When gibbons meet partridges: testing for heterospecific vocal mimicry in a conspecific functional context

Fieldwork for Hainan gibbons was performed in Bawangling Nature Reserve (19°02′-19°08′ N, 109°02′-109°13′ E), Hainan, and for Rickett's hill partridges in Wuyishan Nature Reserve (27°48′-28°00′ N, 117°39′-117°55′ E), Jiangxi, China. Bawangling is located at the junction of Changjiang and Baisha counties in Hainan, south China. The total area of the reserve is about 8,444.3 ha, and the elevational range is 350-1,438 m a. s. l. The area has a tropical monsoon climate with annual precipitation of about 1,600 mm, an average annual temperature of about 21°C, and a relative humidity of around 85% (Qi and Zeng 2011). The reserve is primarily composed of mountain rainforests, which exhibit the characteristics of tropical rainforests (Qi and Zeng 2011). Wuyishan is located in the northwest of the Wuyi Mountain Range in northeastern Jiangxi, China, the total area of the reserve is about 16,007 ha, and the elevational range is 300-2160 m a. s. l. The nature reserve presents a humid subtropical climate and belongs to the forest ecological nature reserve for the conservation of mid-subtropical broad-leaved evergreen forest, and rare animals and plants. The annual average precipitation is about 2,600 mm, annual average temperature is about 14 ºC, and forest coverage rate is up to 92.7% (Liu et al. 2015). Hainan gibbons are an endangered species of primate with a population of approximately 30-40 individuals living in Bawangling (Liu et al. 2020). Gibbons are highly territorial and the vocal structure of the Hainan gibbon is considered to be one of the simplest among gibbons and may represent the basic features of the Hylobatidae family (Geissmann 2002). Hainan gibbon calls could be detected with a high degree of accuracy in the field by observers (Dufourq et al. 2021). They produce variety of call notes that are organized as sequences. During chorus, the male gibbons produce calls that consists of two short notes followed by a long note (Deng et al. 2014). The call that they use to mark their territory and maintain unity within the group against potential rivals is made up of short sounds or notes that are put together to form longer phrases, which can consist of up to six notes and are combined to create songs (Deng et al. 2014). In addition, they also produce alarm calls in cases of threat, which elicit mobbing behavior in conspecifics (Deng et al. 2016). The Rickett's hill partridges is endemic to southeastern China and is currently considered as near threatened (Liu and Chen 2024). Rickett's hill partridges’ song consists of a series of well-spaced, repeated, melodic and plaintive whistles. They use these calls for territorial defense and maintain contact with mated partners. This species is most vocal in the evenings and early mornings (Liu and Chen 2024).Here we examined the similarity in vocalizations between Hainan gibbons (Nomascus hainanus) and Rickett's hill partridges (Arborophila gingica), and further tested their behavioral responses to these calls. Our results indicated that the call measurement based on the peak frequency and duration from the Hainan gibbon is clustered into the Arborophila species group and its typical call note is constructed by one long element. Among 11 Arborophila species, the call note from the Rickett's hill partridge is the closest and most similar call note to Hainan gibbons. In the playback experiment, Hainan gibbons promptly approached the speaker in response to calls from their own species, whereas they displayed no response when calls of Rickett's hill partridges were played. Likewise, Rickett's hill partridges produced a higher number of notes in response to conspecific calls, whereas a significantly lower number was produced when heterospecific calls were played. To sum up, both species showed a stronger reaction to the calls from their own species compared to heterospecific calls. Our study highlights the importance of species-specific vocal signals in conspecific recognition and communication. Future studies should perform comparative analysis of these vocal signals in closely related Nomascus species to investigate whether they can discriminate between conspecific and heterospecific signals.