Nectar Bats vs. Fruit Bats: Which are the Better Pollinator?

Both nectarivorous and frugivorous bats most often come in physical contact with the flowers they visit. However, the extent of physical contact (and thus also the extent of pollen transfer) that these bats make with their food source depends on the morphological and behavioral adaptations they have developed. It is important to note that the mutualisms that have developed between flower visiting bats and flowers are not altruistic: they represent an evolutionary power struggle between plant and animal taxa in order for one to obtain ecological and energetic advantages over the other (Kunz and Fenton 2003). Therefore, many bats have established strategies to increase food extraction from plants and minimize pollen transfer (which is often a negative consequence for bats), and many plants have evolved adaptations to achieve the opposite (since nectar is an energetically costly reward).

Frugivorous bats and opportunistic nectarivores commonly land, perch, and crawl on plants in order to extract fruit and nectar. This increases the overall length and extent of physical contact between bats and their flower food sources, thus increasing the probability of pollen transfer (Tschapka 2003). In contrast, obligate nectarivores have highly specialized morphological (elongated tongue and rostrum, diminished dentition) and behavioral (hovering rather than perching) adaptations that reduce contact with flowers, reducing the probability that pollen will be deposited or picked up (Figure 1; Tschapka 2003). 

Fig 1. Tschapka (2003). (A) Artibeus spp., an opportunistic nectarivore, perching on inflorescence of Calyptrogyne ghiesbreghtiana to extract nectar from flowers. (B) Specialized nectarivore Glossophaga commisarisi hovering over the same inflorescence, but showing significantly reduced physical contact with flowers. Photo accessed 20/5/2015.

In a study by Tschapka (2003) in Costa Rican lowlands, bats were video recorded to observe perching and hovering behaviors on the understory palm Calyptrogyne ghiesbreghtiana, and bats were captured to measure pollen load of bats across families and dietary habits. In addition, plants of C. ghiesbreghtiana were analyzed for fruit set following visitation by bats. The study found that perching bats carried a significantly higher pollen load than did hovering bats (Tschapka 2003). In addition, plants that were visited by perching bats developed more fruits than did plants that were only visited by hovering bats (Fig 2; Tschapka 2003). These results suggest that increased contact caused by perching behaviors cause opportunistic nectarivores to be more effective pollinators than specialized nectarivorous bats. The high pollination efficiency of perching bats also suggests that the morphology of C. ghiesbreghtiana has evolved to favor a pollination system facilitated by opportunistic nectarivores rather than the specialized glossophagine bats. That is, perching bats have caused more successful pollination of the plant, which is reflected in the specific adaptations of that plant to perching bats (Tschapka 2003). In addition, the small, shallow flowers of C. ghiesbreghtiana attract perching bats because they do not require bats to have highly specialized rostrums or tongues to extract nectar. Therefore, perching bats are more likely to visit flowers of c. ghiesbreghtiana to obtain a nectar reward, and in turn, increase the probability of pollination and fruit set (Tschapka 2003). This study exemplifies the coevolutionary processes that occur between flower-visiting bats and plants. 

Fig 2. Tschapka (2003). Mean percent fruit set in C. ghiesbreghtiana visited by perching bats compared to those visited by only hovering bats. Figure accessed 20/5/2015.

References

Kunz, T.H. & Fenton, M.B. 2003, Bat ecology, Paperback edn, University of Chicago Press, Chicago.

Tschapka, M. 2003, "Pollination of the understorey palm Calyptrogyne ghiesbreghtiana by hovering and perching bats", Biological Journal of the Linnean Society, vol. 80, no. 2, pp. 281.