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Female frogs play the field

Multiple partners and nests increase odds that offspring will survive

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6:00pm, September 9, 2008
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The danger of putting all your eggs in one basket is very real for a small Australian frog.

A new study has found that female Pseudophryne bibronii frogs lay eggs fathered by up to eight different males in up to eight different nests. The mothers aren’t being risqué — the extreme behavior actually reduces risk of offspring death, the team reports in the Proceedings of the Royal Society B.

While polyandry — one female mating with multiple males — appears to be a common strategy among animals, eight partners in a row is a new record for vertebrates, says biologist Phillip Byrne of Monash University in Melbourne, Australia, who led the new study. Such dalliances might be more widespread among animals that nest in unpredictable environments or within mating systems where the male is in charge of the nest, the research suggests.

Male P. bibronii frogs make nests in little soil depressions along small waterways that come and go with the rains. Responding to calls from males, females come to mate and lay eggs, but it’s the dads who stay with the nest and the developing young. Nurseries must stay wet enough that the eggs don’t dry out and also must flood early enough that the tadpoles have time to grow into full-grown frogs (but not too early or the eggs get washed away). Because the mom can’t really predict which nests are best — it largely depends on when and how much it rains — there is always a high risk that the whole brood will be killed.

Led by Byrne, who has been tracking a population of these frogs for more than six years, the researchers staked out several nests in Australia’s JervisBayNational Park. For four months, from 6 a.m. to 6 p.m., the team kept track of almost 100 frogs. Byrne’s group also collected fertilized eggs and then reared the froglets. Genetic analyses based on toe tissue samples revealed that, on average, females divided their eggs among the nests of five males. The more nests a female visited, the more of her offspring lived, directly linking her promiscuity to fitness, the researchers report.

That direct link is surprising, comments Malte Andersson of the University of Gothenburg in Sweden. The usual reasons invoked for multiple mates are the odds of mating with a lousy dad or one with inferior genes, rather than seeking protection against nest failure.

Advances in molecular techniques that allow unequivocal paternity and maternity tests have uncovered several cases of polyandry in birds, fish and amphibians, suggesting is the practice is more pervasive than previously thought. This trend is contrary to the paradigm of promiscuous males and monogamous, choosy females, who risk disease and bad genes, among other things, when they have multiple partners.

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