Secrets of love found under the dungheap

Philandering dung beetles have been used by Australian scientists to confirm the theory that promiscuous male animals have larger testicles than their monogamous counterparts.

The beetles proved the perfect subject for experimentation because they have a tendency to form complex love triangles beneath piles of steaming dung. In contrast to most other species, where the males are either wholly monogamous or wholly promiscuous, there are two distinct varieties of male dung beetle: the steady, reliable type that finds a female and guards her jealously, and the dissolute sort that sneaks in and mates with her while the other male is not looking - the dung beetle equivalent of the milkman.

Professor Leigh Simmons, of the University of Western Australia, studied the two different types of male to test the theory that sperm competition spells bigger testicles. The theory is based on the assumption that, in a species where a female may mate with several males, the best way for a male to ensure that his sperm fertilises an egg is to deposit as many as possible in one hit. Promiscuous males ought therefore to produce more sperm and to have bigger testicles than monogamous males.

So it proved with the dung beetles. For although the "guard", as the first type of male is called, is twice as big as the "sneak", they have exactly the same sized testicles. Professor Simmons discovered "sneaks" also have longer sperm. Typically, a 5mm-long "sneak" fires 1mm-long sperm - equivalent to a human producing 1ft-long sperm.

Professor Simmons said: "Dung beetles have a remarkable social life and the interactions between individuals can be incredibly complex. Sneaks are always engaging in sperm competition, while guards do so only occasionally, because they can defend females. Those different selection pressures result in different testes sizes."

On the question of whether the same theory applies to man, he said: "If certain males have a predisposition to a type of behaviour, then we should expect they should invest accordingly in sperm production if they are to reproduce successfully."

In other words, yes.