The echidna, or spiny anteater, is an unusual mammal belonging to the monotremes, the only mammals that lay eggs. This Australian native, covered in protective quills, possesses a reproductive anatomy unlike any other mammal. The echidna has a single organ with four distinct heads, or glans, which function in a specialized, alternating fashion during mating.
The Quadripartite Structure
The male echidna’s reproductive organ is a single shaft terminating in four rosette-like structures called glans. When not in use, the organ is retracted entirely inside a preputial sac within the animal’s cloaca, the single opening for waste and reproduction shared with reptiles and birds. Upon erection, the organ is expelled, reaching approximately seven centimeters in length, and is notable for its bright red appearance and backward-pointing spines along the shaft.
Anatomical studies have revealed the internal mechanism controlling this unique structure. The internal urethral tube, which carries semen, splits near the tip into two branches, and these branches split again, resulting in four distinct passages, each leading to one of the four glans. The echidna’s erectile tissue is structured to allow for a divided blood supply, unlike placental mammals.
The spongy tissue, known as the corpus spongiosum, which surrounds the urethra and keeps it open for the passage of sperm, is separated by a septum. This division effectively splits the single organ into two distinct halves along its length. This allows the echidna to control blood flow to only one half of the organ at a time, meaning only two of the four glans will become rigid and functional during any single copulatory event. The lack of erectile tissue common in other mammals means the echidna’s organ relies on a different mechanism for rigidity, which facilitates this selective erection.
Function and Alternating Use
The quadripartite structure facilitates sperm delivery through a process known as “alternating use.” During a mating session, only one side of the organ—two of the four glans—receives blood flow and becomes erect, while the other two remain flaccid. The functional pair of glans is perfectly positioned to deliver semen into the female’s two-branched reproductive tract.
The alternating action allows the male to switch the blood supply to the other half of the organ for the subsequent ejaculation. This ability allows the male to recover one half while immediately using the other, which reduces the pause needed between successive copulations. Experiments have shown that an anesthetized echidna could ejaculate up to ten times in a row by employing this alternating pattern.
This complex function is driven by the intense sperm competition observed in the wild. During the breeding season, a female is often followed by a line of up to ten males, known as an “echidna train,” for up to six weeks. The male that successfully mates with the female needs a highly efficient way to maximize his chance of paternity. Furthermore, echidna ejaculate contains cooperative sperm bundles of up to 100 individual sperm. The alternating use ensures that the male can rapidly service multiple females or repeatedly mate with the same female, giving him an advantage in a promiscuous mating environment.
Evolutionary Context of Monotreme Anatomy
The echidna’s specialized reproductive anatomy is a direct result of its classification as a monotreme, a group of mammals that diverged from the main mammalian lineage approximately 166 million years ago. Monotremes—the echidnas and the platypus—retain several ancestral traits, the most prominent being oviparity, or egg-laying, and the presence of a cloaca. The name “monotreme” itself derives from the Greek for “single opening,” referring to this multipurpose cloaca for waste, urine, and reproduction.
The male organ in monotremes is unique among mammals because it is used exclusively for the transfer of semen, not for urination, a function handled separately by the cloaca. This separation of functions has allowed the reproductive organ to evolve into a specialized structure. The platypus, the echidna’s only other living monotreme relative, also possesses an unusual structure, though it has only two glans and its shaft is covered in keratinous spines.
Both the echidna’s four-headed organ and the female’s bifurcated reproductive tract are adaptations that reflect an ancient evolutionary path, showing similarities to the reproductive organs of reptiles. This anatomical design differs greatly from the single-shafted organs of marsupial and placental mammals. It highlights the unique reproductive biology that has allowed monotremes to persist for millions of years.