The pursuit of reproductive success in the animal kingdom sometimes calls for extreme measures. But few creatures can match the hermaphrodite flatworm, which scientists have discovered can reproduce by injecting sperm into its own head.

The tiny aquatic worm, Macrostomum hystix, is able to self-fertilise because it produces both eggs and sperm. Although it prefers to reproduce with other flatworms, when no mating opportunities are present it resorts to using its needle-like penis to inject sperm into its own head.

The sperm then swim down the creature’s transparent body to fertilise eggs in the tail region, leading to viable offspring.

Dr Steven Ramm, an evolutionary biologist who led the work at Bielefeld University in Germany, said: “To us it sounds very gruesome, but to them it may be their best option. The alternative is not reproducing at all, so it’s making the best of a bad situation.”

The scientists believe the worms evolved their ability for “hypodermic insemination”, where sperm is injected directly into the body, because it allows individuals to enforce mating and overcomes any adaptation to the recipient’s reproductive tract, such as a plug left by another individual. “By evolving this way, creatures are able to take control over sperm transfer,” said Ramm.

“After developing the hypodermic penis, the worms developed a more simplified sperm, which allows it to be more mobile, and burrow through the body,” he added.



In the study, published in the Proceedings of the Royal Society B, 100 flatworms were split into two batches. The first were kept as isolated individuals, while in the second, worms were kept in groups of three. After four weeks, sperm living in the 1.5 milimetre-long worms was counted using a microscope.

The worms living in groups had more sperm in their tails, while those that were isolated had more sperm in their head or middle regions, suggesting they had swung their sharpened penis around to inject themselves in the head.



The flatworm is not the only animal capable of reproducing alone. A recent study found that female sawfish can become pregnant without contact with males through a mechanism called parthenogenesis.

“Lots of animals are able to self-fertilise, but this is the first example of one that uses a hypodermic appendage to do so,” said Ramm. “Snails can fertilise themselves, but they have connections between the male and female organs. In these worms, male and female organs are kept separate, so they need this system for fertilisation to work.”