The nectar secreted by flowers may be doing more than just providing sustenance to pollinators. Research by Arthur Domingos-Melo and colleagues, published in Annals of Botany, suggests nectar standing crop, the amount of nectar available in flowers, helps to manipulate the behaviour of bats, turning them into more effective pollinators for the plant.
Scientists studied Harpochilus neesianus, a bat-pollinated shrub found in Brazil, to determine how the availability of nectar influences the movement of pollinating bats between flowers. Harpochilus neesianus has flowers that are self-compatible but don’t have that much fruit set when they self-pollinate. The flower tends to avoid this by separating the anthers from the stigmas to avoid accidental self-pollination.
The flowers open in the afternoon but only become fully receptive at dusk, as the bats start flying. The flower attracts the bats by offering them sugars in the form of nectar. They discovered that Harpochilus neesianus produces nectar continuously throughout the lifespan of each flower, but the quantity and quality of the nectar can differ between flowers.
A reason a flower on one plant might have less nectar than a flower on a neighbouring plant is that a bat has visited it and lapped the nectar up. Domingos-Melo and colleagues argue that these visits make a big difference for flowers undergoing anthesis, which are open for pollination. They write:
Due to slow nectar secretion, nectar accumulates in H. neesianus evenly and only at low volumes at the beginning of anthesis; however, over the night a gradient in nectar standing crop volume is established among flowers within a population. Thus, during most of the period of anthesis, nectar standing crop of a H. neesianus population includes flowers with a relatively high nectar volume (a clear consequence of not having been visited yet), but also flowers with only little remaining nectar (a reliable indicator of visitation followed by low nectar production rates due to the negative effect of removal).Domingos-Melo et al. 2023
After nectar removal, the flowers produce less nectar. This pattern of decreased nectar production in response to removal may encourage pollinating bats to avoid revisiting the same flowers. Instead, the bats would be more likely to seek out new flowers with ample nectar and pollen. By manipulating the bats’ perception of nectar availability, Harpochilus neesianus can direct them to the most rewarding and pollen-rich flowers, maximizing the likelihood of successful pollination and reproduction.
The researchers also found a significant correlation between nectar volume and pollen transfer. When flowers had more nectar, they also tended to have more pollen grains remaining in their anthers, indicating fewer visits from pollinators. On the other hand, flowers with little nectar typically had higher numbers of pollen grains deposited on their stigmas, suggesting more frequent pollinator visits.
Domingos-Melo and colleagues argue that the combination of measuring nectar availability and pollen transfer demonstrates how plants use nectar to manipulate bats. They conclude:
To the best of our knowledge, our study includes the largest sampling of nectar ever made for a bat-pollinated neotropical plant, and the only one where nectar standing crop and the two fitness components (pollen export and receipt) were tested for correlations. Our results for H. neesianus suggest how chiropterophilous species occurring at high densities and over large areas could manipulate bats by controlling the pattern of nectar secretion, thus increasing reproductive success of the population as a whole. Harpochilus neesianus exemplifies how bat-pollinated species could increase their out-crossing rate by causing pollinators to move over the night to more distant individuals once the plants in an area have been visited, and flowers with little to no nectar become more common.Domingos-Melo et al. 2023
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Domingos-Melo, A., Cocucci, A.A., Tschapka, M. and Machado, I.C. (2023) “A negative association between nectar standing crop and pollen transfer suggests nectar functions as a manipulator of pollinating bats,” Annals of Botany, 131(2), pp. 361–372. Available at: https://doi.org/10.1093/aob/mcac154.