Bulletin

Issue: March 2005

Research in Action

Kneedeep in water! The message is in the mating croak

In a way some frogs seeking sex partners are the same as humans, only different. The girls go for the boys who can best convince them that they can provide a suitable place for sex and the production of strong healthy offspring.

And in the case of male Australian terrestrial breeding frogs in the Pseudophyrne genus, an ability to tell prospective partners about their desirable characteristics plays a vital role, making their mating call – or mating croak to be more exact – extremely important.

Just what the mating calls convey is the subject of a $270,000 three-year ARC Discovery Project research grant for La Trobe University post doctoral zoology fellow, Dr Nicola Mitchell.

Many species of this genus, including the brightly coloured Corroboree frog, are critically endangered through changes to their habitat all over Australia.

Dr Mitchell, as a result of 12 years working with frogs in Victoria, Tasmania, South Australia and Western Australia, is Australia’s foremost expert on terrestrial breeding species, which constitute about a quarter of Australia’s 230 frog species.

Her research has important conservation implications because it takes in the effects of changes in the hydrology of our landscape caused by increasing salinity, wetland and urban development and climate change. It examines the effects of variable moisture regimes on the physiology and reproductive behaviours of terrestrial breeding frogs.

Dr Mitchell explains that male terrestrial breeding frogs, which nest under leaf litter and rocks, choose nest sites which are usually dry in summer but which flood or become moist in winter.
Like many other creatures, males signal to attract females, employing their distinctive croaks for the same reason that peacocks strut displaying brightly coloured feathers.

Traditional scientific theory is that such behaviours attract females which know instinctively that the best displays come from prospective partners with the best genes which in turn make them the best breeders.

Scientifically this is called signalling theory, but in the case of terrestrial frogs, there are complications which don’t occur with other species.

Female terrestrial frogs ‘cruise’ around the males’ nests and then mate with the one whose croak she finds the most attractive. She lays her eggs in his nest and when the nest is later flooded or moistened, the tadpoles hatch and complete their development in water like other frogs. However, embryos that develop in wetter nest sites do better than those that develop in drier nests.

Because of this unique breeding environment in which males compete vigorously for the best breeding sites, Dr Mitchell believed that signalling behaviour may not simply be dictated by genetics as in other species.

‘My research will attempt to establish why the males in the wetter nest sites call more.

‘Do some males call better because they are stronger or because they are advertising a superior nest?

‘Or is it simply that a male’s high call output has a physiological basis in that moist breeding sites allow the male to take up water through its skin from its nest site, so perhaps it calls better from a moist nest site because it doesn’t have to worry about dehydration.

‘This may mean that males emit “honest” signals of nest quality because they have no choice and the female recognises this.’

Many scientists around the world are studying amphibians, but as far as Dr Mitchell is aware, none are researching the physiological constraints operating on call behaviours.

‘Most studies of amphibians focus on systems where males provide females with nothing but genes. My interdisciplinary approach and focus on a group poorly represented in the literature will challenge current paradigms in sexual selection.’