Vetmeduni: Courtship songs of house mice: Vocalizations influence mating success
During courtship, house mice (Mus musculus) emit ultrasonic vocalizations (USVs) that are beyond the range of human hearing (>20 kHz). Male mice produce approximately 90% of these USVs during interactions with females, and their vocalizations sound like birdsong when made audible for human ears. Researchers from the Konrad Lorenz Institute of Ethology at Vetmeduni recently analyzed the various types of calls emitted by wild house mice during different stages of courtship and copulation. They also examined whether specific calls influenced the males’ mating success.
For the study, the vocalizations of pairs of mice were recorded during different stages of courtship and mating and then their emission rates over time were quantified and each vocalization was classified, based on its distinctive acoustic characteristics in spectrograms. The researchers classified over 53,000 calls, 90% of which were USVs. USVs can roughly be categorized into short, simple, complex and unstructured groups. During each 10-minute recording, the rodents emitted between 0 and 2,000 USVs.
Mouse vocalizations become more complex during courtship and mating
The mice exhibited remarkably complex and dynamic changes in their vocal composition and repertoire, over the four main phases of courtship and mating behavior (depicted in the illustration, along with spectrograms of the typical calls expressed during two seconds of each phase):
- Initially, before they interacted directly, the mice emitted few calls, which were mostly simple types and unusual calls.
- Upon physical contact and mutual sniffing, the mice increased vocal emission, including all five simple USV types.
- Many hours later, when the males began attempting to mate and exhibiting other sexual behaviors, they increased the emission of all four complex USVs, which include frequency jumps and harmonic elements.
- After ejaculation males briefly paused and then resumed vocalizing, but using only simple USV types again during this phase.
Certain vocalizations and their timing correlated with mating success
The vocalizations of the mice were closely timed with the males' mating behaviors, peaking in rate and complexity seconds before the males approached and attempted to mount the females. The USV emission rates increased with males’ sexual behaviors for both successfully and unsuccessfully mating males, but particularly among the males that successfully copulated. "It is impossible to discern cause and effect from such correlations, as vocalizations can promote sexual behavior, or vice versa," explains Dustin Penn, the senior author on the paper from the Konrad Lorenz Institute of Ethology at Vetmeduni. "However, the production of some types of calls during early courtship predicted whether the mice copulated or not, and in cases when male mating attempts led to successful copulation, the vocal sequences started earlier and included more complex syllable types during late courtship."
Broadband vocalizations also played a role in mating behavior and copulatory success
Approximately 10% of the calls emitted by the mice were broadband vocalizations (BBVs), also referred to as "squeaks." These calls occur at lower frequencies (and are thus partially audible to humans), and they have stacks of harmonic overtones, as with typical mammalian vocalizations. BBVs are thought to be emitted primarily or exclusively by females and some have suggested that they serve to fend off unwanted sexual advances from males. "We found an increase in the number of squeaks as the mice made physical contact. However, contrary to what we expected, these calls did not inhibit male mating attempts or copulatory success," says Dustin Penn. The copulating pairs emitted more BBVs with simple linear harmonics compared to non-copulating pairs. The mice also adjusted the timing of their BBVs just before a mating attempt, with emissions increasing in the seconds leading up to an attempt and sharply decreasing one second before mounting. "These findings suggest that female BBVs may also influence mating and that the timing of their emission before a male’s mating attempt - rising and then falling - could signal the female’s sexual receptivity," Penn explains.
Courtship duet: synchronization of USVs and BBVs
The researchers found that, in the seconds just before a male attempted to mount a female, the two types of calls—male USVs and female BBVs—became highly synchronized in their timing. Such synchronization of courtship vocalizations, known as "duetting," has been observed in some songbirds and primates. The communicative function of such courtship duets remains unclear, and the scientists found no evidence that vocal synchronization affected the mice’s mating success.
Conclusion: Overall, these findings reveal that the courtship vocalizations of house mice are far more complex and dynamic than previously thought, even involving courtship duetting. This study provides the first evidence that the vocalizations of house mice influence male copulatory success and that certain USV types emitted during early courtship can predict whether a pair will successfully mate. These results provide another example of how other species can have surprisingly complex communication, but since they live in their own 'sensory worlds' they produce signals that we humans cannot perceive. These results will be of broad interest to researchers studying animal behavior, sexual selection and animal communication, and also neuroscientists investigating how the brain interprets information contained in mouse vocalizations.
Scientific article
The article "Courtship vocalizations of wild house mice show highly dynamic changes and correlate with male copulatory success" by T. Klaus, B. Wernisch, S. M. Zala and D. J. Penn was published in Animal Behaviour.
Link to article
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