Many flowers go out of their way to attract pollinators, providing abundant nectar to feed them, offering lurid colors and exotic perfumes to draw them in, modifying their shape so they can easily land and get cosy and even adding nectar guides to direct them more precisely to their food source. All of this, to ensure pollination. These attractants even include colors our eyes can’t see, but some insects can (read What Bees See for further information). And it works! Study after study with different pollinators confirms that insects quickly learn to home in on the most generous flowers and pollinate them in priority, abandoning those that aren’t as giving.
But now we’re learning flowers have a lesser-known trick: heat.
Some Flowers are Hot, Others are Not!
It’s long been understood that flowers are often warmer than the air around them. Many flowers have specialized cells that capture and concentrate solar radiation and still others practice thermogenesis: they actually produce their own heat. Until recently, though, it was assumed the whole flower heated up equally.
According to a study in eLife, though, many flowers (55% of the 118 species studied) display temperature variations greater than 2º C (3.6º F) within a single flower. The 2º C difference is an important detail, because it was already known that bees (in this case, bumblebees were studied) are able to detect temperature variations of 2º C and greater. The study found flowers using directional temperatures show on average a 5º C (9º F) degree difference between heated and unheated tissues; one even displayed an astounding 11º C (20º F) difference! These warmer cells are only found on part of the flower, directing the bees towards its nectar supply, their ultimate goal in visiting the flower. And bees can detect them thanks to special heat sensors in their legs and antennae.
The path they take as they follow the heat source towards the center of the flower leads the bees to brush past the flower’s stigma, leaving any pollen they may carry and picking up fresh pollen. That done, the flower rewards them with nectar when they reach the “hot spot.”
These patterns were discovered using thermal photography and often show similar patterns to the nectar guides sometimes visible to human eyes.
In the study, the searchers used artificial flowers with different heated patterns, some of which offered nectar and others, plain water. The bumblebees quickly learned to recognize the patterns of those flowers offering nectar and to focus on them exclusively.
Offering heat to visiting insects might also have another use. Bees (and insects in general) need to maintain a certain body temperature to be able to fly in cool weather, so visiting “warming” flowers could be an additional incentive, especially in colder climates and in inclement weather.
Bees and Beyond
Does this thermal attraction apply to other insects beyond bumblebees, such as other bees, hoverflies, butterflies, wasps, etc.? That remains to be seen (I’m sure there are many more studies to come!), but it certainly looks like flowers have more up their sleeve when it comes to attracting pollinators than simply flower color and perfume.
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