A skinny little fish called the pipefish is high on the list of wildlife oddities, for the male of the species is the one which gets pregnant.

The fish is a close cousin of the seahorse and sea dragon, which also carry developing embryos in a unique organ called a brood pouch into which the female deposits her eggs during mating.

The male pipefish can carry between five and 40 offspring in its transparent pouch, and nurtures them to term after a pregnancy lasting between 12 and 14 days.

But a new study shows that the pipefish is more than a piscatorial gender-bender. It also chooses which of the brood survives, on the basis of the mother's attractiveness.

Biologists Kim Paczolt and Adam Jones from Texas A&M University in College Station, Texas studied consecutive broods brought to term by male Gulf pipefish - Syngnathus scovelli.

They made a curious discovery: Some pipefish were excellent dads and other were deadbeats - and the key lay with the male's attraction to his mate.

Male pipefish tended to seek out larger females for mating, the duo found.

If they mated with a less ample female, they often selectively aborted some of the embryos, apparently to save resources for future reproductive opportunities.

"The bottom line seems to be, if the male likes the mum, the kids are treated better," he said.

"Why this occurs, we don't fully understand, but our findings are quite specific about this relationship... If the male prefers the female, he treats their mutual offspring better."

He added: "It's almost as if he is saying, 'Are these babies worth more effort?' If he is not overly fond of the mother, the answer appears to be 'No,' and he invests fewer resources."

Genetic trade-offs and mating choices are commonplace as animals strive to have offspring with the best possible chance of survival. But this is the first time it has been observed after copulation in a sex-reversed species, says the paper, published in the weekly British science journal Nature. Pipefish are widespread in warm sea waters. They grow to around 10 to 12 centimetres in length and look rather like an elongated version of the seahorse.