In a world full of hungry predators, prey animals must be constantly vigilant to avoid getting eaten. But plants face a particular challenge when it comes to defending themselves.

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"One of the things that makes plants so ecologically interesting is that they can't run away," said John Orrock, a zoology professor at the University of Wisconsin–Madison. "You can't run, you can't necessarily hide, so what can you do? Some plants make themselves less tasty."

Some do this either by boosting their production of toxic or unpleasant-tasting chemicals (examples of which include cyanide, sulphurous compounds, or acids) or through building physical defences such as thorns or tougher leaves. But, Orrock added, "Defence is thought to come at a cost. If you're investing in chemical defences, that's energy that you could be putting into growth or reproduction instead."

To balance those costs with survival, it may behove a plant to be able to assess when danger is nigh and defences are truly necessary. Previous research has shown that plants can induce defences against herbivores in response to airborne signals from wounded neighbours.

But cues from damaged neighbours may not always be useful, especially for the first plant to be attacked, Orrock said. Instead he asked whether plants — here, black mustard, a common roadside weed — can use other types of cues to anticipate a threat.

In a presentation on Tuesday (6th August) at the 2013 Ecological Society of America Annual Meeting in Minneapolis, he and co-author Simon Gilroy, a UW–Madison botany professor, reported that the mustard plants can eavesdrop on herbivore cues to mount a defensive response even before any plant is attacked.

Slugs and snails are generalist herbivores, attracted to mustard plants. However, the pests leave evidence of their presence — a trail of slime, or mucus. So Orrock treated black mustard seeds or new seedlings with snail mucus, then tested how appealing the resulting plants were to hungry snails.

His results were surprising. "Getting slimed" made the plants become less palatable to their predators. "That shows that plants are paying attention to generalist herbivore cues and that they turn on their defences before they even get attacked," the professor concluded.

What's more, they used the information in a time-sensitive way. Plants exposed only as seeds were eaten more — which Orrock said is evidence of lower defences — than those exposed as seedlings.

"The more recently they receive the information about impending attack, the more likely they are to use the information to defend themselves," he elaborated. "Not only do they eavesdrop, they eavesdrop in a sophisticated way."

With Gilroy, Orrock is now exploring the genetics — and possibly evolution — of induced defenses. "If selection is strong enough from generalist snail herbivores to drive the evolution of eavesdropping by plants, then it might be far more common than we think," he said.