Very small male zebrafish use sneaky sex strategy

Zebrafish (Image: Azul) Zebrafish are well-studied, particularly in genetic research

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Small male zebrafish adopt sneaky tactics to improve their chances of reproducing, scientists have found.

Male zebrafish fertilise eggs after they are laid, a process of external fertilisation known as spawning.

Scientists from the University of Sheffield discovered that the most diminutive males were able to get between a female that had just laid eggs and larger, rival males.

This gave them first access to the eggs in order to fertilise them.

The findings are published in the journal Ethology.

Zebrafish are widely studied by scientists, particularly in genetic research.

There is now increasing interest in their behaviour, the study of which can shed light on how our distant aquatic ancestors evolved.

Fish society

SMALL, SNEAKY AND SUCCESSFUL

  • Small male mosquito fish (Gambusia holbrooki) are more reproductively successful than larger males, because they sneak up on females without being detected
  • Small male chironomid midges and some damselflies appear to enjoy increased mating success, probably because of they are more agile than their larger rivals
  • In some small mammals, such as chipmunks, smaller males behave more aggressively than larger males when fighting over mates
  • In some freshwater cichlid fish that lay their eggs inside shells, only the smallest males can enter the inner part of a shell to reach the female and avoid larger males that defend these shell nests

The University of Sheffield team that carried out this study was interested in one particular element of zebrafish "society" - their hierarchy.

"You can spot the dominant males," explained Penelope Watt, the biologist who led the study. "You see them patrolling the tank, chasing smaller fish away."

She and her team wanted to find out more about the importance of this dominance in reproduction, so they carried out paternity tests on zebrafish embryos to see if dominant males fathered more offspring.

The team placed a single female into a tank with two males - one dominant male and one subordinate. The female was contained in a plastic cylinder covered in fine mesh, so she could smell the males but could not make contact with them.

After 24 hours, the scientists released the female, allowing the fish to spawn, with males and females releasing masses of eggs and sperm together at the base of the tank.

When the female was ready to lay eggs, males jostled for position close to her, explained Dr Watt. The closer a male was to a female, the better the chance he had of fertilising the eggs she produces.

When the team DNA-tested the resulting offspring, they found, as expected, that dominant males had fathered more offspring than subordinate animals.

But the "subordinates" that had the best chance of fathering offspring were the smallest.

"It's probable that the smaller males are sneaky males and they're more manoeuvrable," the scientists said. "They can get in between the female and the larger male."

Such sneaky mating strategies are seen in other fish species, and this finding supports the idea that, in some species, male fish evolved "sexual tactics" to compensate for their diminutive size.

Dr Watt added: "It's nice to find out something about their behaviour that we didn't expect.

"They're a more complex species than we've given them credit for."

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