Using behavioral and stable isotope data to quantify
rare dietary plasticity in a temperate bat
Before and after photos of a cardón fruit that was visited by bats over the course of one evening. The captured pallid bat shown here was smeared with fruit pulp from its upper ear to its chest, indicating its entire head was submerged inside of the fruit while feeding.
Lesser long-nosed bats have specialized adaptations for nectar-feeding, such as a long rostrum (muzzle), prehensile tongue, and ability to hover. This translates well into fruit-eating behavior: in an incredibly graceful way, this species hovers over the fruit, feeds, and then flies away.
Among mammals, bats play significant roles as seed dispersers: over 250 species of bats feed primarily on fruit, then eject or defecate the seeds as they fly, hence serving as important agents of plant reproductive success. Most frugivorous, or fruit-eating, bats live in tropical regions. However, we see an exception in Mexico, where the lesser long-nosed bat (Leptonycteris yerbabuenae) is adapted to feed on nectar, but also feeds on cactus fruit. It's also not uncommon for nectar- or fruit-eating bats to supplement their diet with insects, but the opposite of this --insect-eating bats supplementing their diet with fruit --is physiologically challenging and therefore very rare.
Previous studies found that in Baja California Sur, Mexico, the pallid bat (Antrozous pallidus) seasonally depends on floral nectar as a food resource, even though it's adapted to specialize on flightless arthropods such as scorpions. Even more surprising, despite having no evolutionary adaptations for nectar-feeding (such as the ability to hover or a prehensile tongue), the pallid bat was found to be a more effective pollinator of cardón (Pachycereus pringlei), the world's largest cactus species, than the nectar-specialist lesser long-nosed bat. My research examined whether the pallid bat's tendency to feed on nectar from cactus flowers during spring extended to feeding on cactus fruit during summer.
We recorded bat foraging behavior at fruits of the cardón cactus and used stable isotope analysis on bat wing and breath samples to quantify the incorporation of fruit into the pallid bat's diet. We found that, despite having a blunt, rounded nose and lacking the ability to hover, the pallid bat visited cactus fruit just as frequently as the lesser long-nosed bat, and removed the same amount of fruit. Carbon signatures also confirmed that pallid bats feed on fruit during summer, providing evidence for the first temperate bat species to directly incorporate fruit into its diet. This work strengthened our understanding of the pallid bat as a key cactus mutualist in parts of the Sonoran Desert.
In contrast, pallid bats evolved to eat arthropods, such as scorpions. They have a rounded face and lack the ability to hover. Seen here is the not-so-graceful way that a pallid bat eats fruit: it wraps its wings around the fruit's outer spines and then sticks its head inside to take a bite before letting go and flying away.
A graphical abstract of our paper, created by Tali Hammond.