Frogs have long been known as great vocal communicators, but now Australian biologists have found evidence that they also communicate by smell.
Until now, scientists had largely ignored olfactory communication in frogs, while concentrating on the more obvious vocal signals.
In the research, published today in The Royal Society's Biology Letters , the authors say that the sophisticated use of scent in salamanders and tadpoles suggest smell may be more important to adult frogs than previously thought.
The researchers from the University of New South Wales in Sydney examined two species of large, ground-dwelling frogs, the great barred frog (Myxophyes fasciolatus) and the striped marsh frog (Lymnodynastes peronii).
They wanted to see whether adult frogs could sense predators by smell; and if frogs were sensitive to the predation risk in olfactory communication. Adult M. fasciolatus tend to spend long periods in the same location, a behaviour that would lead to odour accumulating at mating and calling sites.
That would be an important odour cue to foraging predators, including the red-bellied blacksnake, Pseudechis porphyriacus.
The smaller L. peronii frogs are also potential prey for larger M. fasciolatus individuals.
The researchers used a series of choice trials to determine whether individuals were attracted to the odours or to other frogs, and if individuals were sensitive to the predation risks associated with these odour accumulations.
In a controlled arena, the frogs were given a choice of sand-filled plastic containers as shelter.
The containers were scented differently each time, sometimes with the frog's own scent, sometimes with the scent of another frog of the same species, and sometimes with the scent of the striped swamp frog, L. peronii.
The great barred frogs showed that they were attracted to all frog scents, and generally preferred the scented shelter as compared to the unscented control. When the scent of a red bellied black snake was introduced, the male frogs preferred the site with the scent of another individual of its own species; but shunned sites with their own individual scent, or the scent of the striped marsh frog.
Juggling act between pleasure and pain
Associate Professor Peter Banks of the University of New South Wales, a co-author on the paper, says it's interesting that the frogs favour sites where they are more likely to find mates, even if that represents a greater danger from predators.
"It's all part of the juggling act that they have to do," he says.
Banks says decision making on an olfactory basis hadn't been proved in frogs before.
"Lots of people have focussed on their auditory communication, their calls, because that's the most conspicuous thing," he says. "But frogs have very sensitive skin and they're very sensitive to chemicals in the environment. So it was only logical that adult frogs would be picking up on chemical information such as odour marks or scent marks; but no-one else had [demonstrated] it."
"It's not really that surprising, when you think about it, given the physiology of the animals and the way they behave. They've got preferred sites that they call from, and they'll be leaving chemical traces behind in those spots."
Banks says the research creates a whole range of fresh questions.
"What information might be in these marks? Is it just that it smells like another frog, are there signs of individuality in there, is there information about the breeding status of the animals that are there? How exactly are they using that information?"
According to Banks, research into the use of odour by rodents has been well studied for more than 30 years.
"This paper is the first information we have about [frogs] communicating in this way," he says.