Does flower height matter to bats?

Is the best way for a plant to attract bats as pollinators to hoist their flowers up high?

It’s not just bees that are pollinators. Bats pollinate over 500 species of plants. Obviously, bees and bats have different needs, and you can see this in the flowers. Flowers open at dusk or night bat-pollination. The bats don’t see much colour, so the flowers tend not to waste energy on creating colourful blooms. But they do create large corollas for their flowers. These are something that can easily be found with echolocation. They’ll also use strong scents. These will be of fruit or musty odours to entice bats in. When the bat gets there, they’ll find a lot of nectar and pollen. But are the plants doing anything else to attract pollinators?

Grey-headed Flying Fox covered in pollen. Image: Andrew Mercer / Wikipedia

Diniz and colleagues decided to see if the height of the flowers was significant in attracting bats. They examined the flowers of Crescentia cujete, the Calabash tree, to see if height affected visits or the seed:ovule ratio.

The answer is yes, to an extent. Where the relationship breaks down also gives a clue as to why height is important.

What the team found was that the higher a flower was, the more visits it got – until it reached a threshold. Once you got above two metres, the flowers were much of a muchness. The reason, the authors argue, is that once you get to this height, you’re free of the undergrowth. Flowers over two metres are accessible to any airborne bats.

Diniz and colleagues say, “In this case, the evolution of bat-pollination in trees may provide us with an example of exaptation, because a prior investment in height may have proved secondarily advantageous during the transition to chiropterophily.” In this case exaptation means the height of the tree gains a second function, appealing to bats, after it had already evolved due to other pressures, like access to light. Once the trees had the height, they found themselves in the perfect position to build a relationship with some very efficient pollinators.

Further reading

Diniz, U. M., Domingos-Melo, A., & Machado, I. C. (2019). Flowers up! The effect of floral height along the shoot axis on the fitness of bat-pollinated species. Annals of Botany. https://doi.org/10.1093/aob/mcz116

Fleming, T. H., Geiselman, C., & Kress, W. J. (2009). The evolution of bat pollination: a phylogenetic perspective. Annals of Botany, 104(6), 1017–1043. https://doi.org/10.1093/aob/mcp197