Bats rely on ‘bag of chips’ effect to find prey in a group [with video]

When you sit in a dark cinema theatre and someone opens a bag of chips, everyone in the theatre knows that someone is eating chips and approximately where that someone is. Bats work similarly. When one bat finds a patch of insects, all of the other bats within earshot will realize this. That’s very useful information because bats can use their sonar to detect an insect only when it is very close – within 10 meters - but a bat can hear that another bat has detected an insect from more than 100 meters away.

HFSP Program Grant holder Yossi Yovel and colleagues
authored on Thu, 29 January 2015

Bats use biosonar to detect prey. However, this sound based sensory system is very limited in range only allowing them to detect insects when they are less than 10m away. We show that the social insectivorous Rhinopoma microphyllum bat uses a collective searching strategy flying in large groups and eavesdropping on each other’s biosonar calls. They take advantage of the fact that they can detect when another bat finds food from much larger distances of more than 100m (based on the echolocation calls that are emitted during an attack on prey). R. microphyllum bats are known to forage on queen ants which are hard to find, but aggregate in patches, so finding an ant patch might provide food for many bats.

Photo by Jens Rydell

To perform this study, together with an Israeli company (Lucid inc.), we developed a miniature GPS device that can be mounted on bats and also includes an ultrasonic microphone which allows the bats’ biosonar activity to be recorded. Because bats adjust their biosonar calls according to the task they are performing, recording them allows to determine when a bat is attacking prey, when it encounters another bat and how it reacts to it. This unique ability to infer the behavior of a wild animal makes bats highly advantageous models for studying foraging decision making in the wild.

Our devices log the data on-board, so to retrieve it we have to find them when they fall off the bats after a week or two. The devices carry a miniature RF transmitting unit that allows them to be searched with telemetry antennae. Retrieving the data on the recorders was not easy. We often had to search for the tiny trackers to find them in dense vegetation or wherever they happened to land. We only got 40 percent of the recorders back.

The data we collected was the first of its kind – revealing the full movement and biosonar activity of wild bats over whole nights. For example, we recorded more than 1,100 interactions between tagged bats and their peers. The data suggest that bats intentionally aggregate to improve their chances of finding prey. However, we also found evidence that when their density is too high, bats suffer from a difficulty to detect prey, probably because they must pay attention to other bats that are constantly entering their biosonar ‘field of view’. We thus suggest that social foraging constitutes a trade-off between the benefit of flying in a group and the disadvantage of needing to pay attention to other individuals.

Our HFSP project focuses on multi-modal sensory navigation and foraging in bats. In this project we have developed the methodology that will allow us and our collaborators to pursue experiments dealing with multi-modal sensing in wild bats.

Reference

Bats aggregate to improve prey search but might be impaired when their density becomes too high. (2015) N. Cvikel, K. E. Berg, E. Levin, E. Hurme, I. Borissov, A. Boonman, E. Amichai, & Y. Yovel, Current Biology, 24: 2962-2967.

Link to Current Biology article

Link to New York Times article

Pubmed link