Coding bonus: Bats’ hippocampal cells log spatial, social cues

Cave-dwelling bats spend most of their time doing just two things: navigating and socializing.

“Those two things are not typically separated in their daily life,” says Michael Yartsev, associate professor of bioengineering and neuroscience at the University of California, Berkeley. “95 percent of their life is basically spent in these collective social environments, where they both have to move spatially, and they also have to socialize with other individuals.”

These behaviors are intertwined in the brain, too, a study published today in Science shows. Bats housed in naturalistic laboratory “caves” use a large chunk of the hippocampus, including some of the same neurons that represent an animal’s place in the environment, to encode the distinguishing features of fellow bats, including sex, hierarchy status and affiliation, the study reveals.

The hippocampal “cells are encoding a multiplex of information,” says Angie Selles, assistant professor of biological sciences at the University of Illinois Chicago, who was not involved with the work.

The hippocampus is typically known for spatial navigation and the consolidation of episodic memories. Yet this new study highlights how the same cells in the hippocampus that respond to spatial stimuli also respond to bats interacting with one another and can generate sociospatial cognitive maps. “It brings it to a more holistic view of the hippocampus,” Selles says.

The hippocampus is already known to harbor social place cells, neurons that respond to the position of another animal, according to previous studies in bats and rats. Yet many of these studies were done in pairs of animals rather than in large groups, and so it wasn’t clear how the brain responds to the spontaneous behaviors of others in more complex social environments.

To address this, the new study used laboratory setups that mimic caves, in which multiple bats can roam around and interact with one another in a naturalistic way while electrodes record their brain signals.

The new findings partially replicate those published in 2023 by an independent group, which found identity-coding neurons in both the cortex and the hippocampus of bats in a similar naturalistic lab-based cave. But whereas the previous study used only male bats, the new study used wild bats of both sexes and kept them together 24 hours a day, seven days a week for several months. “It’s as social as it gets,” says lead investigator Nachum Ulanovsky, professor of neuroscience at the Weizmann Institute of Science.

This paradigm “is definitely pushing the field forward and, in a way, setting a new standard, because we do want to move into this very naturalistic behavior,” Selles says.

T

he team employed a “Big Brother” type of setup, Ulanovsky says. Groups of 5 to 10 wild Egyptian fruit bats lived together in a 3-by-3-by-3-meter room outfitted with six cameras and a radiofrequency-based system that monitored tags and unique barcodes placed on the bats to track their movements at all times.

The bats lived together for several months, eventually forming a stable social network. Once the bats were familiar with one another, the researchers mounted electrodes onto one or two bats and measured their their hippocampal brain activity for about three hours a day. No people were in the room, because bats can also respond to and recognize their experimenters.

Nearly 70 percent of the animals’ hippocampal cells turn on during a social interaction, electrode recordings revealed, suggesting the presence of “identity coding cells” that are socially modulated. That is a “super high percentage,” Selles says. “It was super crazy that there were so many.”

The activity of some of these cells correlates with interactions with specific individual bats—so much so that the researchers were able to decode whether a given bat was present in the room or not just by analyzing the neural activity data.

Whether a bat was flying or perched on a net, the same neurons responded to the same fellow bats, independent of the context or the bats’ positions. These neurons also represented distinct types of social interactions, such as grooming or boxing (in which the bats fight).

Neurons in the hippocampus also encoded the sex of the other bats, their hierarchy in the colony and whether they were perceived as friends or foes, the team observed. On average, the neurons responded more strongly to male bats, bats high in the dominance hierarchy, and bats with lower social affiliation, namely ones that were seen as “enemies.”

“These neurons contained a very rich set of information,” Ulanovsky says. Previous studies have examined these features such as sex and hierarchy separately, he adds. But “we found that the same neural network, the same neurons, represented all of these factors together.”

Thanks to this multimodal representation, the hippocampus might help bats “survive and navigate and socialize in this collectively complex environment,” Yartsev says.

Still, the question remains: What might happen with bigger groups of bats? Ulanovsky points out that colonies can sometimes grow into the thousands. “Our study showed you know how things can be really mixed together when you have a group,” Ulanovsky says. “So it’s important to do these kinds of studies—not just the controlled two-animal studies, but also these kinds of studies where you have more naturalistic situation.”