Acoustic battles for the harem: How the calls of Siberian wapiti reveal their status and individuality

Researchers at HSE University, Lomonosov Moscow State University, and the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences have studied the distinctive vocalisations of Siberian wapiti (Cervus canadensis sibiricus) stags during the peak of the mating season, when males produce rutting calls (bugles) to attract females (hinds) and deter rivals. The scientists have discovered how the acoustic parameters of these rutting calls reflect the stag's status—whether he currently holds a harem or is still attempting to acquire one—as well as his individual characteristics. The study has been published in Journal of Zoology.

Rutting calls are a vital component of intra-species interactions in Siberian wapiti, enabling stags to attract female hinds (sometimes entire groups) and intimidate rival males. These calls are integral to the mating season. In the wild, competition for keeping a harem (a group of females) is intense, prompting male ungulates to develop sophisticated strategies that combine physical displays and vocal demonstrations of strength. Acoustic displays are highly effective because they carry over long distances and remain reliable even in low-visibility conditions, such as at night, when deer engage in most of their interactions. Rutting calls enable females to assess a male's status and quality from a distance, while also signalling to rivals whether it is worth challenging him in a fight.

The study findings suggest that rutting calls act as a kind of 'passport,' reflecting the stag's harem-holding status. Compared to peripheral stags that remain outside a harem, harem holders produce shorter rutting calls with a higher minimum fundamental frequency.

The fundamental frequency of sound is a parameter that is subjectively perceived as the pitch of a voice and includes various measurements, such as the minimum, maximum, initial, and final fundamental frequencies. Moreover, even in a single stag, a change in status leads to alterations in the parameters of his rutting calls: when he acquires a harem, the initial and maximum fundamental frequencies of his calls decrease, while the minimum fundamental frequency increases, and the calls become shorter.

Although rutting calls can reflect a male's status and change as it evolves, their individual acoustic parameters typically remain consistent. It was previously believed that each stag has unique acoustic characteristics, a kind of vocal signature, allowing it to be distinguished from other stags. This presumed vocal individualisation is still used in nature reserves to monitor the number of stags by ear. In this method, a trained observer, stationed at a specific point, identifies the number of male deer nearby based on the differences in their voices.

According to Olga Sibiryakova, Associate Professor at the HSE Faculty of Biology and Biotechnology and co-author of the study, although individuality is encoded in rutting calls, it does not always manifest clearly enough: zoologists were able to correctly identify the individual stags in 53.2% of calls, which exceeds random chance but indicates that it is not always possible to identify a specific animal. It is noteworthy that discriminant analysis correctly classified 78.9% of bugles according to the stag's status.

Olga Sibiryakova

'The vocalisations can convey information not only about the individual caller but also about his status, and status information is conveyed much more clearly in the sounds than individuality, which suggests that transmitting status information to others is of greater importance during the mating period,' the researcher notes.

The study findings can be applied to managing deer herds in natural environments, tracking these animals for conservation and hunting purposes, and developing non-invasive methods for studying ungulates. The use of non-invasive monitoring methods can facilitate the work of zoologists and ecologists by allowing them to track the status and behaviour of deer in various conditions without disrupting their natural habitat.