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Fact: Flatworms decide who gets pregnant by penis fencing.

First up, let me clarify: penis fencing is actually the official scientific term for this kind of behaviour, its not just me looking for a great tagline.

Flatworms are hermaphrodites – each worm has the reproductive organs of both a male (i.e. testes, which produce sperm) and a female (i.e ovaries, which produce eggs). This means that when two flatworms meet, they have a bit of an awkward moment: who’s going to be the girl tonight?

Obviously there’s only one sensible solution. The flatworms draw their weapons and fight it out like men. Literally. Each worm has an extendable dagger-like penis with which it attempts to stab the other. If a worm successfully stabs its opponent, it injects its sperm in a process called intradermal hypodermic insemination (hypodermic means ‘under the skin’ – think of a hypodermic needle used in medical injections). The alternative term for this process is ‘traumatic insemination’, which in my opinion describes the whole business perfectly.

Why the fight? It’s down to the Bateman Principle, which states that in almost all animals the father contributes far less resources to producing and raising the offspring than the mother. This is in part because the egg contains much more nutrition for the growing embryo than the sperm, and in part because the mother must often carry the pregnancy to term (and, in many species, look after the newborn babies) while the father can be off on his merry way. Therefore it’s in the interest of each worm to be the father rather than the mother and thus have offspring while investing the minimal amount of effort.

However, it turns out flatworms are actually quite cordial creatures. The vast majority of the time they will engage in bilateral insemination, where they take turns to inject each other with sperm. Perhaps not so traumatic after all.

Another one of Mr Shaunak’s Little Bites of Science
Image under creative common’s license

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