Sexual and individual signatures are encoded in the temporal rate of Cape Gannet Morus capensis display calls

Vocalisations play a vital role in animal communication, as they are involved in many biological functions such as mate selection, individual recognition and care of young. Seabirds often breed in large and dense colonies, making successful recognition between mates or between parents and offspring crucial for reproductive success. Acoustic signals have been shown to play an important role in this regard for several seabird species. Furthermore, most seabird species, including the Cape Gannet Morus capensis, are monomorphic, making sex identification for research challenging. Identifying individual and sexual signatures in their vocal productions could thus facilitate sex identification in the field. This study aimed to better understand the potential use of vocalisations for sex and individual recognition in Cape Gannets by describing the acoustic structure of their display calls at the nest. Vocalisations of nesting Cape Gannets were recorded over a two-week period. Acoustic measurements were extracted from 80 calls (16 individuals) and included 36 variables in both temporal and frequency domains. Twenty acoustic variables showed significant differences in vocalisations between male and female Cape Gannets. However, values of the fundamental frequency and the average of Inter-Onset-Interval (time elapsed between successive sound units) appeared to be the most important acoustic variables for sex determination. Both temporal and frequency parameters showed a potential for individual identity coding, again with the average units’ Inter-Onset- Interval being the most important variable for individual identification for both females and males. This study provides the first evidence of sex-specific and individual vocal signatures in adult breeding Cape Gannets enhancing our understanding of the role of the display calls in mate recognition. From an applied perspective, identified sex-specific differences could potentially be used as a non-invasive method for field-based sex-determination.