A new paper about the reproductive behaviour of the spiny anteater, to be published in the December issue of American Naturalist, makes for fascinating – if slightly disturbing – reading.
The spiny anteater (Tachyglossuss aculeatus) is a primitive mammal with an unusual four-headed penis. The animal is difficult to observe in the wild, and does not readily copulate when in captivity, so exactly how the male uses its penis was a mystery.
Stephen Johnston of the University of Queensland and his colleagues obtained a male spiny anteater which regularly produced erections when handled during public viewing sessions at a zoo, and had been “conditioned to develop an erection to the point where it would ejaculate.” Johnston’s team were therefore able to make the first observations of the animal’s erection and ejaculatory mechanism.
They discovered that the anteater’s ejaculatory mechanism is very strange – it is unique among mammals, and actually cloesly resembles the form of ejaculation observed in some reptile species. Further, they found that the anteater’s sperm forms bundles which consist of up to 100 cells (above right).
The spiny anteater’s penis is bilaterally symmetrical, and the glans is subdivided into four “rosettes”, all of which remain displayed during the early stages of an erection. But as the erection continues, the two rosettes on one side become engorged with blood, and the two on the opposite side retract.
Because the two unused rosettes retract, the erection is symmetrical in appearance (right). The erect penis – which is one-quarter the anteater’s body-length – is therefore fully compatible with the female, which has only two reproductive tracts.
Observation of consecutive ejaculations showed that the left and right sides of the penis are used alternately. But it remains whether or not ejaculation becomes restricted to one side of the penis during copulation; it is possible that sperm travels to the vas deferens on the unused side of the penis without being discharged.
When semen samples were examined using a scanning electron microscope, they were observed to contain bundles of up to 100 spermatozoa. This apparently increases the motility of the cells – although the precise velocities were not measured, large sperm bundles were found to swim more quickly than smaller bundles or individual cells.
Female anteaters sometimes copulate with up to 11 males in quick succession, so bundling may have evolved as a form of sperm competition. Alternatively, it could be a means of storing sperm within the female reproductive tract, or of preventing the cells on the inside of the bundle from maturing before fertilization can occur. A better understanding of sperm bundling should be possible now that the researchers have the ability to collect semen samples regularly.
This is the first time such ejaculatory behaviour has been described in a mammal. The ejaculation mechanism of the spiny anteater is reminiscent to that of the squamate reptiles (lizards and snakes), which have two alternately-used hemipenes that otherwise remain inverted inside the tail.
This study therefore supports the theory that monotremes (the mamalian order to which anteaters belong) have a close evolutionary relationship with reptiles. And further studies of the spiny anteater’s reproductive behaviour may provide useful information about how all mammals evoloved.
Read more about the spiny anteater’s bizarre sex life at New Scientist.
Johnston, S. D., et al. (2007). One-sided ejaculation of Echidna sperm bundles. Am. Nat. 170. doi 10.1086/522847. [Full text]