disclosing their identities. The principles of nature and biology are often used to validate perceptions of what the ‘normal’ human experience is: after all, with millions of species on the planet, what makes us think that we can be the ones to defy the way we are programmed?


In reality, humans are distinct from other creatures not in the way that we exist on the spectrums of sexuality and gender, but instead in the rigidity of the norms we enforce upon ourselves. Not only are the restrictive standards we have constructed actually quite rare from an ecological standpoint, they can be extremely harmful for those who do fall outside the usual. The concepts of sex and gender have formed important parts of recent political agendas in the UK, with many adamant that these are unchangeable aspects of a person’s biology. This discourse has left transgender and non-binary individuals in an extremely vulnerable position.


Gender is, however, seldom as black and white within the natural world as some may have you believe. Investigations within each of the biological kingdoms have yielded numerous examples of organisms breaking the binary. At the moment, it feels particularly important to explore, celebrate, and learn from the complex ways in which different beings experience sex and gender.


To mark Pride month, I have collated some of my favourite examples of non-conforming creatures. These wonderful organisms should serve as a reminder to us as humans that flexibility in gender is inherently natural, and is both a remarkable and essential aspect of life on Earth.


You Change Your Mind like a… Moss Changes Sex?

The process of transitioning is a lengthy and arduous one requiring careful consideration for people whose birth sex does not align with their gender. Ironically, many other living things change their sex rather flippantly, making the decision as if it were comparable to what to have for dinner!


Tetraphis pellucida, the pellucid four-tooth moss, can readily become either male or female depending upon the density of its colony at any precise moment. This species competes with others to cover the wood in its habitat - when it does so sparsely, individuals tend to be female and reproduce asexually. However, as the colony becomes more dense, the population becomes dominated by males who produce gametes. This strategy, known as sequential hermaphroditism, acts to make sure that the population’s reproductive success is maximised in all situations.


Though this might seem strange to us, many organisms can change their sex to fit certain situations. This is often referred to as environmental sex determination, in which this characteristic is defined by conditions such as access to light or nutrients as opposed to genetics. Plants, which often possess combinations of male and female characteristics anyway, may change their sex in response to stress. An investigation into a species of maple tree has found that severely damaged males often re-grow as healthy females following a period of stress. Whilst transgender individuals make up a relatively minor proportion of the human population, this phenomenon is much more common within other species - in just four years of monitoring, over half of the maple trees studied changed their sex!


Environmental sex determination is also important in many species of reptile, with the temperature at which eggs are incubated playing a crucial role. The importance of this factor is demonstrated by the bearded dragon, as genetically male individuals can be born as females if eggs are kept above 32℃. Though this seems bizarre enough in itself, the circumstances get even stranger: despite now being female, the incubated lizards look and behave like males. As these ‘transgender’ reptiles are bolder and more active than other females, this odd occurrence has been suggested to be an adaptation to a warmer climate in which both being female and possessing male characteristics are beneficial for survival.

 The bearded dragon. Image Credit: MrsKirk72 on Pixabay.


Many other members of the animal kingdom can alter their gender. Around 500 species of fish have been found to change gender during their adult life! In some cases, sex changes are necessary to maintain healthy and functional populations: schools of clownfish, for example, are headed by a dominant female. When this female dies, the male who was formerly her second-in-command becomes female in order to take on her role. This maintains a necessary hierarchy and ensures the group is never left without a leader.


The Animal (Drag) Kingdom

In addition to changing sex entirely, it is quite common for animals of one gender to develop the appearance or characteristics of another. These drag queens and kings of the wild world often develop unique strategies to increase their chances of surviving and reproducing, and play important roles in ensuring the success of their species.


Though all sunfish are born a relatively dull colour, most males develop vibrant colouration as they age, whilst females remain comparatively drab. Some males, however, mature earlier than they are expected to - as they are forced to devote more of their energy and resources to producing sperm at a younger age, they are not able to adopt the standard bright appearance. These individuals are known as satellite males and look very similar to females of the same species. Although this may seem like a disadvantage, the colouration of satellite males allows them to evade aggression from the larger, colourful males which hold territories. Their unsuspecting appearance allows them to sneak into a nest and fertilise another couple’s eggs before slipping away unnoticed. Female mimics can be seen throughout the animal kingdom, ranging from birds to crustaceans.

