Swans do have penises, placing them in a small minority among birds. Roughly 97 percent of bird species lack an external male organ entirely, relying instead on a brief cloacal contact (often called a “cloacal kiss”) to transfer sperm. Swans belong to the order Anseriformes, the waterfowl, which along with a handful of other ancient lineages retained a functional intromittent phallus from the ancestor they share with reptiles. The story of why swans kept this anatomy while nearly all other birds discarded it turns out to involve embryonic programming, an unusual hydraulic mechanism, a striking evolutionary arms race between males and females, and even organs that change size with the seasons.
Which Birds Still Have a Penis, and Why Swans Make the Cut
The external male phallus appears to be the ancestral condition for amniotes, the group that includes reptiles, mammals, and birds. Among birds, it persists in only the most ancient lineages. The Paleognathes, a group that includes ostriches, emus, rheas, kiwis, and tinamous, all have some form of phallus. Beyond them, only certain Neognaths in the order Galloanseriformes, which encompasses ducks, geese, swans, and some gamebirds, retain one. When the massive radiation of Neoaves occurred, giving rise to the “modern” birds that now dominate the planet, the external male genitalia were lost entirely.1Current Biology. Bird genitalia
Swans, as members of the family Anatidae alongside ducks and geese, sit squarely in the group that kept the phallus. Not all Galloanseriformes carry the same version, though. Tinamous and other Paleognathes have spiraled intromittent penises, while many galliforms (chickens, turkeys, quail) have a reduced, non-intromittent structure that is not externally visible. Among waterfowl, coiled phalluses of varying length are the norm, with ostriches being the notable oddity: their penis has a solid collagenous shaft and is not inverted inside the cloaca.1Current Biology. Bird genitalia
Why Most Birds Lost the Organ in the First Place
The disappearance of the penis across most bird lineages is not a case of the organ never forming. In chicken embryos, researchers discovered that a genital tubercle, the precursor structure of the phallus, does begin to develop. It simply does not finish. The tubercle undergoes programmed cell death, or apoptosis, and regresses before hatching. In duck embryos, by contrast, the same structure continues growing into a fully functional penis.
The molecular trigger turned out to be a bone-growth signaling molecule called Bmp4. In chick embryos, Bmp4 is expressed in the developing genital tubercle in a pattern that drives the cells to self-destruct. When researchers blocked Bmp signaling in chick genitalia, the cells survived and the phallus regression was prevented. Going in the other direction, when they activated Bmp signaling in duck genitalia, the duck tubercle underwent the same apoptosis pattern seen in chickens.2PubMed. Developmental basis of phallus reduction during bird evolution In effect, the difference between a duck hatching with a penis and a chicken hatching without one comes down to whether a single signaling pathway is switched on or off at the right developmental moment.
This finding suggests that phallus loss in birds was not a single dramatic event but rather the result of a change in gene regulation that could be toggled. Swans, like their duck and goose relatives, simply never acquired the Bmp-driven regression that most other birds did.
How the Waterfowl Penis Actually Works
If you are picturing something that operates like a mammalian penis, the waterfowl version is fundamentally different. In most vertebrates, from reptiles to humans, erection is driven by blood filling vascular spaces in the organ. Birds underwent a major evolutionary transition: their erection mechanism switched from blood-based to lymphatic.3Journal of Zoology. The erection mechanism of the ratite penis Lymph fluid, rather than arterial blood, inflates the structure. This is a wholesale shift in plumbing that happened as birds diverged from their reptilian ancestors, and it applies to both the large-penised waterfowl and the ratites like ostriches.
In waterfowl specifically, the penis is normally kept inverted inside the cloaca, coiled up like a spring. When deployed, it everts outward in an explosive fashion. Researchers studying Muscovy ducks, which have a penis about 20 centimeters long, found that full eversion takes an average of just 0.36 seconds and reaches a peak velocity of about 1.6 meters per second. The collagen structure of the organ is unusually thin and not arranged in the rigid orthogonal pattern found in mammalian penises, making it highly flexible when erect rather than stiff.4PubMed Central. Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia
The speed and flexibility of eversion matter because, as it turns out, female waterfowl have evolved anatomical countermeasures. But before getting to that, it is worth noting that the waterfowl phallus typically spirals in a counter-clockwise direction. This chirality is not random; it plays directly into the coevolutionary dynamics between males and females.
The Sexual Arms Race Between Male and Female Waterfowl
Waterfowl are unusual among birds for having high rates of forced copulation. In many species, males that are not a female’s chosen partner will attempt to mate by force. This has set up one of the most vivid examples of sexually antagonistic coevolution in the animal kingdom: males evolving longer and more elaborate penises, and females evolving vaginal anatomy specifically designed to thwart unwanted intromission.
Female waterfowl vaginas are not simple tubes. A comparative study of 16 waterfowl species found two distinctive anatomical features: dead-end sacs and clockwise coils. Both appear to function as obstacles to the counter-clockwise spiraling male phallus, making successful penetration difficult without the female’s active cooperation. Across species, the degree of vaginal elaboration is positively correlated with phallus length. In other words, species where males have longer penises are also the species where females have more complex vaginal defenses.5PubMed Central. Coevolution of Male and Female Genital Morphology in Waterfowl
Researchers tested how well this female anatomy works in practice by attempting to evert Muscovy duck penises into glass tubes of different shapes. When the tube was straight, eversion succeeded easily. A counter-clockwise spiral tube, matching the chirality of the penis, also allowed successful eversion. But a clockwise spiral tube, mimicking the direction of female vaginal coils, or a tube with a sharp 135-degree bend, mimicking a dead-end sac, significantly impeded eversion.4PubMed Central. Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia The female anatomy physically blocks unwanted mating, giving the female substantial control over which male actually fertilizes her eggs, even in species where forced copulation attempts are common.
