Hawkmoths Smell Flowers With the Tips of Their Tongues

May 25, 2016 | Erica Tennenhouse

Hawkmoth nectaring on wild tobacco
Photo credit: Hawkmoth nectaring on wild tobacco. Courtesy of Danny Kessler.

Newly discovered olfactory neurons aid in close range perception.

A flower shop can create bouquets of huge, brightly colored flowers, but the selective breeding that has led to their beauty has also in many cases rendered these flowers scentless.

In the wild, floral scents still abound in thousands of plants. Although the main purpose of these aromas is to attract pollinators, broadcasting one’s scent can also draw in unwanted visitors.

“Many plants face the problem, that floral traits which attract friends such as pollinators also attract foes such as nectar thieves, or even worse, herbivores,” Danny Kessler from the Max Planck Institute for Chemical Ecology told The Science Explorer. Additionally, constantly producing scent can be taxing on a plant because the process requires a lot of energy.

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Kessler and his colleagues wanted to know if plants might be able to avoid some of these risks by safely reducing the amount of scent compounds that they produce without forfeiting vital pollination services.

They focused on the wild tobacco plant. Its flowers produce a very simple scent that attracts the plant’s major pollinator, the hawkmoth. Previous work on this duo suggested that tobacco plants lacking scent are “invisible” to hawkmoths. But precisely how the hawkmoths detect the scent, and how that scent affects their behavior, was unknown.

The researchers genetically silenced the scent in some tobacco plants and allowed hawkmoths to choose between unscented plants and their scented counterparts. Surprisingly, the hawkmoths were equally likely to visit both plants.

“It was stunning to see that neighboring plants — one emitting floral scent and one not — attracted moths in the same way,” said Kessler.

However, the plants lacking scent produced very few mature seeds. Although hawkmoths visited them frequently, they didn’t spend long enough on unscented flowers to successfully pollinate them.

It was clear that scent was an important consideration for hawkmoths once they got up close, likely because odor is known to reflect the potential amount of nectar available in a given flower.

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Hawkmoths are easily identified by the long appendage emanating from their heads, known as a proboscis. Its great length prevents the hawkmoth’s antennae, which contain olfactory receptors, from getting close enough to differentiate the scent of one flower from another, or to even resolve the differences in scent between neighboring plants.

The fact that hawkmoths appeared to be assessing scent once they were in close proximity to a tobacco flower was at odds with the idea that their antennae cannot perceive scent at close range.

The proboscis is known to be an efficient nectar-sucking machine, and because it is capable of tasting sugar, it has been likened to a tongue, explained Kessler.

However, after conducting a series of choice tests and assessing its neurophysiology, anatomy, and genes, the researchers discovered for the first time that the proboscis is also capable of perceiving flower scent. It appeared that the proboscis effectively combines the functions of a tongue and a nose.

Smelling flowers at close range with their proboscis helps hawkmoths forage efficiently, which allows them to continue to provide a vital ecosystem service.

The findings were published in the journal eLife.

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