Bat Mothers Use “Motherese” When Communicating With Their Pups
- 演化之聲
- Mar 12
- 3 min read
In the tropical rainforests of Central America lives a bat known as the greater sac-winged bat (Saccopteryx bilineata). This species has a remarkably complex social system, and its communication is equally sophisticated. It is one of the few mammals known to possess vocal learning abilities, meaning that its vocal development is not determined solely by genetics but is shaped through experience and social interaction during growth. Colonies typically consist of one or several harem males together with multiple females and their young, while additional peripheral males remain outside the central harem structure. Females are the only individuals that directly care for the young, whereas males mainly defend territories and compete for mating opportunities. Within this social framework, vocal signals are not merely tools for navigation and echolocation; they also function as channels for social interaction and identity communication.
Researchers discovered that mother bats produce a special type of call when interacting with pups whose vocal systems are still developing. These sounds, known as maternal directive calls (MDs), resemble what humans often call “motherese,” a speech style adults use when talking to infants. These calls occur especially when mothers reunite with their pups or when young bats engage in daytime vocal practice.
Field recordings were conducted over three breeding seasons at several locations in Central America, including Barro Colorado Island and Gamboa in Panama, as well as the Curú Wildlife Refuge in Costa Rica. The bats in these colonies had been studied for many years and were accustomed to human observers. Individuals could be identified using colored rings placed on their forearms, allowing researchers to record vocal interactions from close range.
Acoustic analyses revealed clear differences between the sounds females direct toward pups and those they use when addressing adult bats. When communicating with their offspring, mothers produce calls with a softer timbre and slightly lower pitch. The average peak frequency of these pup-directed calls is about 36.5 kHz, whereas calls directed toward other adults average 44 kHz. This shift in tone resembles the way human parents naturally soften and modify their voice when speaking to babies.
Interestingly, these maternal calls do not appear to contain an individual vocal signature. In other words, pups do not rely on this sound to recognize their own mother. Instead, the recognition system works largely in the opposite direction: pups produce distinctive isolation calls that allow mothers to identify them. Because mothers can already distinguish their own offspring through these calls, the pups themselves do not need to identify the mother individually.
This observation suggests that maternal directive calls likely serve a different function. Rather than acting as identity signals, they may operate as a form of positive social feedback. When young bats practice their vocalizations or begin attempting short flights within the roost, mothers may produce these calls in response, potentially encouraging further vocal practice. During development, pups imitate adult vocal patterns and gradually refine their calls, eventually acquiring the characteristic vocal features of their colony before weaning.
Adult males also participate in vocal interactions with pups. Both harem males and peripheral males were observed producing full isolation calls after hearing pups vocalize. However, these male calls are not directed toward any particular pup and occur without physical contact. The interaction therefore appears to be a general acoustic exchange rather than a targeted parental response.
Despite this lack of individual targeting, acoustic comparisons revealed that pups' isolation calls resemble those of adult males from the same colony more closely than those from other colonies. This pattern suggests that adult males may help shape the colony-specific vocal signature shared within a group. Through repeated exposure to male calls, young bats may gradually align their own vocalizations with the colony's acoustic “dialect,” allowing these group signatures to persist across generations.
Male isolation calls also appear in adult social interactions. In aggressive encounters, subordinate males sometimes emit their isolation call, after which dominant individuals may cease their attack. Males also produce these calls when courting newly arrived females, possibly signaling their group affiliation. In both cases, a shared colony-specific vocal signature may reduce conflict or facilitate social integration.
Studies of the greater sac-winged bat therefore reveal a complex system of vocal development shaped by multiple social influences. Female bats appear to use a form of infant-directed vocalization comparable to human “motherese,” while adult males may help transmit group-specific vocal traits across generations. Together, these findings suggest that vocal learning in bat societies is embedded within a rich network of social interactions that guide how young animals acquire the sounds of their community.
Author: Shui-Ye You
Reference:
Fernandez AA and Knörnschild M. (2020). Pup Directed Vocalizations of Adult Females and Males in a Vocal Learning Bat. Front Ecol Evol.
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