Roosters do have male reproductive organs, but they look nothing like what most people picture. Instead of external genitalia hanging outside the body, a rooster’s testes sit deep inside his abdominal cavity, and the vast majority of the mating process relies on a brief cloacal contact rather than penetration. This internal setup surprises many people who assume all male animals are built roughly the same way, and it raises real questions about how roosters actually fertilize eggs, what role testosterone plays in all that crowing and strutting, and why birds evolved such a different arrangement in the first place.
Where the Testes Are and How They Work
A rooster has two bean-shaped testes located high in the abdominal cavity, near the kidneys and along the backbone. Unlike most mammals, whose testes descend into an external scrotum to stay cooler than core body temperature, poultry testes develop and produce sperm right there inside the body at full avian body temperature, which hovers around 41°C (about 106°F).1PubMed Central. Morpho-Histology and Morphometry of Chicken Testes and Seminiferous Tubules among Yellow-Feathered Broilers of Different Ages That is several degrees warmer than what mammalian sperm can tolerate, yet roosters manage just fine under normal conditions. However, when ambient temperatures spike during heat waves, the extra thermal load does push sperm viability down. Research on broiler breeders has shown that elevated body temperature alone reduces sperm quality and contributes to infertility during heat stress.2PubMed. Elevated body temperature directly contributes to heat stress infertility of broiler breeder males
Inside those testes, tightly coiled seminiferous tubules are where sperm cells are produced. The process of spermatogenesis in chickens appears to kick off earlier in development than in many mammals, with meiosis initiating relatively quickly and a specialized barrier forming alongside the earliest stages of sperm maturation.3BMC Genomics. Single-cell RNA sequencing uncovers dynamic roadmap during chicken spermatogenesis The size of these tubules and the rate of sperm production are not fixed. They respond strongly to how many hours of light a rooster gets each day. Roosters exposed to longer photoperiods (around 16 hours of light) show significantly larger seminiferous tubule areas and higher sperm counts compared to those kept under shorter day lengths.4PubMed. Photoperiodic effect on the testicular transcriptome in broiler roosters
No Penis? How Mating Actually Happens
The most common question people have about rooster anatomy is whether they have a penis, and the honest answer is: barely. Roosters belong to an order of birds (Galliformes) that has largely lost the intromittent phallus their ancestors once possessed. Research estimates that roughly 97% of living bird species lack a functional intromittent penis, and chickens are firmly in that majority.5Current Biology. Developmental Basis of Phallus Reduction in Birds What roosters do have is a small, rudimentary genital tubercle, a nub of tissue that never develops into a full organ.
Instead, roosters mate through what is sometimes called a “cloacal kiss.” Both the rooster and the hen have a cloaca, a single opening that serves as the exit for the digestive, urinary, and reproductive tracts. During mating, the rooster mounts the hen, she everts her cloaca slightly, and the two cloacal openings press together long enough for sperm to transfer. The whole event lasts only seconds. It is not an elegant process by mammalian standards, but it works remarkably well. In commercial breeding, semen can also be collected from roosters using an abdominal massage technique and then artificially inseminated into hens, bypassing the cloacal kiss entirely.6PubMed Central. Timing and Interval Effects of Repeated Inseminations by Roosters on the Fathering of Chicks
One detail worth noting for anyone who has ever tried to sex day-old chicks: even though adult roosters lack a prominent phallus, newly hatched male chicks do have a tiny genital structure that differs subtly from females. Male chicks display a phallus with three small cone-like projections, while female chicks show only two. This distinction is so slight that it traditionally required trained human sexors to detect, though newer technology using optical imaging and machine learning is beginning to automate the process.7Poultry Science. Automated chick gender determination using optical coherence tomography and deep learning
Why Chickens Lost Their Phallus
The evolutionary story behind the missing rooster penis is one of the more fascinating findings in developmental biology. Chicken embryos actually start growing a genital tubercle, the precursor to a phallus, just as duck embryos do. The difference is what happens next. In chickens, a signaling protein called Bmp4 ramps up across the entire length of the developing genital tissue, triggering a wave of programmed cell death that causes the structure to shrink away before it ever becomes functional. In ducks, which retain a full corkscrew-shaped penis, Bmp4 expression stays restricted to the base of the tubercle and never reaches the tip, so the tissue keeps growing.8Current Biology. Development and Evolution of the Avian Phallus
Researchers confirmed this was not just a correlation by experimentally blocking Bmp signaling in chicken embryos. When they did, the cells that would normally die survived, and the phallus stopped regressing. Running the experiment in reverse, they activated Bmp signaling in duck embryos and saw chicken-like cell death and shrinkage begin.9PubMed. Developmental basis of phallus reduction during bird evolution The phallus was lost not because the genetic blueprint disappeared, but because a molecular “off switch” evolved to destroy the tissue mid-development. It is a striking example of evolution working by subtraction rather than addition.
