Predation has shaped the whole of the living world, as prey respond to evolutionary changes in predators, and vice versa. The earlier prey can detect a predator, the greater their possibility of surviving, either by fleeing or hiding. In many aquatic species, prey detection can occur from beyond the grave – for example, Daphnia (“water-fleas”) develop hard protective structures (“inducible defences” – see picture below) when in the presence of water that had contained fish that had eaten Daphnia. The predators have been chemically “labelled” by their prey, enabling other Daphnia to protect themselves.
A study about to be published in Behavioural Ecology and Sociobiology shows that these kind of effects can also involve prey learning, even before they have hatched out of the egg. Maud Ferrari and Douglas Chivers investigated the behaviour of woodfrog (Rana sylvatica) tadpoles in response to a predator, the Tiger salamander. These tadpoles show no innate prey recognition, but if they are exposed to the smell of salamander before they hatch from they egg, they avoid predators when they have turned into tadpoles, by reducing their movement. [It may seem counter-intuitive to talk of "smell" in water, but many aquatic animals (including fish) have clearly distinct smell and taste pathways. I'll blog about this in the future.]
Ferrari and Chivers have taken this a step further, by seeing whether the tadpole embryos can actually learn the threat posed by a predator. To test this, they presented frogspawn with salamander odour coupled with cues from injured tadpoles, for two hours a day over five days, then tested the tadpoles for their responses to the predator.
They found that the amount of freezing shown by tadpoles was dependent on the concentration of the injured tadpole cues coupled with salamander smell, demonstrating that the embryos had learned to associate the two signals. There also seemed to be a time of day effect – the effect was stronger when learning took place in the afternoon, compared to the morning or early evening – although further experiments need to be done to confirm this.
In some species (especially insects), early experience can lead to the non-genetic transmission of preferences from one generation to another. This study shows that interactions between predator and prey can be extremely complex, involving innate responses, and learned changes in behaviour, which may even occur before birth.
Original article (open access).
BBC page, with quotes from Ferrari.