The prostate gland is often associated with human male health, but its presence, form, and function vary considerably among male mammals.
What is the Prostate and What Does It Do?
The prostate is an accessory gland of the male reproductive system, located below the urinary bladder and encircling the urethra. In humans, it resembles a walnut, producing fluid for semen.
This milky, slightly alkaline prostatic fluid makes up 25-30% of human semen volume. It contains substances like prostate-specific antigen (PSA), proteolytic enzymes, citric acid, and zinc. These components neutralize the vaginal tract’s acidic environment, maintaining sperm viability, motility, and transport. Muscle tissue aids in forcefully expelling semen during ejaculation.
Diversity of the Prostate Across Mammals
Most male mammals have a prostate, but its characteristics vary significantly. In humans, it is a single, compact organ with distinct zones. Dogs have a bilobed, ovoid prostate, their sole accessory sex gland, producing substantial fluid for territory marking.
Rodents, like rats and mice, have multi-lobed prostates (ventral, dorsal, lateral) supporting rapid reproductive cycles. Some rodent and bat species’ prostatic fluid contains a coagulant forming a “mating plug” after copulation. Carnivorans and boars have particularly well-developed prostates.
In contrast, some mammalian groups lack a functional prostate or have only rudimentary structures. Monotremes, like platypus and echidnas, lack a fully developed prostate or seminal vesicles, possessing rudimentary glandular tissue around their urethra. These egg-laying mammals feature a cloaca, a single opening for reproductive, urinary, and digestive systems.
Some carnivores (bears, weasels, ferrets, walruses, sea lions) also lack a prostate. Cetaceans (whales, porpoises) have only diffuse urethral glands as a prostate. Marsupials vary; some lack a prostate, while others, like the koala, have proportionally larger prostates than placental mammals, serving as the primary source of seminal fluid.
Evolutionary Development of the Prostate
The prostate gland’s evolutionary history begins in the male reproductive tract, originating from epithelial buds in the urogenital sinus during embryonic development. Androgens consistently influence this process. The prostate gland is unique to mammals, with no equivalent periurethral glands in reptiles.
This organ likely appeared around 65 million years ago, coinciding with broader mammalian development. Its evolution links to internal fertilization and the need for a sperm-supporting fluid environment. Varying prostate forms and functions across mammalian lineages reflect diverse evolutionary pressures and adaptations, with sexual selection playing a part as larger accessory glands correlate with increased sperm competition.