Why this happened is less settled. One hypothesis is that females drove selection against the phallus, preferring males that needed cooperative mating rather than forced copulation. Ducks, geese, and ostriches, the minority of birds that kept their penises, tend to have higher rates of forced mating. But this remains speculative, and the picture is complicated by the fact that phallus loss happened independently in several bird lineages at different times.
Testosterone, Combs, and the Visible Signs of Maleness
Even without prominent external genitalia, roosters are visually unmistakable. The large red comb and wattles, the iridescent hackle and saddle feathers, the spurs, and the crowing are all driven by testosterone produced in those internal testes. Research on broiler chickens has found that circulating testosterone levels are significantly higher in roosters with large combs, and that testosterone levels correlate with testicular weight.10PubMed Central. Relationship between comb development, immune regulation, growth hormone, testosterone, and growth traits in Tianfu broilers The comb is not just decoration; it functions as a kind of external scoreboard of a rooster’s hormonal status and overall health. Hens use comb size as one cue when selecting mates.
The relationship between light exposure and testosterone adds another layer. Longer daylight hours do not just boost sperm production; they also promote testosterone synthesis in the Leydig cells of the testes. The mechanism involves suppression of melatonin, the hormone associated with darkness and sleep. When days are longer, melatonin production in the testes drops, which in turn lifts a brake on the hormonal pathway that generates testosterone.11Biology of Reproduction. Prolonging photoperiod promotes testosterone synthesis of Leydig cells by directly targeting local melatonin system in rooster testes This is why backyard chicken keepers often notice their roosters becoming more vocal, more aggressive, and more sexually active as the days lengthen in spring.
The Sperm’s Journey After Mating
Once sperm leaves the rooster, it travels through the vas deferens, a paired duct system running from each testis down to the cloaca. Along the way, the sperm cells pick up coating proteins secreted by the Wolffian duct lining. These proteins bind to the sperm surface and appear to play a role in how sperm behave once they reach the hen’s reproductive tract.12PubMed. Specificity of sperm-binding Wolffian duct proteins in the rooster and their persistence on spermatozoa in the female host glands
What happens on the hen’s side is equally remarkable. Hens possess specialized sperm storage tubules located at the junction between the uterus and the vagina. Sperm can survive inside these tubules for days or even weeks after a single mating, gradually being released to fertilize eggs over time.13PubMed Central. Unique Physiological Mechanisms of Sperm Storage and Prolonged Sperm Survival in Hen Oviducts: A Review The sperm count in these storage structures is highest shortly after mating and gradually declines; by about two weeks, numbers have dropped substantially, and fertility decreases accordingly.14PubMed Central. The sperm storage capacity in hens was correlated with the morphological differences of the oviduct and uterus-vagina junction This storage system means a hen does not need to mate before each egg is laid. A single successful mating can fertilize a string of eggs across many days.
