Plants can hear approaching pollinators and will produce sweeter nectar as a result.
A study by Veits et al, (2019) shows that flowers from the evening primrose plant (Oenothera drummondii) produce sweeter nectar within 3 minutes of being exposed to the sound of a pollinator.
This study provides the first documented evidence that plants can rapidly respond to pollinator sounds in an ecologically relevant way.
Between 2014-2016, scientists from Isreal conducted 3 experiments. They exposed primroses to various sounds of different frequencies, and then measured the concentration of sugar in their nectar. They compared high frequencies, low frequencies, silence, and a recording of a bee. They also measured the number of vibrations that occurred across the petals of the primrose flower as a result of playing the sounds.
What they found
Veits and her team (2019) found that the average sugar concentration of nectar increased by a factor of 1.2 in flowers exposed to pollinator‐like frequencies (‘Bee’ and ‘Low’ sound signals), in comparison with flowers exposed to ‘Silence’ or ‘High’.
They also noted that the flowers vibrated similarly in response to both the hovering of a live bee and to the playback of a bee. Furthermore, the amplitude of the vibrations depended on the presence of intact petals. That is, it significantly decreased when petals were removed. Suggesting that the petals have something to do with receiving the signal.
Both the vibration and the nectar response were frequency‐specific. Meaning, the flowers responded and vibrated to pollinator-like sounds, but not to higher frequency sounds. Veits et al, (2019).
Why is this a thing?
It’s definitely a cool discovery, but why is it happening?
- Why do plants listen to their pollinators?
- And why do they produce sweeter nectar when they hear them?
Thankfully, the study suggests some answers to those questions, too.
As a population, plants need pollination, a.k.a reproduction, to survive. And 87.5% of flowering plants rely on animal pollinators to facilitate that (Ollerton et al. 2011).
The main pollinators of evening primroses are hawk‐moths and bees (Eisikowitch & Lazar, 1987). Hawk‐moths pollinate the primroses at night and early morning, whereas bees pollinate at dusk and morning. Notice how their feeding times don’t overlap? That’s a strategy that has evolved over time so that the moths and bees don’t have to compete with each other for nectar. This is called resource partitioning. It allows the two species to occupy a different ecological niche, and therefore coexist within the same ecosystem.
Plants have evolved countless ways to attract their pollinators. In order to increase pollination, and therefore fitness. Some plants use signals such as colour, odour and shape, some use food rewards of nectar and pollen, most use a combination of all of those things (Willmer 2011).
The reason why the primrose would tune into the sounds of a bee or moth’s wingbeat is that like with most things in evolution, it benefits their survival to do so.
This is because by increasing the sugar concentration of their nectar, plants can enhance the learning process of their pollinators, and increase the tendency of them to visit their flowers (Cnaani et al. 2006). In other words, the bees will learn that the primrose is a nutrient-rich food source, and so will consistently return. Hence, increasing the primroses chances of pollination.
A sweeter reward can also increase visit duration by pollinators. However, nectar with too high sugar levels can become too viscous. Enhanced nectar rewards can also result in a higher number of flowers visited per plant, or encourage pollinators to visit other plants of the same species, which facilitates outcrossing.
To put it simply, that means that the effort taken by the primrose to sweeten its nectar, if timed correctly and delivered to the correct recipient, will pay off.
Also, Veits et al, (2019) state that bees have been shown to be capable of perceiving differences in sugar concentration, as small as 1‐3%. Therefore would definitely be able to detect a difference in nectar.
If a primrose produced sweeter nectar to any old sound (such as bat wingbeat or bird song), it will not see an increase in pollination because neither bats nor birds pollinate them. So it makes sense that they’re tuned in and listening to the specific sounds of their pollinators. Veits et al, (2019) show this frequency-dependent response in the results that are mentioned above.
So how does a plant hear things anyway?
First of all, we can hear things by detecting and interpreting vibrations on our eardrums, and similarly, plants can detect vibrations too. It is believed that the ‘ear-drums’ of plants, so to speak, are within the flowers – or more specifically, the petals.
Veits et al, (2019) found that the primrose flowers vibrated mechanically in response to the sounds, suggesting a possible mechanism where the flower serves as an auditory sensory organ (an ear).
They also state that whilst their results suggest that flowers are important for plants to hear pollinators, they cannot exclude the possibility that other parts of the plant may also respond to pollinator sounds. Or that other parts of the plant may serve as sensory organs for sounds at other frequencies.
They go on to suggest that the petals of flowering plant species may have evolved to detect sound, it’s something that is not exclusive to the primroses. They suggest that the resonance frequency of a flower will be dictated by its size, shape and density, which could all be under natural selection. If that is the case, they would expect that plants with ‘noisy’ pollinators – such as bees, moths and birds – would have evolved large ear‐like flowers with proper mechanical frameworks making them sensitive to the sounds of their pollinators. Veits et al, (2019).
The bigger picture
This study shows that plants can to some extent hear things. Not only that but can respond to those sounds in ecologically relevant ways. This means that plants could be affected by other non-tested sounds too. Sounds such as herbivores, other plants, the weather, even anthropogenic noises like machinery or cars, etc. The ability to detect and respond to sound frequencies could differ amongst plant species too.
However, A primrose has not been able to evolve in the presence of many anthropogenic noises that they are exposed to today.
So this leaves me with a few questions to end on.
First, if an anthropogenic sound mimics that of a bee, in this case, could that lead to the primrose expending energy on producing sweeter nectar in response, for it to not be taken by a pollinator, and therefore for it not to pay off?
And secondly, could an anthropogenic sound overpower the sound of a pollinator and therefore lead to information being missed? and hence, lower chances of pollination and lower fitness in the plant?
References:
, & ( 2006). Flower choice and learning in foraging bumblebees: effects of variation in nectar volume and concentration. Ethology, 112, 278– 285.
& ( 1987). Flower change in Oenothera drummondii Hooker as a response to pollinators’ visits. Bot. J. Linn. Soc., 95, 101– 111
, & ( 2011). How many flowering plants are pollinated by animals? Oikos, 120, 321– 326.
Veits, M. , Khait, I. , Obolski, U. , Zinger, E. , Boonman, A. , Goldshtein, A. , Saban, K. , Seltzer, R. , Ben‐Dor, U. , Estlein, P. , Kabat, A. , Peretz, D. , Ratzersdorfer, I. , Krylov, S. , Chamovitz, D. , Sapir, Y. , Yovel, Y. and Hadany, L. (2019), Flowers respond to pollinator sound within minutes by increasing nectar sugar concentration. Ecol Lett. doi:10.1111/ele.13331
( 2011). Pollination and floral ecology. Princeton University Press, Princeton, NJ.
