Facultative Nectarivory: The Pallid Bat

We have seen that many nectarivorous bats have evolved highly specialized morphological and behavioral adaptations to maximize their feeding efficiency. However, there are cases of facultative nectarivory and frugivory in some bat species that could be demonstrating a gradual evolution towards more obligate feeding patterns. Such is the case of the pallid bat (Antrozous pallidus), a primarily insectivorous microbat from Central and southern North America (Figure 1). A. pallidus has been observed visiting flowers and drinking the nectar of the cardon columnar cactus (Pachycereus pringlei) in northwestern Mexico, and represents the first case of facultative nectarivory in a bat outside of the New World microbat family Phyllostomidae (A. pallidus is in family Vespertilionidae) (Frick et al. 2012). Such behavior from an insectivorous bat may represent the evolutionary processes that took place nearly 40 million years ago, when obligate nectarivory first emerged in bat lineages (Kunz and Fenton 2003).


Figure 1. Tuttle, M. 2014. Pallid bat (Antrozous pallidus) sipping nectar from a flower of a columnar cactus. A. pallidus is most typically a gleaning insectivore, snatching arthropods and insects off the ground. https://www.thedodo.com/the-work-of-a-real-batman-843781039.html; retrieved 19/4/2015.


In a recent study in northwestern Mexico, experimental results show that A. pallidus actually deposits more pollen at cactus flowers per visit than the obligate nectarivore the lesser long-nosed bat (Leptonycteris yerbabuenae), suggesting the pallid bat may in fact be a more effective pollinator than a highly specialized nectarivore (Frick et al. 2012; Figure 2). This is mainly due to the feeding mechanisms of the pallid bat; since it is not specialized to hover and quickly extract nectar like the lesser long-nosed bat is, the pallid bat lands on the flowers and plunges its face and tongue into the flower to extract nectar. This increases physical contact between the bat and the flower, increasing the chance of pollen transfer at each flower visit (Frick et al. 2012).

Fig 2. Frick et al. 2012. Average pollen loads on cactus flower stigmas per visit by A. pallidus and L. yerbabuenae. A. pallidus, the facultative nectarivore, deposited significantly more pollen at cactus flowers per visit than did L. yerbabuenae. Reprinted from Frick et al. (2012); retrieved 19/4/2015.


This behavior of facultative nectarivory by a primarily insectivorous bat could exemplify the evolutionary processes that take place to develop obligate nectarivory. Whereas the highly specialized lesser long-nosed bat has adaptations to increase feeding efficiency and reduce metabolic costs, the pallid bat still expends a significant amount of energy while feeding on nectar due to a lack of traits specialized for nectarivory (Frick et al. 2014). It is possible that the pallid bat could eventually develop more adaptations specific to nectarivory given the proper selective pressures and mutations.

Behavioral Adaptations of Nectar Bats

Whereas nectar bats have clearly evolved some impressive morphological adaptations to optimize nectar feeding, many species also show behavioral modifications to support obligate nectarivory.
The most widespread behavioral adaptation in nectar bats is the ability to hover over (rather than landing on) flowers, much like hummingbirds do while feeding. While hovering is an energetically expensive behavior, it allows nectar bats to visit a greater number of flowers per night and thus improves foraging efficiency. In addition, bats that feed by hovering can visit a larger range of plant types, including less robust flowers that could otherwise not be visiting by bats that land to feed on fruit or nectar (Tschapka and Dressler 2002). Nectar extraction  by hovering bats is quick and effective, and thus allows the bat to consume as much as 135% of its total body weight in nectar each night (Tschapka and Dressler 2002).


Figure 1. Melton, C. Mexican long-tongued bat (Choeronycteris mexicana) hovering and extracting nectar from agave flowers. http://www.nearfamous.com/Pages/NectarBats.html; retrieved 12/4/2015.


In addition to hovering behaviors, nectar bats show a particular curiosity superior to other bat species. Nectar bats in Central America are known to inspect emerging branches and mist net poles for potential flowers to feed on. Nectar bats born in captivity also readily investigate protruding objects in the hope of finding a food source (Tschapka and Dressler 2002). 

The aforementioned behaviors are examples of adaptations the developed from a long history of coevolution between nectar bats and plants. A particularly intriguing example of behavioral/physiological coevolution can be found in the case of nectar bats and the tropical plant Mucuna holtonii, which will be discussed in detail in a future blog post. 


References

TSCHAPKA, M. and DRESSLER, S., 2002. Chiropterophily: On Bat–Flowers and Flower 
Bats. Curtis's Botanical Magazine, 19(2), pp. 114