Comparative analysis of long-range calls in equid stallions (Equidae): are acoustic parameters related to social organization?

Abstract

Evolution of long-distance communication in equids may correspond with species-specific types of social organization. To compare harem-forming species (type I) with those that do not establish permanent social units (type II), we conducted a comparative analysis of stallion long-range calls in seven species/breeds of equids: two breeds of domestic horses (archaic and modern breeds) and five wild species: Przewalski’s horse, kiang, Somalian ass, Grevy’s zebra, Grant’s zebra). Acoustic features allowed assigning calls of stallions with 92% average classification success to the correct species. The duration of the call clearly separated horses (type I) from type II species: kiang, Somalian ass and Grevy’s zebra. Accordingly to its harem social system (type I), the pattern of long-range call in Grant’s zebra deviates from that of its relatives in the direction of horses. Frequency of the first dominant band that was associated with body size separated modern horses from the archaic breed and Przewalski’s horse. Playback experiments confirmed that equids, especially the type II species, respond strongly to conspecific calls but also to calls of other equids.