This dynamic has real consequences for female reproductive investment. Research looking at how these defenses relate to other traits found that egg size is negatively correlated with both penis length and the number of vaginal spirals. The apparent trade-off between egg size and morphological defenses against forced copulation is strongest in monogamous species, suggesting that females in pair-bonded species divert resources toward anatomical protection rather than larger eggs.6Journal of Avian Biology. The role of female investment in a sexual arms race
Where Swans Fit in the Arms Race
Swans are famously monogamous. They form long-term pair bonds, and forced copulation is much less frequent in swans than in many duck species like mallards or Muscovy ducks. This matters because the intensity of the genital arms race varies enormously across waterfowl. Species with high rates of forced copulation tend to have the longest penises and the most elaborate female vaginal anatomy. Species where pair bonds are strong and forced copulation is rare tend to have shorter phalluses and simpler female tracts.
Swans fall toward the less extreme end of the spectrum. They possess a functional phallus, yes, but it is not as dramatically elongated or elaborately spiraled as those found in the most contentious duck species. The correlation between mating system and genital elaboration holds across waterfowl as a group: the more conflict between the sexes over mating, the more elaborate the genital anatomy becomes on both sides. In relatively peaceful species like swans, the evolutionary pressure to escalate has been lower.
A Penis That Changes Size With the Seasons
One of the more surprising features of waterfowl genital anatomy is that the penis is not a fixed structure. It grows and shrinks with the breeding cycle. In mallards, penis length increases at the start of spring, when birds enter breeding condition, and decreases again in late summer as the reproductive season ends. These changes track with levels of androgen hormones.7BioOne (The Auk). Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl
This seasonal plasticity is not just about hormones cycling on autopilot. Research has shown that the degree of penis growth can be influenced by social environment, particularly the level of male-male competition. When more rival males are present, the competitive pressure appears to drive greater investment in genital size. This is phenotypic plasticity in action: the same bird can grow a longer or shorter penis depending on reproductive context, rather than being locked into one size by its genes alone.
For swans, this means the anatomy you would observe depends on when you look. A male swan examined outside the breeding season would have a substantially smaller phallus than the same bird measured in spring. This is common across waterfowl, but it is easy to overlook, because most people (understandably) are not inspecting swan genitalia at different times of year.
How the Breeding Season Reshapes More Than Anatomy
The seasonal changes extend beyond the visible organs. The cloaca, the shared opening for the reproductive, urinary, and digestive tracts in birds, hosts a microbial community that shifts dramatically with breeding condition. A study of wild birds found that males transitioning into breeding condition increased their cloacal microbial diversity by about 38 percent, with the number of observed microbial types rising by roughly 58 percent. When males shifted back out of breeding condition, diversity dropped again. Interestingly, female microbial communities did not show the same pattern of change during either transition.8Oxford Academic. The Cloacal Microbiome Changes with the Breeding Season in a Wild Bird
Why this matters for waterfowl health is still being worked out, but the implication is that the reproductive tract environment is not static. As the phallus grows and hormones shift, the microbial ecosystem of the cloaca transforms as well. This could affect susceptibility to infections, the transfer of microbes between mates during copulation, and potentially even fertility. In captive swan populations, where veterinary staff monitor reproductive health, understanding these seasonal shifts is relevant for timing health assessments and managing breeding programs.
Veterinary Concerns in Captive Swans
Phallus prolapse is one of the more common reproductive emergencies seen in captive waterfowl, including swans. Because the organ is normally inverted inside the cloaca, injuries, infections, or handling stress can cause it to evert and then fail to retract. A prolapsed phallus that remains exposed becomes swollen and prone to tissue damage, which can quickly become a serious welfare problem. In collections and sanctuaries that keep swans, staff are typically trained to recognize prolapse early so the organ can be cleaned, gently reduced, and, if needed, temporarily sutured to keep it in place while the bird heals.
Other reproductive health issues in captive waterfowl include cloacal infections and injuries from aggressive mating, particularly in mixed-species enclosures where larger males might interact with smaller females of different species. Because the phallus is lymphatic rather than blood-vascular, injuries tend to produce swelling patterns that are different from what a mammalian vet might expect, and the tissue is more fragile. Any facility keeping swans benefits from having a veterinarian who is familiar with the specifics of waterfowl anatomy, rather than extrapolating from more commonly treated birds like parrots or raptors.
Why This Anatomy Gets So Much Scientific Attention
Waterfowl genitalia have become a model system in evolutionary biology for studying sexual conflict, a situation where the reproductive interests of males and females diverge. The research is not just about cataloging weird anatomy. The interplay between male phallus length, female vaginal complexity, mating system, egg investment, and seasonal plasticity gives researchers a system where they can test big evolutionary questions about how competing selection pressures shape bodies over time.
Swans, specifically, are useful because they represent the low-conflict end of the waterfowl spectrum. Comparing them to high-conflict species like Argentine lake ducks, which hold the record for the longest penis relative to body size among all vertebrates, lets researchers see how the same underlying anatomy responds to very different mating pressures. The fact that the difference between having a penis and not having one comes down to a single molecular signal during embryonic development makes birds an unusually clean system for studying how complex organs are gained and lost across evolutionary time.2PubMed. Developmental basis of phallus reduction during bird evolution
For the casual observer who just wanted to know whether swans have penises, the broader context is this: swans sit at a fascinating intersection of ancient anatomy, active coevolution, and ongoing scientific discovery. They kept something that nearly all other birds threw away, and the reasons they kept it, and the form it takes, tell us a lot about how evolution works when the interests of males and females collide.