In the animal kingdom, the penis can be spiny, split, corkscrew-shaped, and even detachable. They are among the most diverse structures in biology. The human penis, which is very uniform, is an anatomical exception. Understanding why penises evolved — and why they’re so different — also helps explain why humans have one. The penis appeared as a solution to a simple problem: how to perform internal fertilization.
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The first animals lived in the sea before our ancestors began living on land half a billion years ago. Today, many aquatic animals still release sperm and eggs into the water. However, as organisms migrated to Earth, a new mechanism became necessary to transport sperm into the female body, and the penis appeared.
But here’s a twist: not all wild animals use it. About 97% of bird species do not have a penis. Instead, they reproduce through the “cloacal kiss” – a short connection between the cloaca, a single opening serving the digestive, urinary and reproductive systems, and through which sperm are transferred.
This “kiss” requires choreography. For most birds, reproductive success depends on precise timing, elaborate courtship, and impeccable body alignment. Animals with penises have an anatomical shortcut: they can deposit sperm directly onto the target, even if the encounter is brief or awkward.
In other words, the penis is just one among many evolutionary solutions. But when evolution chooses one of these possibilities, the possibilities multiply. It is a classic example of convergent evolution, where different lineages develop similar structures in response to the same stresses.
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In some species, penis size is determined by environmental factors and access to sexual partners. Barnacles, a crustacean associated with rocks, have the largest penis proportional to their body in the entire animal kingdom; It can reach eight times its size. This allows them to “hunt” for partners around them. For those wondering: the largest penis ever, at 2.5 to 3 metres, belongs to a blue whale.
The banana slug, a hermaphrodite, has a thick penis about the size of its body, which is adapted to deposit sperm deeply and increase the chances of fertilization. Sometimes he gets stuck in withdrawal and his partner bites him to get him out. However, the slug usually recovers and lives.
Often, penile structures evolve to deal with sperm competition, when multiple males mate with the same female and their sperm compete internally for fertilization. In these species, the penis becomes a competitive tool.
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The domestic cat, for example, has backward-facing spines on its penis. They stimulate ovulation in the female – ensuring sperm find a ready egg – and even discourage mating with other males, because they make retrieval painful.
Bedbugs go even further: they use a dagger-like penis to penetrate the female’s abdominal wall and deposit sperm directly into the body cavity. This painful fertilization gives the male a shortcut, but inflicts a significant physiological cost on the female. Although rarely fatal, injuries take time and energy to heal.
In no other species does this evolutionary conflict appear as clearly as in ducks. Some males have a spiral-shaped penis that extends in less than half a second. This is a reaction to the fact that the female vagina evolved with dead-end pouches and spirals that rotate in the opposite direction. This is a classic example of mutual antagonistic sexual evolution in which male traits that increase fertilization are countered by female adaptations that limit male dominance.
In many reptiles, evolution has resolved the problem of mating position, that is, the physical position and alignment of bodies during copulation, with a pair of reproductive ducts. Snakes and lizards have hemispheres, which are two separate organs, only one of which is used for sexual intercourse. This redundancy likely evolved to provide flexibility, allowing mating from both sides, and may be an adaptation for maximum success in short mating periods.
In mammals, the penis can be strengthened by a bone: the diaphysis. Found in species such as dogs, chimpanzees and walruses, it allows penetration without relying on blood pressure. This structural support is useful in species where mating is prolonged or where mechanical stimulation during copulation is necessary to stimulate ovulation, in clumsy or prolonged mating such as in walruses, and when the female’s anatomy or behavior favors longer copulation.
What does this penis reveal about humans?
Compared with this impressive diversity, the human penis appears conservative. But this simplicity is deceptive. Unlike many mammals, humans do not have a beak. Instead, an erection depends solely on blood flow. This mechanism may reflect a change: from short, frequent copulations, common in species with high sperm competition, to longer relationships based on emotional bonding. In this context, visible and hydraulic erections serve not only as procreative, but as a sign of arousal and health.
The shape of the human penis may also reflect adaptation to sperm competition. Researchers believe that a slight widening of the glans at the crown — the prominent boundary between the glans and the body — may help displace sperm from other males during sex. This is not uncommon among mammals, but may be particularly important in humans because sexual intercourse and ovulation do not always coincide, leaving more room for competition. Human sperm can survive up to five days in a woman’s reproductive tract.
The glans and frenum (its lower surface) contain a large number of nerve endings that are sensitive to touch. This sensitivity not only provides pleasure, but also provides real-time feedback, allowing responses to subtle changes in movement, pressure, and interaction with the partner. This type of sensory feedback may have played a role in amplifying mutual sexual involvement.
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A genetic study published by Nature magazine in 2011 revealed that humans lost the DNA sequences responsible for the development of penis spines, which are small keratin protrusions found in chimpanzees and monkeys, which increase friction and stimulate the female during mating. These spines potentially increase stimulation and shorten the duration of intercourse. Their loss among humans may reflect a shift from competition to cooperation.
This is related to another key aspect of human reproductive evolution: cryptic ovulation. Unlike many mammals, female humans do not clearly indicate when they are fertile. In response, males have developed strategies based on sustained sexual access, emotional bonding, and mate monitoring. Thus, the human penis is not just a reproductive organ, but rather part of a broader behavioral system associated with trust, intimacy, and long-term partnerships.
* Michelle Spear is Professor of Anatomy at the University of Bristol, United Kingdom.
*This article is republished from The Conversation under a Creative Commons license. Read the original text.