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Evening primrose flowers can hear approaching bees and quickly make their nectar sweeter in response to the sound.

Lilach Hadany and colleagues at Tel-Aviv University, Israel, collected nectar from flowers before and after exposing them to a range of sounds, including recordings of bees and synthetic noises.

Within three minutes of exposure to bee sounds or artificial sounds聽of a similar frequency, the flowers increased the concentration of sugar in their nectar by 20 per cent on average. There was no change in sugar levels in flowers played no sound, or higher-frequency sounds.

Bees are highly sensitive to differences in sugar concentration, preferring to go after higher calorie nectar. By improving the rewards on offer, plants may benefit by encouraging the pollinators to spend longer visiting the plant, or to visit more flowers of the same species.

Enhancing sugar levels when pollinators approach might help a plant save energy in the long run, and reduce the risk of nectar being degraded by microbes or stolen by ants. 鈥淣ectar can be a significant energy investment, and thus keeping a constantly high level of sugar can be wasteful,鈥� says Hadany.

How plants detect the sound of bees is unknown. However,聽using highly sensitive laser instruments, the researchers found that the evening primrose flowers vibrate聽when played recordings of bee or moth sounds.

Hadany thinks that flowers may receive sound pressure in聽a similar way to ears. When petals were removed from flowers, they vibrated less when played the sound clips, suggesting petals may help receive or amplify pollinator sounds.

Previous research has found that some plants can hear the vibrations of caterpillars eating their leaves, and retaliate with repellent chemicals. Some flowers only release pollen efficiently when bees cause them to vibrate at a specific frequency 鈥� a technique called buzz pollination.

鈥淭his is a very neat and well-thought study that adds to the increasingly growing body of evidence that plants are able to detect sounds in their environment and respond selectively in ways that are ecologically meaningful and of adaptive value,鈥� says Monica Gagliano at the University of Sydney, Australia.