I spent last week at a bird conference in North Carolina, a state where gender identity has been at the forefront of conversation. Between scientific talks, I sat down with a group of conference attendees to discuss how we could better include underrepresented voices in our research. Inevitably, as always seems to happen when scientists attempt to discuss non-scientific concepts, we found ourselves drawing on examples from our work—in our case, wildlife biology.

We tend to think of gender expression, and especially gender non-conformity, as uniquely human. However, many other species gain distinct advantages by projecting an appearance that doesn’t “match” their biological sex. Snakes, lizards, beetles, fish, and birds, to name a few, all exhibit “transgender” behaviors in which males imitate females to gain advantages, including reduced competition, better access to territory, and improved mating opportunities.

The most widespread example in the bird world may be a phenomenon called delayed plumage maturation. In many songbird species, young males keep female-like plumage for a few years after maturity in an effort to escape detection by older, more powerful males. By posing as females, they avoid becoming targets for aggression, allowing them time to grow and gain experience. Female-like males may also benefit from the element of surprise, sneaking unnoticed into the territories of older males and, sometimes, attacking them. While both these explanations of delayed plumage maturation have logical support, they have been difficult to verify in the field. Young males don’t always experience less aggression than males in mature plumage, and a study in which some young male songbirds were painted to look like older males found that the painted birds gained territory more quickly than those with female-like plumage.

In some species, however, we witness permanent female mimics: males that continue to resemble females throughout their lives. For one European bird of prey, the marsh harrier, 40% of adult males look and act like females. This species is highly territorial, and female-like males can avoid maintaining their own territories while sneaking onto other males’ territories to mate with the resident females. The ruff, a European shorebird, is the only other bird species known to have permanent female mimics. Male ruffs don’t attract females by guarding territories; instead, they gather to display in communal areas known as leks. Females who visit the lek can choose potential mates from among displaying males. As a result, most male ruffs have evolved elaborate plumage to show off… except for a few, which look exactly like the (much drabber) females. It’s hard to see why this undercover approach would offer an advantage in a species in which males gain mates by showing off. It’s possible that drab males hanging around the edges of a lek site can find mating opportunities with females by getting in between showier males and their conquests. However, there’s evidence that the ruffs themselves can recognize biological males even when they resemble females.

Most of the examples of what researchers call deceptive sex signaling in animals are of males posing as females. The reverse—females that imitate males—is a much rarer phenomenon. The aggression typically associated with male behavior may be to blame. Male mimics do exist in some species, however. In hummingbirds, male mimicry by females is more common than the reverse, and in hyenas, some females even develop male-like genitalia. Interestingly, in both these species, females guard territories and are aggressive toward other females.

While plumage and other physical characteristics are fixed by evolution, what may be more incredible is that some species also use deliberate behavioral tactics to imitate the opposite sex. The Australian giant cuttlefish, a brightly-colored cephalopod, provides a particularly dramatic, and clever, example. Smaller males are at a physical disadvantage in securing mating opportunities. However, by changing their pattern and posture to imitate a female, they can slip unnoticed beneath the gaze of larger males—and then mate with their female partners! The process is incredible to watch, both for its physical beauty and profound intelligence.

While it’s difficult to draw analogies between the gender-disguising ruses of non-human animals and human gender identity, the comparison highlights just how much we rely on visual cues in making inferences about one another (and about the creatures we encounter). With animals as with humans, it’s often wise to look more closely.

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