Abudefduf sexfasciatus Female fish use test eggs to gauge the parenting skills of prospective fathers Female scissortail sergants allow potential mates to fertilize a small batch off eggs and then monitor their parenting skills to decide if they are good enough to deserve a full clutch.

When studying filial cannibalism* in scissortail sergeants, ecologist Andrea Manica** of the University of Cambridge noticed that some females approached a male’s nest, deposited a small amount of eggs, and then left.

This aroused his curiosity and he decided to provide the males with ceramic tiles to use as nest sites. Once a female has deposited a small clutch on a ceramic tile, Manica either left the eggs alone or rotated the tiles to move the eggs.

The tiles that were left alone turned out to be popular; two-thirds of the females returned to deposit a full clutch of eggs later. The tiles that had been rotated by Manica were much less desirable and only a quarter of the females returned to lay a new batch.

Overall, this method of testing potential fathers seems to be rather rare in the population researched by Manica. Out of 421 females, only 7.4 percent laid test eggs before depositing a full batch. Manica also noticed that the method was used mainly at the onset of the breeding cycle. Later in the cycle, the amount of eggs already inside a nest seemed to suffice as indicator.

The female fish probably use these test eggs when they don’t have much to go by. As a strategy, to me it makes lots of sense. There are probably lots of other species that do that,” said Manica.

The Scissortail sergant (Abudefduf sexfasciatus) is a large damselfish native to coral reefs in the Indo-Pacific. Also known as the Striptailed damselfish, it can be recognized on its black striped tail and sides. In this species, the eggs are cared for by the male fish who must not only resist the urge to eat his own offspring but also be brave and skilled enough to protect them from being eaten by other predators.

The study has been published in Animal Behaviour.

* Filial cannibalism is when an adult eats the young of its own species. In many species of fish, adults won’t hesitate to eat even their own immediate offspring.

** http://www.zoo.cam.ac.uk/zoostaff/manica.html