Frisky frog Bibron's Toadlet shares the love like no other

SURVIVAL'S mysteries sometimes take biologists in surprising directions, in Phillip Byrne's case enabling him to claim an impressive title for the Bibron's Toadlet as the most promiscuous female vertebrate.

SURVIVAL'S mysteries sometimes take biologists in surprising directions, in Phillip Byrne's case enabling him to claim an impressive title for the Bibron's Toadlet as the most promiscuous female vertebrate.

Until now, this honour has resided with shore birds such as the spotted sandpiper and the bronze wing jacana, who may have up to four or five partners. The female Pseudophyrne bibronii can manage up to eight.

Byrne, a behavioural ecologist at Monash University, in collaboration with the Australian National University's Scott Keogh, studied a 100-strong group of the frogs in Jervis Bay National Park, on the NSW south coast, in 2005.

Four months of overnight shifts, seven days a week - and DNA testing - revealed the female of the species deposited eggs in the nests of up to eight males.

In the world of the Bibron's Toadlet, breeding takes place soon after the first autumn rainfall, when the males, having built their nests in the leaf litter of eucalypt and casuarina trees, begin vociferously calling in an attempt to attract a mate.

A female selects a male's nest and as she releases her eggs he fertilises them. He then takes responsibility for them until they hatch, which normally would be at about five or six weeks, but could be up to five months later.

"In the recent past, insights into mating behaviour could only be gained by directly observing the animals, but females are generally covert operators so DNA techniques make it much easier to establish paternity and detect promiscuous behaviour," Byrne says. He says evidence is building that females in many species choose multiple partners in a bid to provide offspring with the best chance of having a genetically superior sire.

What is new is that, in the case of the Bibron's Toadlet at least, promiscuity may be linked to giving the young the best possible nest in which to develop. This is particularly important because the nests are vulnerable to weather.

"The eggs enter a state of suspended development until heavy winter rainfall floods the nest site and triggers hatching," Byrne says.

"But a lot of nests fail, either because they never flood or they flood prematurely, so it is likely to be very difficult for the female to select a nest that will succeed."

A further complication arises if the nest becomes too wet and eggs hatch prematurely, before there is enough sustained moisture to support them.

"We don't know yet whether females are choosing their mates indiscriminately, or whether they have preferences for particular characteristics of the male or his nest site," Byrne says.

Byrne's next step is to work out how important to the female's choice is the male's mating call, that is, whether it is an indicator of genetic quality.

Their work has implications for conservation. "Understanding reproductive behaviour is critical for the management of threatened populations in nature, as well as the establishment of successful captive breeding programs," he says. His and Keogh's findings regarding the Bibron's Toadlet were recently published in the esteemed journal Proceedings of the Royal Society of London.