Individual hens vary in how many storage tubules they have and how large those tubules are, which translates directly into differences in fertility duration. Hens with more and larger tubules maintain higher sperm counts for longer and stay fertile for more days after a single insemination. This variation matters in both commercial breeding programs and backyard flocks where rooster-to-hen ratios are not always ideal.
What Intensive Breeding Has Changed
Modern broiler breeder roosters are genetically selected for fast growth and heavy muscling, traits that have come with unintended reproductive side effects. A review of the scientific literature on mating behavior in these birds found that decades of selection for productivity have reduced or eliminated normal courtship displays in many commercial lines. The result is higher rates of forced copulation and increased injury rates among hens.15Current Biology. Aggressive Mating Behavior in Roosters (Gallus gallus domesticus): A Narrative Review of Behavioral Patterns In heritage and free-range breeds, roosters typically perform an elaborate courtship ritual involving wing-dragging, tidbitting (picking up food and calling hens over), and circling before mounting. Many commercial broiler males skip these steps entirely, which creates both animal welfare and fertility problems.
This is one reason the poultry industry relies heavily on artificial insemination for certain lines. In turkey production, AI is nearly universal because modern toms are so heavy-breasted they can barely mount hens at all. Chickens have not reached that extreme, but the trend toward larger, faster-growing birds pushes in the same direction. For backyard flock owners raising heritage breeds, natural mating remains efficient and largely trouble-free, which is part of why many small-scale keepers find roosters so much easier to manage than commercial breeding literature might suggest.
What Happens When You Remove the Testes
Caponization, the surgical removal of a rooster’s testes, has been practiced for centuries and provides a vivid demonstration of just how much a rooster’s behavior and body composition depend on those internal organs. Without testosterone, capons stop crowing, lose their aggression, and no longer develop the large combs, wattles, and spurs characteristic of intact males. Their feathers often take on a softer, more hen-like appearance.
The changes go beyond behavior and looks. Research on caponized crossbred chickens found that castration improved body weight, feed conversion, and carcass traits compared to intact roosters. The meat from capons was more tender, had better water absorption, and scored higher in sensory evaluations. Capon carcasses were also lighter in color and more yellow, with a higher total protein content and a more favorable ratio of omega-6 to omega-3 fatty acids.16Annals of Animal Science. Effects of Caponization on Growth Performance and Meat Physicochemical Properties of Crossbred Chickens Capon meat was historically considered a luxury product in European cuisine, and it still commands a premium in parts of France, Spain, and Italy. The practice has largely fallen out of favor in the United States and United Kingdom due to animal welfare concerns and the dominance of fast-growing broiler breeds that reach market weight before sexual maturity makes a difference to meat quality.
Birds That Kept Their Penises
Chickens are far from the only birds, but they are a useful reference point for understanding how unusual avian reproductive anatomy can get. Among the roughly 3% of bird species that do retain a functional intromittent phallus, ducks and geese are the most familiar examples. Male Muscovy ducks and Argentine lake ducks sport remarkably long, corkscrew-shaped penises that can rival the bird’s own body length. Ostriches, emus, cassowaries, and tinamous also have penises, reflecting the ancestral condition that most bird lineages later lost.
The contrast between ducks and chickens is especially instructive because the two groups are relatively closely related within the broader Galloanserae clade. As described earlier, the same Bmp4 signaling pathway is present in both, but the timing and location of its expression differ just enough to produce completely opposite outcomes: a rooster with a vestigial nub versus a drake with a fully developed spiral organ. That a single molecular dial can produce such different anatomy in close relatives underscores how evolutionary changes in gene regulation, rather than the invention of entirely new genes, drive some of the most dramatic differences in animal body plans.
For anyone keeping mixed poultry flocks, this anatomical difference has practical implications. Duck mating involves actual penetration, which means drake-on-hen crossbreeding attempts (while they do not produce viable offspring) can injure chickens more severely than rooster mating does. Flock managers who house ducks and chickens together sometimes need to separate drakes during breeding season for exactly this reason.