Female promiscuity and the terrestrial toadlet

For the majority of vertebrates, the egg is best. Most are oviparous – egg-layers – and their success depends very much upon the quality of the nest as well as the mate.

In unpredictable environments, some birds, fish and amphibians like to spread risk by practicing polyandry – the female mates with many males rather than sticking to one, or the more common practice of polygamy.

One Australian species of frog, Pseudophryne bibronii or Bibron’s toadlet, likes to do it this way too, and then some. This frog practices sequential polyandry, in which the female mates with multiple partners, leaving each male to tend their brood while she continues on to another.

(Another type is simultaneous polyandry, in which the female keeps only two males at her pleasure.)

In what the authors describe as the most extreme example of sequential polyandry yet recorded, P. bibronii females have been found to mate with up to eight different males.

In a paper published in the Proceedings of the Royal Society B, Dr Phillip Byrne of Monash University and Dr Scott Keogh of ANU write that there are a number of reasons why this may be so. Their main conclusion, however, is that the practice protects against nest failure.

This small frog lives in the eastern states of Australia, laying eggs in soil depressions constructed by the male near waterways. The males call out to the females, advertising themselves and their nest, and remain with the eggs as they develop.

The tadpoles live in suspended animation until a flood arrives and they are able to hatch. The survival of the tadpoles depends on the timing of the flood and the level of rainfall, if it arrives at all.

Unpredictability like this means frogs need to take out an insurance policy. By spreading her eggs amongst multiple nests, the female insures against disaster if some of the nests fail, the researchers say.

“Paternity analysis confirmed that females are extremely promiscuous,” they write. “On average, females divided their eggs among the nests of five males.”

Females that divided their eggs amongst more nests sites – and therefore more males – had much higher offspring survival rates.

“Our study advances our understanding of female promiscuity by being the first to show that promiscuous females can safeguard against choosing fathers that provide poor homes for their offspring,” Byrne said.

“It is becoming increasingly apparent that females in many animal species choose to mate with multiple partners as a safeguard against choosing a genetically inferior sire, but insurance against a father who provides a lousy home is a novel and potentially widespread explanation for the evolution of female promiscuity.”