Pipefish Syngnathus typhle are relatives of seahorses, and both species have rather unusual sex lives. Instead of the male donating sperm to the female, the female transfers eggs to the male; he then fertilises them and keeps them inside a pouch on his front. He is the one who is pregnant, and the one who gives birth.
Laboratory experiments had helped scientists understand some details of pipefish mating behaviour, but what really happens in the wild remained a mystery. To investigate, Adam Jones from the University of Georgia, USA, and colleagues took a number of male and female pipefish from a Swedish fjord, and compared small sections of DNA, called microsatellites, in females, males, and the embryos they carried. For the first time, it was possible to match the eggs carried by males to the females who had donated them, and find out exactly who had mated with who.
They found that there is a multi-male, multi-female mating system: a male accepts eggs from several females, and a female doesn’t put all her eggs in one basket, mating with a number of different males. This increases her chances of leaving at least some of her eggs with a male of high quality.
As well as this, however, they found something that surprised them. While most males accepted ten or so eggs from each of three or four females, some males had an extra one or two eggs from an yet another female. This is odd, because mating is a difficult process - why go to the trouble only to transfer one egg?
The researchers suggest that these anomalies could indicate the activities of ‘sneaky’ females - less successful individuals, who resort to surreptitiously depositing a few eggs into a male’s pouch while he is busy being courted by a more attractive female.
(Behavioural Ecology and Sociobiology vol.46, pp357-365)
Background
As well as male pregnancy, pipefish are unusual in a number of other ways. Female pipefish produce more eggs than any one male can fertilise. The opposite is true in most species - males, because they don’t have to care for the developing young, produce enough sperm allow them to mate with many females, while females produce relatively few eggs at a time. Pipefish females also develop ornamental markings to attract mates - normally, it is the male who displays to attract dull-looking females. These role reversals allow biologists rare chances to test theories about the underlying rules of sex evolution.