Sex life of a seahorse

They've long been thought to be the most faithful creatures on earth, but now scientists are lassoing these tiny beasts to see how monogamous they really are. Steve Connor takes a look at the trusty steed of the ocean

Each day they shadow dance and pirouette together in romantic displays of affection. Seahorses are renowned for forming strong pair bonds in apparently monogamous relationships that have to be constantly reinforced with a bit of twice-daily synchronised swimming.

Seahorses are also famous for practising an unusual form of extreme feminism. The male rather than the female gets pregnant and gives birth to live young - the only known instance of such behaviour in the animal kingdom.

Their reputation for fidelity is to be put to the test in an experiment being conducted this summer at the Weymouth Sea Life centre in Dorset, where members of the public will be encouraged to watch out for any adulterous behaviour.

Sarah Leaney, the centre's display supervisor, said that individual seahorses are being harmlessly tagged with little necklaces so that it is easier to tell them apart - and see who is courting whom.

"We're really trying to find out whether seahorses mate for life. It should be possible to monitor any infidelity as it happens," Ms Leaney says.

There are about 35 known species of seahorse in the world, two of which live in British coastal waters. The smallest species, the pygmy seahorse, which is just 16mm long, was only discovered in 2003.

Most seahorses are thought to form strong pair bonds that last for a breeding season, or even for life. However a few species, such as the big-belly seahorse (Hippocampus abdominalis), are believed to be more promiscuous.

"We'll be tagging our big-belly seahorses to see if they really do swap around as people think," Leaney says. "The males have these really big bellies which they use to attract females. While the males are incubating the eggs, the females get ready to lay the next batch."

Male seahorses have special pouches on their abdomens for incubating the eggs that they have fertilised internally. After an appropriate period of courtship - sometimes lasting up to nine hours - female seahorses deposit their eggs directly inside the male pouch using a tube-like ovipositor.

This direct insertion into the male pouch ensures that no stray sperm can get to the eggs before the male in question can mix them with his own sperm.

For the male this is important because the incentive for raising the brood himself is knowing for certain that the eggs have been fertilised by him and not by another male.

To all intents and purposes, the pouch of a male seahorse is a just like the womb of a female mammal. The fertilised eggs get embedded in the pouch wall and are bathed in a fluid that provides nutrients and oxygen, as well as carefully regulating salinity levels to make sure the embryos develop normally.

After two or three weeks of "gestation", the male seahorse goes into a form of labour, complete with contractions and heavy gulping. "Our female visitors get quite amused when they see this," says Ms Leaney.

Once the live young emerge from the pouch, male seahorses usually take no further interest in them. The young are perfect, miniature replicas of their parents and have to feed and fend for themselves. In the wild this means there is a very high infant mortality - sometimes 90 per cent or more.

At Weymouth, the survival rate is higher, with about 40 per cent of live young reaching adulthood. Captive breeding such as this is important because every species of seahorse is now on the Red List of endangered species - seahorses are a favourite of quack Asian remedies and an estimated 20 million are netted each year.

Why the seahorse is the only animal that goes in for male pregnancy has taxed the best minds in zoology. Conventional evolutionary theory states that it should be the female that invests more heavily in parental care because of the inherent differences in investment already made in making a minuscule sperm compared to a relatively massive egg.

If, however, a male can be absolutely sure that an egg has been fertilised by his own sperm, then he may have a good incentive to invest in that egg's future welfare. Transferring unfertilised eggs directly from the female's body cavity into the male pouch makes this highly likely - although it is still theoretically open to cheating by an adulterous female.

There must also be something about the ecology of the seahorse that makes it better to go in for strong pair bonding and male pregnancy. They are poor swimmers, and use their prehensile tail to grip on to seaweed to prevent them from being swept away.

Perhaps once you've found a suitable mate in this sort of enclosed environment, it is better to hang on to them rather than taking the risk of searching further afield.

For now, the reasons for their fidelity - and indeed that fidelity itself - remain uncertain. All that we can be sure of is that their fascinating appearance and graceful swimming will continue to charm their human admirers.