If you think feeding your picky toddler is tough, spare a thought for Insa Wagner. ‘Chiroderma villosum [hairy big-eyed bats] insist on fresh figs’, says Wagner from the University of Ulm, Germany, adding that windfalls and frozen fruits are unacceptable to the fussy eaters. Wagner initially became interested in the bats’ diet when she noticed their fiddly eating habit – it took them nearly an hour to eat one fig. She also says, ‘Most bats use fruit as chewing gum: they take a bite, chew it until all of the sugar is gone and then spit out the fibrous remains’, adding that the animals often swallow intact seeds along with the juice or spit the seeds out untouched. However, in addition to processing the fruit pulp to extract the juice, the big-eyed bats separated the seeds from the fruit flesh before pulverising them with their teeth and spitting the fragments away. Could the bats be chewing the goodness out of the seeds for nourishment? Wagner, her PhD supervisor Elisabeth Kalko, and Marco Tschapka, also from the University of Ulm, decided to find out.

But working with the bats and their favourite figs proved challenging. Figs fruit asynchronously, so there are always some trees bearing ripe fruit in a forest: the challenge was finding them. ‘I had a list of about one-hundred fig trees [Ficus obtusifolia] along the coastline of the Barro Colorado Monument that I had to check at least once a week for nearly ripe fruit’, recalls Wagner, who foraged constantly for ripe figs to satisfy her bats. And collecting the fig-mad bats was also tricky. Wagner explains that hairy big-eyed bats are relatively scarce compared with the more common Artibeus watsoni (Thomas's fruit-eating bat). However, after patiently staking out ripe fruit trees and collecting individual animals, Wagner was ready to start investigating how they masticated each mouthful and how efficiently they extracted the seeds’ nutrients.

Analysing how 12 bats chewed their fig meals, Wagner realised that the big-eyed bats spent three times as long chewing the seeds as they did the fruit pulp. And when she compared the number of seeds in the pellets of discarded pulp produced by C. villosum with the number of seeds in the fruit pulp pellets of A. watsoni, the C. villosum fruit pellets were almost seed free. She also found that the A. watsoni poo contained four times as many intact seeds as that of C. villosum. So, instead of distributing the seeds in their faeces, the big-eyed bats appeared to be consuming them, but were the bats deriving any benefit from the seeds?

Teaming up with Irene Tomaschewski and Jörg Ganzhorn from the University of Hamburg, Germany, Wagner analysed the nutrient content of the seed remains and discovered that the big-eyed bats were extracting almost all of the goodness from them (over 85% of the lipids, protein and sugars). And when Wagner tested the bats’ fig preferences, she found that the bats always preferred F. obtusifolia figs, which have a higher seed content than the figs of other species. What is more, the bats were only interested in the seeds from strangler figs, which are covered in a slimy gel that the seed uses to attach itself to the trunk of a host tree to grow on.

Chiroderma villosum acts more as a seed predator than a seed distributor’, says Wagner, who also suspects that the gel coating on F. obtusifolia seeds helps the bats to separate the seeds from the fruit flesh, allowing them to extract more from their fig meals than other bats.

J. U.
E. K. V.
Cheating on the mutualistic contract: nutritional gain through seed predation in the frugivorous bat Chiroderma villosum (Phyllostomidae)
J. Exp. Biol.