BioEd Online

Female fish often entrust males with eggs for safekeeping — but how can they be sure that the males are up to the task?

An ecologist has now come up with evidence to support a 17-year-old hypothesis suggesting that some females try out potential mates with a small batch of 'test' eggs before breeding with them.

Males may fail as fathers for a number of reasons. They may be unable to defend eggs left in their care, for example, or even decide that the eggs would make a tasty snack.

Andrea Manica, of the University of Cambridge, UK, was studying the latter — a behaviour called filial cannibalism — in scissortail sergeants (Abudefduf sexfasciatus) in Malaysia when he noticed that some females would approach a male's nest, deposit a small number of eggs, then skedaddle. Manica wondered whether the females were testing the males.

So he provided males with ceramic tiles to use as nest sites and patiently waited for females to lay small clutches of eggs on the tiles. Then Manica — who spent up to six hours at a time underwater watching the fish — either left the tiles alone, or rotated them to move the eggs.

When the eggs were undisturbed, two-thirds of females who laid test eggs came back and laid a full clutch, he reports in Animal Behaviour¹. However, when the eggs were moved — implying that the father had been inattentive — only a quarter of females returned and laid a full clutch, ready for the male to fertilize.

Overall, only 7.4% of the roughly 421 females actually laid test eggs before breeding, and this strategy was used mainly at the start of the breeding cycle. Later on, the number of eggs in a nest seemed to provide a reliable enough indicator that the father was up to the task. "The female fish probably use these test eggs when they don't have much to go by," says Manica. "As a strategy, to me it makes lots of sense. There are probably lots of other species that do that."

A fishy tale

Back in 1992, ecologists Sarah Kraak and Eric van den Berghe had suggested that Mediterranean blenniids (Aidablennius sphynx) might use this kind of screening process².

"I had seen that females usually lay several hundred eggs, and initially I actually ignored these cases where only 1-10 eggs had been laid," says Kraak, who now works in the science of fisheries management at University College Cork in Ireland. "I thought it was irrelevant and mentioned it only casually to van den Berghe. I owe it to him that I started to pay attention to the phenomenon and collect the data for the 1992 paper; he came up with the actual hypothesis."

"I feel flattered that someone finally picked up on that 17-year old hypothesis and presents evidence for it in another species," she adds.