Hawk moths extract nectar using a long proboscis, and some flowers fit their proboscis better than others. According to a new Nature Communications study, hawk moths recognize the scent of flowers that match the length of their proboscis. Not only does this maximize the moths’ energy gain, it also makes sure pollen is transferred between plants of the same species. 

Many flowering plants rely on the animals that feed on them or their nectar to help spread their pollen. In that way, pollinators drive the evolution of floral traits, leading to the development of what's called "pollination syndromes." That’s when flowers have shapes, colors, and scents that match the preference of their pollinators. 

A Max Planck Institute for Chemical Ecology team led by Bill Hansson and Markus Knaden hypothesized that there must be a flower that perfectly fits the tobacco hawk moth (Manduca sexta), even though this moth is a generalist that visits a variety of plants. Tobacco hawk moths are huge, and they beat their wings more than 30 times a second while hovering over plants to drink nectar with their 8-centimeter-long (3.2-inch-long) proboscis. Nectaries are often located at the very end of the long petal (or corolla) tube, and the moths can’t afford to waste energy on flowers with nectar they can’t reach. 

To test their hypothesis, the team analyzed sugar concentrations using gas chromatography and mass spectrometry to calculate the calorie content of flower nectar from seven different Nicotiana tobacco species. The flowers ranged in length from under 1.5 centimeters (0.6 inches) to over 11 centimeters (4.3 inches). For the most part, all the flowers they studied provided the same amount of calories: Longer flowers had more nectar, but the nectar of shorter flowers was more concentrated. 

While energy gain is measured as calories acquired from nectar, energy loss is determined by measuring carbon dioxide the moths breathe out. The researchers developed a small wind tunnel equipped with multiple cameras that tracked the movements of single moths. You can watch a video of a moth visiting a flower in a wind tunnel here. 

Hungry moths had the strongest response to the scent of Nicotiana alata flowers – which are about 7.5 centimeters (3 inches) long – and they could easily reach its nectar with their proboscis. When visiting other flowers, the moths had to spend an excessive amount of energy since they couldn’t drink as effectively. Since all moths in the experiment were encountering flower scents for the first time, their preferences were innate, not learned. And the innate preference of these moths was for the scent of the best-fitting flower – which offered them the best energy gain.