Flowering plants will go to great lengths to attract pollinators. As humans, we reap the aesthetic benefits of these advertising campaigns in the visual beauty that flowers offer and their perfumed scents. All this time, however, we've been missing another language of flowers – the way some have developed complex temperature patterns that enchant bees, providing pollen holders with an extra communication channel in a crowded market.
According to a paper in eLife, 55 percent of 118 plant species studied with thermal imagers revealed temperature variations greater than 2ºC (3.6ºF) within a single flower. Amongst these, the average difference was almost 5ºC (9ºF), and the most extreme internal difference reported was 11ºC (20ºF). The 2ºC figure is significant because bees have been shown be able to detect temperature variations of this size.
When the authors created artificial flowers with either the outer petals or a rectangle in the center being hotter than the rest, they found bumblebees could sense the difference. After being trained to associate one pattern with food and the other with plain water, the bees would choose flowers with the food pattern.
Having cues that appeal to multiple senses can help flowers stand out during periods where there is a profusion of pollen on offer, all chasing a limited stock of over-worked insects. The patterns also serve a purpose from the insects' point of view. Pollinators may recognize their favorite flowers from those they find less appealing based on temperature patterns, even when their colors and smells are too similar to distinguish. First author Dr Heather Whitney of the University of Bristol said in a statement: “The presence of multiple cues on flowers is known to enhance the ability of bees to forage efficiently, so maximizing the amount of food they can take back to sustain the rest of their colony.”
An additional incentive is that collecting pollen on a warm flower can help an insect maintain the temperature needed to fly on a cold day.
Sometimes flowers, particularly those pollinated by beetles, generate their own heat through chemical reactions, but most rely on sunlight. Due to this, large internal variations are more common among flowers that grow in sunny locations. In these cases, plants form these patterns through a combination of the heat-trapping capacities of darker pigments, the ability of some plants to track the Sun across the sky, and the different exposure of shaded and exposed petals.
The use of temperature is just one way plants appeal to pollinators in ways that escape us. Bees can see in the near-ultraviolet, so some flowers that look white to us are actually ultraviolet to attract them.