Ornithorhynchus may not appear a fierce warrior with his edentate bill and webbed feet, but with one swift jab of his hind leg, he can incapacitate any enemy long enough for mating to occur. Larger, more experienced males utilize battle as means of mate-acquisition. Younger, more naïve males choose to adopt stealth as their strategy by exploiting temporal shifts in mate-seeking. During breeding season, elevated testosterone levels induce more frequent male-male conflict as breeding partners are sought. Male platypodes do not rely solely on these massive testes to optimize their fitness. He whose testes are largest, produces the fastest sperm he whose sperm swims fastest, fertilizes. It is thought that sperm competition is likely the culprit.
Let it be known: despite their condition, male monotremes possess some of the largest testes in the world relative to their size. Termed “the testicondid condition,” platypodes are among birds, reptiles, and only a few other mammals that possess this primitive trait. They also possess ascrotal testes that reside within the abdomen. While the bifid, spiny penis is the sexiest of platypus reproductive structures, it is not the only morphology that is a little different in monotreme males. It is matched only by the bifid penis of the male, which possesses asymmetrical glans that fit within the female’s uteri in a perfect “lock and key.” To aid fertilization, the upper third of each bifid is covered in keratinized spines that induce ovulation as well as flowery papillae that bring sperm closer to the follicles. Such asymmetry, while common in many avian and some reptilian species, is rarely exhibited in mammals. On the left is a functioning ovary on the right lies a small embryonic sac of primordial follicles. In females, the cloaca opens into a urogenital sinus that bifurcates into two reproductive tracts. Retained from reptilian ancestors, this single aperture serves as both the exit point for excretory material and the exchange point for gametes. Unlike most other mammals, platypodes house their reproductive organs within a true cloaca. Particularly in reproductive biology, platypodes blend the structures of mammals, reptiles, and birds to make for some of the strangest sex and reproduction in the animal kingdom. One of three extant species in the basal order Monotremata, it exists as an amalgamation of inter-class characteristics that engender “Frankenstein’s monster” in taxonomic classification. The platypus ( Ornithorhynchus anatinus ) is a zoological enigma.
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