Jumping on the bandwagon: Bats prefer food they smell on their roost-mates’ breath

Bats and humans are a lot alike, at least in one sense: we both rely on our friends for recommendations on which products to buy. For us, it’s second nature—when in doubt about which thing to buy, we ask a friend what they have. But before you bite the bullet—or the fruit, as it were—you’d want to know whether you could trust the information you’d be getting. How reliable is your friend’s review?

Everyone's looking for a good place to eat. [1]

For fruit-eating bats, the process of picking up recommendations is never ending. Bats like Uroderma bilobatum, or the Tent-Making Bat, live in small social groups of typically 2-10, but up to 60, individuals and eat fig fruits. Fig trees can be found most anywhere, and can produce a large amount of food at one time (enough for the whole gang). However, individual trees rarely have ripe fruit at the same time—so for bats, the location of dinner shifts around a lot. The bats feed on multiple species of figs, which they can identify by the individual smell of the different fruits, and which typically have ripe fruit for 5-7 days. Ripe trees thus represent a feeding bonanza, but only for a few days. Then, the bats are out of luck. As the supply dwindles, the bats will begin flying recon missions, trying to find the next lucky strike.

U. bilobatum in a roost [2]

Close up of a captured bat [3]

For a small bat, flying around trying to find fruit is a big investment of energy! Luckily, they don’t all need to fly everywhere—the benefit of group living is that they can split the search effort. Letting your friends do the work and learning from their efforts is a time-honored strategy, for both bats and humans. In times of scarcity, the ability to learn from your friends where dinner can be found may be critical, and may explain why bats form groups in the first place. In order to successfully use their roost-mates' “recommendations”, though, bats would have to be able to determine whether it actually meant food were available.

In a study published in Behavioral Ecology, researchers tackled the question how U. bilobatum uses social information. They investigated whether U. bilobatum would prefer novel food (something they hadn’t previously encountered) consumed by roost-mates (in captivity and in the wild) and whether or not they would discern between odor on a roost-mate’s breath (indicating that the fellow bat successfully ate the food) versus on their fur (indicating they simply had contact with the food).

To test this, they performed four experiments:

First, the researchers needed to see if simple familiarity with a novel food smell would make the bats more likely to prefer it to an unfamiliar novel food. To do this, they placed individual bats in cages next to bananas mixed with candy-flavored oil. The bats could see and smell the food, but not eat it. After half an hour of presumable frustration, the food was removed and the bats were given two dishes, one with bananas mixed with the familiar flavor, and one mixed with a different novel flavor. Despite having previously smelled the first flavor, the bats showed no significant preference for either flavor—simply having smelled a food before didn’t make the bats more likely to eat it.

Next, “demonstrator” bats were removed from the captive roost and fed bananas and novel-flavored sugar solution. They were each then introduced into a different cage with an “observer” bat, and the two bats were allowed to interact—generally the two bats would sniff each other and rest together. The observer was then given the choice of the demonstrator’s novel food, or a different flavored novel food. With few exceptions, observers approached and ate the food consumed by the demonstrators, showing that they rely on other bats to “recommend” certain foods.

Observer bats preferred to eat the food they demonstrator bats had eaten. [4]

 The researchers then essentially repeated the previous experiment with wild bats. They caught demonstrator bats and fed them novel-flavored foods, then released them back to their roost. Two days later, other individuals from the same roosts, observers, were caught and offered the novel food that the demonstrator had eaten or a different novel food. As with the previous experiment, bats showed a preference for the demonstrator foods, indicating that there was also social information exchange in nature—an important result, because captivity may significantly alter behavior.

Just like in captivity, wild bats preferred food their roost-mates had eaten. [5]

Finally, it was important to see if the bats assessed the quality of the information—do the bats distinguish whether the demonstrator has actually eaten the food? This is important because if the demonstrators haven’t actually eaten the food, the observer could be in for a wild goose chase trying to find food that isn’t actually available—maybe the fruit wasn’t ripe, or it was unpalatable. To assess this, two bats were chosen as demonstrators. The first was fed novel-flavored sugar water, while the second had a different novel odor applied to its fur, and was fed unflavored sugar water. They were then put in a cage with an observer bat. Would the observer prefer food scents that were on the breath of a bat or on its fur? Would it distinguish the two?

As it turns out, they do distinguish. When offered the two types of novel foods, the observers overwhelmingly preferred the novel food that had been on the breath of the demonstrator that had been fed the novel flavor. The bats were more apt to take the “recommendation” of a fellow bat that had actually eaten the food, compared to one that had simply been around it.

Observer bats preferred food they smelled on other bats' breath over the smells on their fur. [6]

For fruit-eating bats, the ability to use social information to quickly and accurately locate food may be critical. In lean times, when ripe fruit is scarce and spread out, the roost may use the collective experiences of its members to identify the best dinner spot. Small fruit-eating bats spend enormous amounts of energy in flight, so spending time searching for a food source that doesn’t exist could impose a hefty fitness cost. Thus, the ability to distinguish between the best “recommendations” of roost-mates may be an adaptation to deal with the difficulty of finding new fruiting fig trees. Bats, like humans, rely on their friends’ opinions to choose a place to have dinner.

Watch the primary author of the study, Teague O’Mara, explain the bats!

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Journal citation:

O’Mara, T.M., Dechmann, D.K.N., Page, R.A. (2014). Frugivorous bats evaluate the quality of social information when choosing novel foods. Behavioral Ecology 25(5). 1233-1239. doi:10.1093/beheco/aru120. http://beheco.oxfordjournals.org/content/25/5/1233.full.pdf+html

Media Credits:

[1] Photo by Yelp, inc: https://flic.kr/p/9rAJFA

[2] Photo by Charles Sharp: http://commons.wikimedia.org/wiki/File:Common_tent-making_bats.JPG

[3] Photo by Roger Le Guen: https://flic.kr/p/egvUF2

[4] Graph from O'Mara et. al. http://beheco.oxfordjournals.org/content/25/5/1233.full.pdf+html

[5] Graph from O'Mara et. al. http://beheco.oxfordjournals.org/content/25/5/1233.full.pdf+html

[6] Graph from O'Mara et. al. http://beheco.oxfordjournals.org/content/25/5/1233.full.pdf+html

[7] Video by Virtual Field Trips http://www.youtube.com/watch?v=dZxDsyZooYI