The ultrasonic songs of mice contain distinctive individual signatures

Photo: Bettina Wernisch/Vetmeduni Vienna

Photo: Bettina Wernisch/Vetmeduni Vienna  1

House mice emit complex ultrasonic vocalizations or USVs, which are above the range of human hearing. A recent study from the Konrad Lorenz Institute of Ethology (University of Veterinary Medicine Vienna) found that the USVs emitted by wild male mice contain distinctive individual signatures that are stable over time. Their findings show that these signals potentially mediate individual recognition.

Laboratory mice are the most important model species for the biomedical sciences, and their USVs are increasingly used as a model to study autism and other neurological disorders. Ethologists study USVs, as a model to test ideas about animal communication and sexual selection.

In house mice, USVs are mainly emitted by males during courtship and mating. Dustin Penn, Sarah Zala and their students are studying the vocalizations of wild house mice to determine their functions. They previously found that females are attracted to recorded playbacks of male courtship USVs. The aim of this study was to determine what kind of information is contained in males' complex courtship vocalizations.

The complex courtship vocalizations of male mice

In their recent study published in Animal Behaviour, the researchers recorded male mice over three weeks, before and after presenting the males with female scent. They used their newly developed software (Automatic Mouse Ultrasound Detector or A-MUD) to detect more than 24,000 vocalizations, and then classified each call into one of 15 categories, depending upon its particular acoustic features.

They found that male mice emitted few if any vocalization without female stimuli. Yet, immediately upon being presented with female scent, most males began vocalizing, and they emitted a large number and variety of different types of USVs. A few males did not vocalize when presented with female scent, but these silent types were rare.

Male ultrasonic calls show high individual variation and consistency

The first author of the study, Maria Adelaide Marconi, explains that they found "very high individual variation in the number and types of USVs that males emitted, as expected, and yet, the males' ultrasonic vocalizations also showed surprising individual consistency over time”. Since the variation between different males was greater than the variation within individual's calls, their results show that USVs contain distinctive individual signatures.

USVs are surprisingly complex at many levels of organization, and the researchers found signatures of individual identity at every level that they analysed, including emission rates, mean frequency and duration, the number of different types of USVs, and even the length of silent intervals between calls. Moreover, using a machine learning algorithm, they were able to assign approximately 90% of recordings to the correct individual.

Implications of findings and future plans

These findings help to understand the communication functions of the complex vocalizations produced by mice in the wild. Penn and Zala explain that nearly all of the research on USVs is conducted with inbred strains of laboratory mice, and it is often assumed that these calls are "affect displays," which provide signals about an individual's current emotional state. There is increasing evidence to support this idea, including results from their own lab. Most research is focused on changes in USV emission within individual mice, and variation between individuals is often viewed as bothersome 'noise' in the data. The authors point out that their findings show how USVs can simultaneously provide information about an individual's state and its identity. They are currently investigating the consistency of individual signatures in USVs over a male's life span and under different social contexts, and also whether they control individual recognition.

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