Why ducks have corkscrews, cats have spikes and humans have a 'simple' penis: evolutionist explains

Understanding why evolution needed the penis in the first place, and why it varies so much between species, helps us better understand human anatomy.

The author of The Conversation, biologist Michelle Spear, reminds us that the penis arose as a solution to one simple problem: how to ensure internal fertilisation.

Why the penis was needed in the first place

The first animals lived in the ocean, long before our ancestors came to land about half a billion years ago. Many marine organisms still just dump sperm and eggs into the water, where fertilisation takes place.

But with the transition to land, a new mechanism was needed - a way to deliver sperm directly into the female's body. That's how the penis evolved.

In land animals, however, this is not the only way. Some 97 per cent of bird species have no penis at all. They mate by means of what is known as "cloacal kissing" - a brief contact of the only orifice through which both excretion and reproduction take place in birds.

Such mating requires precise "choreography": a fraction of a second, complex mating rituals and perfect matching of postures. Animals with a penis get an anatomical "simplification" of the task: sperm can be delivered quickly, even if the contact is short and not too precise.

The penis is only one solution to the problem of fertilisation. But once evolution "chose" it, the options began to multiply. This is an example of convergent evolution: different groups of animals independently arrive at similar solutions under the pressure of similar conditions.

Records and extremes: from barnacle crayfish to banana slugs

In some species, penis size and shape are a response to environmental conditions and access to mates.

• In barnacle crayfish, the penis is the absolute champion of length relative to the body: it can reach up to eight times the length of the animal itself. Attached to one place, these crayfish literally "look" for partners around with the help of the elongated organ.

• Therecord for absolute size is held by the blue whale: its penis can reach 2.5-3 metres.

Thebanana slug is a hermaphrodite with a thick penis as long as its body. It is needed to deliver sperm deeper and increase the chance of fertilisation. Sometimes the organ gets stuck during extraction - then the partner simply chews it off. The slug usually survives and heals.

Sperm competition: spikes, knives and corkscrews

In many species, the shape of the penis reflects sperm competition, where several males mate with the same female and their sperm literally compete inside her body. In such cases, the penis becomes a tool of competition.

• In domestic cats, the penis is covered with spines pointing backwards. They stimulate ovulation in the female - increasing the chance that sperm will meet a ready egg. But they also make "uncoupling" painful, making it less likely that the female will immediately approach another male.

• In bedbugs, the male goes even further: his penis resembles a dagger. He pierces the female's abdominal wall and injects sperm directly into the body cavity. This "traumatic coitus" bypasses the usual sexual pathways, but at a high cost to the female: she has to spend resources to heal the wounds.

A particularly vivid picture of evolutionary "weapons competition" is that of ducks. In some species, males have spiralled penises that can shoot out in a fraction of a second. This is thought to be a response to evolutionary changes in females: their vaginas are becoming increasingly tortuous, with dead-end branches and spirals twisted in the opposite direction. Such an example is called sexual antagonistic coevolution, when "offensive" traits in males cause reciprocal "defensive" changes in females.

Dual organs in reptiles and the ossicle in many mammals

In many reptiles, evolution has solved the problem of mating posture in a different way. Snakes and lizards have hemipenis - two sex organs - and only one is used each time they mate. This "redundant system" gives more flexibility in position choice and may increase the chance of fertilisation in brief encounters.

In many mammals, the penis is "reinforced" by a bone, the baculum. Dogs, chimpanzees, walruses and a number of other species have it. The presence of the bone allows an erection to be maintained without relying solely on blood pressure. This is useful where mating is prolonged, the position is awkward or extra stimulation is needed for ovulation.

What all this says about the man

Against the background of such a "zoological fantasy", the human penis seems very simple. But this simplicity is deceptive.

Firstly, humans do not have a baculum. An erection is achieved only by the flow of blood. This may be due to the transition from brief, frequent mating with high sperm competition (as in many primates) to longer sexual encounters and stable emotional unions. In such a system, an erection is not just for fertilisation, but a signal of arousal and health, important for long-term relationships.

The shape of the human penis can also carry traces of sperm competition. According to one hypothesis, the slight enlargement of the glans and the pronounced "corona" (corona) at the border with the shaft help to partially displace the sperm of the previous partner during friction. This is particularly relevant for humans, as intercourse and ovulation rarely coincide perfectly, and sperm can survive in the genital tract for up to five days.

The glans and frenulum contain a high density of nerve endings, making them particularly sensitive. This is important not only for pleasure, but also for real-time feedback: the organ responds to changes in pressure, movement, and interaction with a partner, which can enhance mutual engagement.

In 2011, a genetic study published in Nature showed that humans have lost sections of DNA that control the development of penile spines, which chimpanzees and macaques, for example, have. In these species, the spines increase friction and stimulate the female, shortening the duration of the act. Their disappearance in humans is thought to be due to a shift from competition - to co-operation and pair bonding.

This fits in with the idea of 'hidden ovulation' in females: unlike many mammals, humans do not explicitly display a fertile period. In response, male strategy is evolutionarily associated with prolonged intimacy, emotional attachment and protection of the partner, rather than just brief mating at the peak of ovulation.

As a result, the human penis is not just a reproductive organ, but part of a complex behavioural system associated with trust, intimacy and long-term partnership.