The echidna has a four-headed penis

An alert reader sent me the following video, which in terrifying and graphic detail shows the remarkable four-headed penis of the echidna, or spiny anteater. (Remember that we discussed this species earlier in the week).

(Doesn’t that penis resemble the alien bursting forth from the guy’s stomach in the eponymous movie?)

This video derives from research described in a 1997 article in The American Naturalist. The methods and materials section is interesting . . . .

In 2005, Currumbin Wildlife Sanctuary (Gold Coast, Australia) came into possession of a 17‐year‐old captive male echidna that had become habituated to human presence as part of an interactive public display. Zookeepers noted that, on handling, this animal would readily produce an erection. Over a period of 2 weeks, zookeepers at Currumbin Wildlife Sanctuary conditioned the echidna to develop an erection to the point where it would ejaculate. In preparation for semen collection, the echidna is placed in lateral recumbency on a clean surface of the floor of its enclosure. Using a closed fist, the zookeeper gently pushes his hand into the lower abdomen of the animal, at which time the echidna usually responds by pushing its cloaca up against the zookeeper’s fist and developing an erection (fig. 1).

But the ejaculatory behavior of the generously-endowed male is unique:

During the early stages of erection, the glans penis displays all four rosettes, but as the erection continues, two of the rosettes retract, leaving the remaining rosettes fully engorged and slightly rotated, to give the fully erect penis a symmetrical appearance (; figs. 1, 2A). The penile morphology of the erect echidna penis is therefore, at this point, completely compatible with the anatomy of the cranial portion of the female’s urogenital sinus. In this form, the engorged penis would appear to deliver the semen directly adjacent to the female’s oviductal ostia. Ejaculation commences approximately 20 s after the penis has become fully erect. When erect, the penis was about one‐quarter of the echidna’s body length. Erection and ejaculation typically lasted between 10 and 15 min. During ejaculation, the semen pooled into the cups of the rosettes as a white viscous fluid (fig. 2A).

What is going on here? Is there any explanation for this bizarre ejaculatory behavior? One possibility arises from examining the echidna’s sperm:

Semen samples contained bundles of up to 100 spermatozoa that were joined at their apical extremity and were observed to swim progressively forward in a vigorous and co‐ordinated pattern.

To the authors (and also me), this cooperation among sperm implies one thing: sperm competition.  For by working as a team, a group of sperm can swim faster than individual sperm, something that’s already been observed in the opposum, in which many sperm join with another to form pairs in the ejaculate. If females can be inseminated by more than one male, anything that gives a male’s sperm a leg up in the race to the egg will be favored by natural selection.  In WEIT, I describe several adaptations in other species that may have been favored by sperm competition (these include “copulatory plugs” in rodents and “penis scoops” in damselflies — features of a male’s penis that he uses to remove the sperm of a previously-mating male before he ejaculates himself).  Features of the echidna’s biology indeed suggest that sperm competition is likely.  For one thing, they form “gravy trains”:

Perhaps the most likely is that the formation of bundles is associated with the evolution of some form of postcoital sperm competition (Jones et al. 2004). This idea is supported by the comparatively large size of echidna testes, the high number of extragonadal sperm (Jones et al. 2004), and the observation that male echidnas can form mating trains of up to 11 animals behind an estrous female, with the dominant male in the front of the line (Rismiller and Seymour 1991; Augee et al. 2006).[JAC emphasis].  In addition, mating can be a prolonged event (30–180 min; Augee et al. 2006) in the echidna and may be associated with a copulatory tie (Rismiller and Seymour 1991); both these phenomena are likely to be mechanisms that exclude copulatory opportunity for male rivals.

So the multi-headed penis may also play a role in reproductive competition between males, though we don’t know how. One website claims that the mystery of the four-headed penis has been “solved”:

Now the mystery of the four-headed penis has been solved, revealing another reptilian trait: male echidnas ejaculate with just two heads (half of the penis) at a time.

This resembles very much the way lizards and snakes ejaculate: they have a double penis (named hemipenis), but only one of the two penises is used during the copulation, while the other will effectuate the next copulation/ejaculation. Marsupials (another primitive group of mammals) are now in this matter something between monotremes and placental (evolved) mammals: they do not use half of the penis for mating, but still have a double headed penis, while the echidnas have a reptilian joined hemipenis, with each part of the penis in a marsupial-like fashion.

But ancestry from reptiles, who have two-headed penises, is not an evolutionary explanation of the echidna’s equipment, for their four-headed organ is a novel feature.  And how having such an organ helps you win “sperm wars” against other males is not clear.  Perhaps another alert reader can suggest an answer.