The male (left) and female (right) bluegill sunfish. Image Credits: Bclegg77 and FredlyFish4 on Wikimedia Commons.


The male mimic, though less frequently occurring, can also be present in many species. The White-Necked Jacobin is a species of hummingbird common in the Americas, the juveniles of which sport brilliant blue and white plumage. Males generally retain this colouration into adulthood while females do not. Curiously however, around 1 in 5 females have been found to remain brightly coloured well into adulthood. Males of this species are known to be extremely hostile towards females attempting to feed within their territories - by closely resembling males, these individuals are less likely to experience aggression from territory holders, and are often able to feed for longer than females of the usual colouration. Gender non-conformity can hence provide enormous rewards.


Beings Outside the Binary

The clear-cut categories of male and female constructed by humans are rarely observed elsewhere in the biological world. Though we are beginning to view gender as more of a spectrum, some species have been deconstructing the binary since their evolution.


Fungi, for example, are far too complex in their systems of sex and mating to be categorised so simply. As opposed to whole chromosomes like the X and Y we are familiar with, sex is genetically controlled in fungi by small sequences of DNA called ‘mating-type loci’. Individuals are able to mate with any members of the same species with a slightly different code at one of these places in the genome. Depending on the number and size of these sequences that determine sex, there are thousands of possible combinations, and hence sexes, in some species of fungi. The fungus currently holding the record for the greatest number of genders is the split gill mushroom, Schizophyllum commune, the genome of which gives rise to over 23,000 sexes.


Intersexuality in humans is not regularly discussed, yet species possessing a combination of male and female characters have been identified across the animal kingdom. In butterflies, this can take a most peculiar and beautiful form: 1 in 10,000 butterflies are what is known as ‘bilaterally gynandromorphic’, split straight down the middle into male and female halves. This can include different wings, legs and antennae on each side of the body. Though some believe that this may occur when two different sperm fertilise the same egg, nobody is quite sure how this happens. Intersexuality has been observed in many organisms, ranging from deer that never shed the velvet from their horns, to whales possessing both male and female reproductive organs.

A bilateral gynandromorph of the Colour Sergeant butterfly, with a characteristically female left wing and a male right wing. Image Credit: Atanu Bose Photography on Wikimedia Commons.


Even when the categories of male and female are more defined, the two sexes often do not look quite how we would typically expect. The best-known example of an animal possessing characteristics associated with the opposite sex is the male seahorse, which carries and gives birth to its young. However, females with typically male features are often less discussed. The spotted hyena illustrates just one of the unusual ways in which this can occur, with females possessing what is, essentially, a penis.


The matriarchal structure of spotted hyena societies has likely selected for high levels of testosterone amongst females over evolutionary time. Whilst this allows dominant females to become more aggressive and successful in obtaining positions of power, it has also gradually increased the size of the clitoris and fused the labia to form a pseudo-scrotum. Not only does this structure appear remarkably similar to a phallus (with the exception that it is often larger than that of a male!), hyenas can copulate, urinate and give birth all through their pseudo-penis. The spotted hyena is the only female mammal to lack an external opening, though it is not the only one to possess a phallus: the females of some elephants, lemurs and primates have also been found to develop a pseudo-penis.

The female spotted hyena. Image Credit: alicave on Pixabay.


An abundance of living things have defied what humans consider to be the norm since their evolution, and have actually been quite successful by doing so. When the natural world contains so many examples of beings deconstructing the gender binary, it seems ludicrous that we consider humans doing that exact thing to be unnatural. Perhaps those using the grossly oversimplified concept of “biological sex” to support harmful propositions ought to take even the briefest of glances into the biological world.


About the Author: Alex Powell is an Ecology and Conservation Biology student at the University of Sheffield, with particular interests in zoology, animal behaviour and climate change. He hopes to pursue a career in ecological research and volunteers with the RSPB in his spare time. You can find out more on his LinkedIn page.

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