Wild frog-eating bats have surprisingly long memories

It appears that when it comes to a dinner bell, some bats can remember for a long time what a good feed sounds like.

New evidence of long-term memory in wild bats has this week been published in the journal Current Biology.

In the new study, 49 wild frog-eating bats (Trachops cirrhosus) were trained to fly towards a specific ringtone to receive a food reward. When eight of the bats were recaptured between one and four years later, they all still flew towards the sound, while 17 control bats which had not undergone training largely did not react to the ringtone.

The result was “surprising” for lead author May Dixon, who led the study as a graduate student at the University of Texas at Austin and now works as a postdoctoral researcher at the Ohio State University (OSU), US.

“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,” says Dixon. “Four years strikes me as a long time to hold on to a sound that you might never hear again.”

While long-term memory has been studied in several animal species – there’s evidence that memories can last up to 10 years in sea lions and 20 years in dolphins – many of these experiments have focused on animals raised in captivity. The new data from frog-eating bats helps fill a gap in our knowledge of memory in wild animals.

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“The environment is different, and the brain is different, in the wild versus captivity,” explains co-author Gerald Carter, an assistant professor in evolution, ecology and organismal biology at OSU. “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.”

“We don’t have systematic understanding of long-term memories in nature yet,” Dixon adds.

“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.”

There’s another burning question here: how exactly do you train a wild bat to respond to a ringtone?

As it turns out, the researchers leveraged the bats’ own hunting strategy. In the wild, frog-eating bats find male túngara frogs – one of their favourite prey species – by the frogs’ distinctive mating calls.

In the lab, bats captured in the wild were played a recording of the frog’s mating call, and further enticed to fly towards the sound with a piece of baitfish placed above the speaker.

Over time, the frog calls were mixed with, and eventually replaced by, a specific ringtone linked to the food reward. The bats were also exposed to three other ringtones that were not associated with food, and microchipped so that they could be identified later.

In subsequent years, Dixon was able to recapture some of the microchipped bats and test whether they remembered the food-associated ringtone. All the recaptured bats flew towards the sound and all but two retrieved the baitfish reward. However, many of the bats also flew towards a sound from their training that wasn’t associated with food.

The researchers hope their work will shed light on the importance of memory in the animal kingdom.

“It’s not always true that being the smartest or having the longest memory is actually advantageous,” says Dixon.

“Research has shown that fruit flies selected for improved memories can’t compete as well against other fruit flies. That’s why we want to figure out when these skills are actually going to help animals and when they could be a liability.”