11.12.2020: Males often vocalize during courtship in a wide variety of taxa, including insects, amphibians, birds and mammals, but the courtship songs of male house mice are ultrasonic, and inaudible to human ears. Studying the ultrasonic vocalizations or USVs of mice therefore requires special microphones and spectrograms to visualize the characteristic features of different calls. Researchers from the Penn-Zala Group at the Konrad Lorenz Institute of Ethology (University of Veterinary Medicine, Vienna) recently developed an improved method for automating the detection of mouse USVs, which they now used to investigate a way of inducing male mice to vocalize.

It has long been suggested that exposing a male mouse to a female, known as ‘sexual priming,’ a day or more before recording induces them to emit more USVs. However, the evidence for this idea has been mixed and inconclusive. Moreover, it was not known whether sexual priming alters just the amount of calling or the types of calls that males produce, or how long the effects of priming last.

USVs measured and evaluated

The researchers conducted a study with wild-derived house mice and allowed males to briefly interact with a female for five minutes (sexual priming). Afterwards, they recorded male vocalizations either 1, 10, 20, or 30 days later while presented with another female. To detect vocalizations in these recordings, the researchers developed an improved version of their algorithm, Automatic Mouse Ultrasound Detector (or A-MUD), in collaboration with Peter Balazs from the Acoustic Research Institute (Austrian Academy of Sciences). They evaluated the performance of this method, which is freely available for scientific use. The researchers measured males’ vocalization rate and spectro-temporal features and manually classified the vocalization into 15 syllable types.

Sexual priming affects vocalization

They found that after interacting with a female, males emitted nearly three times as many USVs, they emitted a larger diversity in the types of USVs, and produced vocalizations that were longer and had other spectro-temporal features. The researchers also showed that males who had not previously interacted with a female had the most distinctive repertoire composition. Some effects lasted longer than others. USV length increased 1 day and repertoire diversity increased 1 and 20 days after priming, other effects were found when comparing unprimed to all primed males irrespective of the time since priming.

“Our results show that the vocalizations of wild-derived male mice are not only context-dependent, as they previously found, but they also depend upon previous social experience and perhaps the contexts of these experiences” explained lead author Sarah Zala. Dustin Penn added that “the effect of sexual priming on male courtship vocalizations is likely mediated by neuro-endocrine-mechanisms, which may function to advertise males' sexual arousal and facilitate social recognition”. Moreover, the results also provide a method for inducing male mice to vocalize for future studies.

The article “Primed to vocalize: wild-derived male house mice increase vocalization rate and diversity after a previous encounter with a female” by Sarah M. Zala, Doris Nicolakis, Maria Adelaide Marconi, Anton Noll, Thomas Ruf, Peter Balazs and Dustin J. Penn was published in Plos one.