By Victoria Gill
Science reporter, BBC News
Wood frog tadpoles "freeze" in response to the odour of a predator
Frogs learn to recognise the smell of their enemies while they are still developing as embryos, say scientists.
Researchers in the US and Canada found that woodfrog embryos were able to learn the "level of threat" posed by their future predators - salamanders.
Embryos put into water containing the odour of a salamander and the odour of injured tadpoles learned that the predator's smell was a threat.
And the stronger the odour, the more dramatic the tadpoles' reaction.
Maud Ferarri, the biologist from the University of California (UC) Davis in the US who led the study, said that this type of learning had already been seen in fish, larval amphibians and larval mosquitoes.
"This, though, is the first time we document that embryos can do it," she told BBC News.
She and her team reported their findings in the journal Behavioural Ecology and Sociobiology.
To make the "chemical cues" for their experiment, Dr Ferrari and her colleagues combined crushed tadpoles with water that a tiger salamander had been swimming in.
"We poured these two chemical cues in to the water surrounding the woodfrog egg mass," explained Dr Ferrari.
"The embryos presumably sample the water and can 'smell' the cues."
The scientists used different concentrations of "injured tadpole odour" to find out if the embryos would equate a higher concentration of damaged tadpole to a higher level of threat.
Once the embryos hatched into tadpoles, the researchers were able to test their response to to the salamander cue alone.
"A very common anti-predator behaviour is freezing," said Dr Ferrari.
"So, to test our tadpoles, we put them in a container with water, and [measured] how much they moved before we introduced the salamander odour and how much they moved [afterwards].
The tadpoles that were exposed to a higher concentration of the injured tadpole odour stayed motionless for longer in response to the salamander cue.
This, the researchers say, shows that they had learned that the predator was more dangerous.
"The results are pretty dramatic," said Dr Ferrari, "tadpoles go from swimming non stop, to not moving at all for several minutes."
This ability to learn at such an early stage of development makes evolutionary sense, Dr Ferrari added.
"For many species of prey, learning is the only way they have to recognise predators," she said.
"Since [this] seems so fundamental to survival... there must be selection for learning to occur as early as possible."