The World of Animal Genitalia: Bizarre, Fascinating, and Complex

There are few things Charles Darwin got wrong. Most of us can quickly point to his theory on inheritance (sans genetics), but a lesser-known oversight was his idea on the evolution of genitalia. Yes, genitalia! It turns out Darwin believed that genitalia can only evolve under natural selection, but not sexual selection.  It was not until a century later that evolutionary biologists started to consider genital evolution in light of sex, starting with the significant contributions of Dr. William Eberhard in the 1970s. Today, genitalia are known to be among the most diverse and rapidly evolving traits, and sexual selection is implicated in their evolution.

Under the sexual selection framework, male genitalia can evolve (1) to stimulate females to take up sperm, (2) for sperm competition, and/or (3) to hold onto resistant females. Given the complex nature of male and female sexual interactions, these three hypotheses are likely operating simultaneously. For example, some of these interactions occur inside the female, making it difficult to determine the role males and/or females play in the outcome. So, let’s say sperm succeeds at reaching the egg, it is hard to tell if it is because males have superior sperm competitive abilities, females choose sperm from the best males, or a bit of both. Although determining the exact mechanism(s) is difficult, evolutionary biologists have shown a role for sexual selection in the evolution of genitalia time and time again and in different organisms, ranging from birds to insects to fish to mammals. So, to kick-start your interest into the world of animal genitalia, below are just two examples from an impressively long, and sometimes bizarre, list.

Over the last decade, Dr. Patricia Brennan, now at the University of Massachusetts, and her colleagues have been studying the reproductive biology of ducks, which has one of nature’s most bizarre systems. Here, males have a long, cork-shaped phallus that twists clockwise, while females have a reproductive tract (called the oviduct) that twists counterclockwise. So, why would genital parts that twist in exactly opposing directions evolve? Using different glass shapes, resembling the oviduct, Dr. Brennan revealed that the counterclockwise glass shape actually slows down the eversion of the clockwise turning phallus of the males. (See the high-speed videos here: Oh, it is probably NSFW.) It turns out that during the mating season, males harass females, even the ones that have paired off, and such harassment can often lead to injury and even sometimes death. So, to fight back, females have evolved a reproductive tract that makes it harder for males to simply waddle up to them and mate. It’s definitely not all fun and games when it comes to duck sex!

Now, let’s move onto everyone’s favourite pest, bed bugs. To many of you, bed bugs are a living nightmare: destroyed furniture, itchy and bloody bites, and many sleepless nights. To evolutionary biologists, bed bugs are one of the most beautiful examples of male-female coevolution! At the University of Sheffield, Dr. Michael Siva-Jothy has dedicated his career to understanding the fascinating reproductive biology of bed bugs. Here, males use a needle-like intromittent organ (called the paramere) to traumatically inseminate the body cavity of females. Yes, to mate, males stab females and not even at their reproductive tract! Not surprisingly, these traumatic inseminations come at a high cost for females, often leading to infections, lower fecundity, and shorter lifespan. Unable to avoid the males, females have evolved a secondary reproductive tract (called the paragenitalia), which contains blood cells associated with immune responses, as a counteradaptation. Interestingly, males have played along and restricted their traumatic inseminations at the paragenitalia, thereby lowering the cost of mating on females. Why would males take part in this strategy, if they can pierce elsewhere? One hypothesis is this strategy ensures that females are just fecund enough to ensure male paternity and hence, male fitness. This is, hands down, way beyond tough love.

Alongside ducks and bed bugs, there are plenty of other cool examples of genital evolution, including the spikey penis of male seed beetles and claws at the tip of the intromittent organ of male guppies! At the University of Toronto, we are fortunate enough to have a number of experts studying reproductive biology in light of evolution and in very different systems, including, but not limited to, Dr. Maydianne Andrade (Department of Biological Sciences; UTSC) – spiders, Dr. Spencer Barrett (Department of EEB; St. George) – water hyacinth, Dr. Asher Cutter (Department of EEB; St. George) – nematodes, Dr. Darryl Gwynne (Department of Biology; UTM) – crickets, Dr. Helen Rodd (Department of EEB; St. George) – guppies, and Dr. Locke Rowe (Department of EEB, St. George) – water striders.

Although the rapid, divergent evolution of genitalia is a general evolutionary pattern, we have still have a poor understanding of the mechanisms (i.e., the three sexual selection hypotheses) underlying this pattern. Evolutionary biologists are now focused on studying the function of genital traits via various and rather creative experimental approaches, including the use of lasers. So, stay tuned to the truly bizarre, fascinating, and complex world of animal genitalia, there’s plenty more to come! Love it or hate it, the evolution of genitalia is, at the very least, a great conversation starter on a first (maybe hold off until the second) date.


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