Most bird species, approximately 97%, do not have external male genitalia, setting them apart from most other vertebrates. However, a small but notable group of birds, including waterfowl and certain flightless species, retains a fully functional copulatory organ. This anatomical variation results from a deep evolutionary divergence, where most avian lineages lost the structure while others maintained the ancestral trait due to specific reproductive strategies.
How Most Birds Mate: The Cloacal Kiss
The reproductive method used by most male birds relies on the cloaca, a single posterior opening. This opening serves as the exit point for the digestive, urinary, and reproductive tracts in both sexes. During the breeding season, hormonal changes cause the cloaca in both the male and female to swell, creating a temporary bulge known as a cloacal protuberance.
Mating is accomplished through the “cloacal kiss,” involving rapid, precise contact between the two swollen openings. The male bird mounts the female, and both momentarily evert their cloacas, pressing them together to transfer sperm. This contact is brief, often lasting less than a second, though it may be repeated to ensure successful insemination.
Despite lacking an intromittent organ, this method effectively transfers sperm. The female’s ability to control the timing and success of the “kiss” influences paternity, as the male requires her cooperation to align the openings. The sperm is transferred directly into the female’s reproductive tract, where it can be stored for days or even weeks, depending on the species.
Avian Anatomy: The Birds That Still Have a Phallus
The approximately 3% of bird species that possess a copulatory organ belong primarily to two groups: waterfowl (ducks, geese, and swans) and ratites (ostriches and emus). These organs are referred to as a phallus and differ significantly from the mammalian penis in both structure and mechanism of erection.
Unlike the mammalian penis, which becomes erect through an influx of blood, the avian phallus uses an engorgement of lymph fluid for extension. This process, known as lymphatic erection, results in an organ that is less rigid and shorter-lived in its erect state. When not in use, the phallus is hidden within the male’s cloaca, only everting rapidly during copulation.
In waterfowl, the phallus often features a long, corkscrew shape with a distinct spiral groove that guides the sperm. Some species, such as the Argentine lake duck, have a phallus that can be nearly as long as the bird’s entire body. Ratites, like the ostrich, possess a more conical, non-spiral phallus, which utilizes the same lymphatic fluid mechanism for extension.
Why Most Birds Lost Their Penises
The evolutionary reduction of the phallus in most bird species is considered a puzzling anatomical change in the vertebrate lineage. Scientists have identified the specific genetic mechanism responsible for halting the organ’s development during the embryonic stage. All male bird embryos initially develop a genital tubercle, the precursor structure to external genitalia.
In species like the chicken and quail, the Bone Morphogenetic Protein 4 (BMP4) gene is activated in the tip of this developing tubercle. The expression of this gene triggers apoptosis, a form of programmed cell death. This cell-suicide program causes the genital tubercle to regress and wither away completely before the bird hatches.
In contrast, in species that retain the phallus, such as ducks and emus, the BMP4 gene is not activated in the same location, allowing the genital structure to continue its growth. This developmental change suggests the phallus loss was due to the de novo activation of a cell death signal, not a failure to grow.
An evolutionary theory suggests that the loss of the phallus may have been a selective advantage for female birds. Species with a retained phallus, particularly waterfowl, often engage in forced copulations, bypassing female cooperation. By removing the intromittent organ, female birds gained greater control over which male fertilized their eggs, a concept known as cryptic female choice. This increased female control over paternity could have driven the selection for males with reduced or absent external genitalia.