A new study, led by scientists Heather Whitney, has found that a wide range of flowers produce not just signals that we can see and smell, but also ones that are invisible such as heat.
It has been known for a while that certain pigments allow flowers to trap heat. A warmer flower could be a big advantage for a flying insect. Often they need a minimum temperature to fly. A rest stop that keeps them relatively warm is a desirable location. But pigments are more complicated than single colour flowers in the visible spectrum. Could this also be true of the effects of pigment on temperature? This would effectively make heat patterns.
A study with a heat-sensitive camera revealed that most of the flowers they examined, including many common in gardens, such as poppies and daisies, had complex patterns of heat across their petals, echoing the colourful patterns that we see with our own eyes. On average these patterns were 4-5°C warmer than the rest of the flower, although the patterns could be as much as 11°C warmer.
Were these signals that the bees could use, or side effects of the coloured pigments? To find out, Whitney and colleagues made artificial flowers that copied these heat patterns but did not include the corresponding colour patterns. Some flowers distributed their heat in a circular pattern, while some were set up as bars. Both test patterns were heated to the same temperature so that it wasn’t the difference in temperatures that acted as the cue. To identify the flowers they had to distinguish the heat patterns.
They then set up tests. Some bees learned the circular patterns had the reward of a nice sugar solution. Others were trained to think it was the bar pattern that was target flower. Some control bees were trained at all, so when they were let loose in the test chambers, the scientists could see if there was some sort of signal from the reward flowers that they’d not accounted for.
While the artificial flowers looked identical to human eyes, and we are not able to tell them apart, they found it was a different case for foraging bumblebees. Bumblebees, who visit a wide range of different flowers, were able to use the patterns to distinguish between different flowers and the rewards that they provide.
Heather Whitney, from the University of Bristol’s School of Biological Sciences, said: “The presence of multiple cues on flowers is known to enhance the ability of bees to forage efficiently, so maximising the amount of food they can take back to sustain the rest of their colony.”
However, plants don’t make light to make their patterns, they reflect or absorb it. The same is true for the heat patterns. The authors of the paper note this is a potential problem for insects in the future, saying: “[T]hese findings are potentially important given current concerns about climate change. If pollinators are partly reliant on subtle differences in temperature across the surface of a petal, then even small changes in the temperature of the environment could have a large and unanticipated influence on how efficient bees and other pollinators are when they are visiting flowers with hidden heat patterns.”
Harrap, M. J., Rands, S. A., Hempel de Ibarra, N., & Whitney, H. M. (2017). The diversity of floral temperature patterns, and their use by pollinators. eLife, 6. https://doi.org/10.7554/eLife.31262