Researchers acquainted 49 frog-eating bats with a series of ringtones to attract their attention, and trained the bats to associate the ringtones with a tasty snack, a baitfish. Eight of the same bats were captured between one and four years later, and exposed to the ringtone again. All of the frog-eating bats flew towards the sound, with six of them flying directly to the speakers and grabbing their rewards, demonstrating that the frog-eating bats expected to find food. The researchers were surprised to discover the long-term memory of the frog-eating bats. Control bats without previous training on the ringtones were unmoved by the exposure.
Lead author of the study May Dixon says, “I was surprised. I went into this thinking that at least a year would be a reasonable time for them to remember, given all the other things they need to know and given that long-term memory does have real costs. Four years strikes me as a long time to hold on to a sound that you might never hear again. The environment that previous generations experienced can be extremely different from the environment an animal is born into – and it may also change throughout an animal’s life. Trying to figure out how animals use learning and memory is one way to figure out how they’re going to make it in a life full of change in the modern world.”
Humans have a tendency to assume that long-term memory gives species an intelligence advantage. Dolphins, elephants and tortoises are considered especially intelligent because of their long-term memories. However, memory flexibility, or adaptive forgetting may be important for survival in nature. Long-term memory comes with a cost, as demonstrated by previous studies on fruit flies. Dixon says, “It’s not always true that being the smartest or having the longest memory is actually advantageous. Research has shown that fruitflies selected for improved memories can’t compete as well against other fruitflies. Just because it’s useful for humans to be so smart and have such good memories doesn’t necessarily mean it’s going to be the best thing for other animals. That’s why we want to figure out when these skills are actually going to help animals and when they could be a liability.”
The frog-eating bats were captured for a series of cognition tests. In the first phase, they were exposed to a highly attractive sound that they might encounter in nature, the mating call of a túngara frog, which is one of the preferred prey species of the frog-eating bats. Bats who flew towards the sound were rewarded with a baitfish placed on a mesh above the speaker.
A frog-eating bat grabbing its treat from a mesh on top of a speaker playing the ringtone. (Image credit: Andrew Quitmeyer)
Over time, the sound of the mating call of the frog was mingled with a ringtone, and then entirely replaced, but the reward remained the same. Researchers then introduced three other ringtones that were not associated with any reward. The bats were able to discern the differences between the ringtones that provided the rewards and the ones that did not, and eventually did not fly towards the unrewarded sounds. The bats were microchipped, then released back into the wild.
Between one and four years later, the bats were recaptured and identified through their microchips. In follow-up tests, all the eight bats flew towards the rewarded sound, and were able to differentiate between the reward ringtone and a novel one. Many of the bats though, did fly towards an unrewarded sound from the initial trial. 17 untrained bats exposed to the same ringtones, twitched their ears but did not fly towards the sounds.
Dixon says, “The study taught us a lot because there are relatively few studies of long-term memory in wild animals, and we don’t have systematic understanding of long-term memories in nature yet. If we can collect additional data on different species of bats, we could pick this apart and see what life histories select for long memories.”
The paper references 39 previous studies that have documented the memories of species ranging from fish, birds, bats and goats to primates. The longest duration studies, of tortoises for nine years, sea lions for 10 years, and dolphins for 20 years, were all conducted on animals that lived in captivity. Co-author of the study Gerald Carter says, “Being able to study memory in the wild is important. You can’t necessarily extrapolate from the wealth of data we have on animals in the lab to what they’re facing in the wild, where there are many more things they have to remember. The environment is different and the brain is different in the wild versus captivity.”
A paper describing the findings has been published in Current Biology.