Harmonious orgy is winning formula for frogs

Female frogs that simultaneously mate with a dozen males produce stronger offspring, new research finds.

HAVING MULTIPLE SEXUAL PARTNERS at the same time is often frowned upon, but in the frog world, the more the merrier.  

New Australian research shows that individual females of the grey foam-nest tree frog (Chiromantis xerampelina) which mate with up to 12 partners at the same time, produce more resilient offspring than those individuals that mate with just one partner.

In what is documented as the most extreme form of 'polyandry' (sex with multiple male partners) in vertebrates, the unusual mating process lasts a few hours during a single night.

The harmonious orgy or 'simultaneous polyandry' begins when a female starts to release eggs onto a tree branch. Males then cluster around her and fertilise the eggs by producing sperm which they whip into a foamy 'nest' with their hind legs.

It's tiring work for the inundated female who periodically leaves the nocturnal action to rehydrate before returning to the nest.

Twelve's a crowd

"It's a very unusual system in which males do not attempt to usurp one another in their efforts to access the female," says Associate Professor Martin Whiting from Macquarie University in Sydney. "While females mate with multiple males, they don't actively solicit males in the way that promiscuous females from other species would."

Martin and coworker Dr Philip Byrne from the University of Wollongong compared offspring of the polyandrous tree frog females, found in the in the Limpopo Province of South Africa, to their more conservative single-partner counterparts.

They found that the offspring of mothers which had multiple partners had a better survival rate than tadpoles born of single-partnered frogs.

Hedging bets

It's not yet confirmed whether this mating method confers better genes or whether the offspring are nurtured into developing better survival skills. But, say the researchers, there must be some benefits to having multiple partners, which brings risks of disease transmission, injury, and increased risk of predation.

"It's almost certainly a genetic benefit," says Martin. Mating with multiple partners, he argues, acts as both fertilisation and survival insurance, as it can result in a broader array of genes in offspring. In turn, over long periods it promotes a diversity of genes that can help frogs cope in the face of changing environmental conditions.

"By covering their bases, these frogs are essentially bet hedging," Martin says.

The results "provide wonderful insights into the sex lives of these frogs", says genetics expert Professor Michael Jennions from the Australian National University in Canberra. "The next exciting step will be to confirm that 'good genes' are indeed at play and to work out how exactly females disproportionately obtain sperm from better quality males."

While it's not known how many frog species practice polyandry, studies have suggested that even among other species - including mammals - there are significant benefits for females who mate with multiple males, says Martin.  It gives a whole new meaning to the Ancient Egyptians' belief of frogs as a symbol of fertility - perhaps they really were onto something.  

The research was published earlier this month in the journal Behavioural Ecoology.

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