A mother bat has one crucial challenge: find her own babies among hundreds of other bats in the colony. Making sure she cares for and nurses only her own offspring is important to ensuring their survival. Among Japanese house bats, mothers keep track of their pups using the babies' isolation calls. To understand how these calls affect the mothers, the researchers monitored changes in the heart rate of mother bats presented with calls from their own babies and other bats' babies. Although the mother bats didn't react to calls from other bats' pups, the sound of their own pups made their hearts beat faster.
"The inspiration for our study began with a striking observation in our laboratory colony. When a pup was separated from their roosting area, only the biological mother immediately rushed over to bring it back," said Dr Kazuki Yoshino-Hashizawa of Doshisha University, lead author of the article in Frontiers in Ethology. "We knew that pups emit isolation calls — acoustic distress signals — when separated. But seeing this strict behavioral selectivity made us wonder: what is happening inside the mother when she hears these calls?"
Calls for help?
The scientists captured four pregnant bats from a colony roosting near Doshisha University. Each bat gave birth to a litter of three pups. When the pups were two days old, the scientists recorded samples of their isolation calls. Because some of the litters contained no females, the scientists recorded calls from only male pups, to rule out any variation based on the sex of the pup.
"Isolation calls are specific vocalizations emitted by pups when they are separated from their mothers, functioning like a human baby's cry," said Yoshino-Hashizawa. "When a pup is lost or separated in a dark colony, these calls alert the mother to locate them. Pup calls have individual signature features, allowing mothers to tell them apart."
The pups grew quickly: at about eight days old they started to produce their first echolocation calls, and after about 12 days they all stopped uttering isolation calls. By the time they were 30 days old, they were able to fly and produce echolocation calls like adults, and they no longer roosted next to their mothers. At this point, the scientists recorded the pups' echolocation calls.
About 40 days after the births, the researchers separated the mothers from their offspring, set up an ECG for heart rate recordings, and played them the different calls. The call of an unfamiliar adult male was also played, to see how they would react to another adult's call.
Hearts beat faster
The researchers found that mothers' heart rates increased in response to their own pups' calls, but not those of pups that weren't their own.
"The fact that the mother's strong heart rate increases in response to her own pup's call suggests that these vocalizations instantly physiologically prime the mother to engage in maternal behavior," said Yoshino-Hashizawa.
But unexpectedly, mother bats' heart rates increased most in response to echolocation calls from pups, even though these pups were older and independent. The scientists hypothesize that this could be due to familiarity — the mothers knew their pups' most recent calls better — but further experiments will be needed to explore this.
The scientists also found that the call of an unfamiliar adult male also elicited a strong heart rate reaction. This could be because the bats perceived the male as a threat: a faster heartbeat shows that the bats react, but scientists can't tell if it's a positive reaction or not. Heart rate variability could be more informative, but the baseline variability of bats' heart rates is not currently well understood, so we don't have the parameters to understand what significant changes might look like. A larger sample size might also reveal more; the scientists worked with a small number of bats, using specialist statistical analysis to compensate for the sample size, because pregnant bats are difficult to care for in captivity.
"While measuring overall heart rate allows us to gauge the intensity of physiological arousal, it cannot distinguish between positive emotions, such as maternal motivation, and negative ones, such as stress or alarm," said Yoshino-Hashizawa. "To fully decipher the emotional valence of these vocal responses, future research will need to incorporate heart rate variability analysis, which offers deeper insight into autonomic nervous system modulation."
