The question of whether male ducks, or drakes, possess testicles has a definitive answer: they do, and these organs are fundamental to the reproductive cycle of waterfowl. A drake’s testicles are the primary reproductive organs, responsible for producing sperm and the hormones that drive mating behavior. Unlike most mammals, a drake’s gonads are kept entirely within the body cavity. This internal placement is a characteristic shared across nearly all bird species, setting avian reproductive biology apart.
The Internal Placement of Male Duck Gonads
The male duck’s paired testicles are positioned high within the body, situated against the dorsal wall near the anterior lobes of the kidneys. This intra-abdominal location means drakes lack a scrotum, the external sac used by most mammals to regulate testicular temperature. The absence of an external scrotal sac is consistent with avian anatomy, as birds maintain a higher average body temperature than mammals.
The testicles are typically oval or bean-shaped and appear pale, often a creamy white or yellowish color, due to their rich vascularization. While paired, the two gonads are often asymmetrical; the left testis is frequently situated slightly higher in the body cavity and may be marginally larger than the right one. They are suspended from the abdominal wall by a fold of peritoneum called the mesorchium, which also carries the blood vessels and nerves supplying the organ.
This internal location allows the testes to be maintained at the bird’s core body temperature, which is suitable for spermatogenesis in avian species. In contrast, the testes of most mammals require a lower temperature than the body core to produce viable sperm, necessitating the external scrotum.
Seasonal Fluctuations in Testicular Size
The size of a drake’s internal testicles is not static, undergoing one of the most dramatic physiological changes observed in the animal kingdom. These gonads are highly dynamic organs, expanding significantly during the breeding season and regressing during the non-breeding period. This cyclical change is regulated by the photoperiod, meaning the increasing duration of daylight hours in the spring triggers the growth phase.
During the peak reproductive season, a drake’s testicles can increase in mass by hundreds of times their non-breeding size, sometimes expanding up to 300 times their quiescent volume. This massive increase reflects a surge in spermatogenic activity and hormone production, preparing the male for mating. The gonadal growth is driven by the release of gonadotropin hormones from the pituitary gland, which is stimulated by the perception of longer days.
Once the breeding season concludes, the photoperiod shortens, signaling the end of the reproductive phase. The testicles undergo rapid regression, shrinking back down to their minimal, non-functional size. This reduction conserves metabolic energy that would otherwise be spent maintaining large, active reproductive organs.
Sperm Transport and the Duct System
Once sperm is produced within the seminiferous tubules of the internal testicles, it is transported through a specific duct system to the cloaca for delivery. The first step is the epididymis, a small, compact structure that receives the sperm from the testis. In drakes, the epididymis is smaller and less complex than the equivalent organ found in most mammals, reflecting differences in sperm maturation and storage.
From the epididymis, the sperm travel into the vas deferens, a pair of coiled, muscular tubes that function as the primary transport and storage route. The duct is highly convoluted, allowing it to contain a substantial length despite its short apparent size. The muscular walls of the vas deferens propel the sperm toward the cloaca, the common opening for the reproductive tracts.
The vas deferens terminates at a small structure near the cloaca where sperm can be held until copulation occurs. This system is optimized for efficient, rapid transport of mature sperm from the deep abdominal cavity to the external delivery mechanism.
The Unique Anatomy of the Drake’s Phallus
The male duck’s external reproductive organ, known as the phallus, is highly distinct and functions as an intromittent organ, unlike the simple cloacal contact used by most other birds. When retracted, the organ is coiled and inverted within the cloaca, remaining hidden until copulation. This phallus is characterized by a remarkable spiral or corkscrew shape, which rotates in a counter-clockwise direction in species like the Muscovy duck.
Erection is achieved not through a rush of blood, as in mammals, but through a rapid influx of lymph fluid, operating as a lymphatic hydrostat. This hydraulic mechanism allows for fast eversion of the organ, a process that can occur in under half a second and reach speeds of up to 1.6 meters per second. The rapid nature of the eversion is necessary for effective copulation, which is often brief in waterfowl.
The phallus can reach impressive lengths, such as the Muscovy duck’s average of approximately 19 centimeters, and over 40 centimeters in the Argentine lake duck. This substantial, spiraled structure is believed to be the result of an evolutionary arms race with the female duck, whose vagina often possesses a complex, opposing spiral structure and blind pouches.