Dolphins frequently use sound to perceive the underwater environment, look for prey, avoid predators, communicate with their peers, and so on. Narrowband FM whistles are thought to be used primarily for communication between individuals in animal societies, similar to human language. Studies have suggested that dolphins may have dialects between different geographical populations of the same species as humans. Therefore, whistle signal characteristics can reveal the kinship between different geographical populations of dolphins, providing corroboration or supplementation for information obtained from genetic or morphological data.
The Chinese white dolphin (also known as the "Indo-Pacific camel dolphin"; Sousa chinensis) is one of the four species of the camel dolphin genus, assessed as "vulnerable" by the International Union for Conservation of Nature (IUCN), and is a national first-class protected animal in China. It is mainly distributed in shallow waters near the shore from the eastern Indian Ocean through Southeast Asia to the mouth of the Yangtze River in central China. In China, Chinese white dolphins are mainly scattered along the southeast coast, mainly including Xiamen Bay, the west coast of Taiwan Island, the Pearl River Estuary, Zhanjiang, Beibu Bay and the southwest sea of Hainan. At present, it is not clear whether there is individual communication or gene flow between the populations of Chinese white dolphins distributed along the southeast coast of China. The population of Chinese white dolphins in the southwestern waters of Hainan was newly discovered in 2014. Little is known about their acoustic signature characteristics and their similarities and differences with the whistles of other populations in the adjacent seas.
Recently, the Li Songhai research team of the Institute of Deep Sea published a population in waters southwest of waters in marine mammal science entitled "Whistle characteristics of a newly recorded Indo-Pacific humpback dolphin (Sousa chinensis). The cover paper "Hainan Island, China, differ from other humpback dolphin populations" reveals the differences between the newly recorded whistle characteristics of the Chinese white dolphin population in the southwest waters of Hainan And the whistle calls of other Chinese white dolphin populations and other species of camel dolphins, and points out that there is environmental noise adaptability in the whistle signal characteristics of chinese white dolphins.
Cover of the current issue of the journal (Photo of Chinese White Dolphin by Dr. Lin Mingli in Sanya Yacheng on June 2015)
The study identified six signal types from 226 whistle signals collected in the southwestern hainan region (Figure 1), including straight line, up-sweep, convex, concave, down-swept and sinusoidal. Of these, the straight type is the most common and the convex type is the least. The basic whistle frequency of the Hainan Chinese white dolphin population can reach up to 21 kHz, which is similar to the whistle calls of other geographical populations, but its whistle duration is higher than that of most other geographical populations, and the whistle inflection point complexity is lower than that of other geographical populations except the Sanniangwan population in Guangxi, indicating that the whistle calls emitted by the Hainan Chinese white dolphin population are simpler than all other geographical population whistles except the Sanniangwan population in Guangxi.
Fig. 1 Acoustic spectra of the six whistles of chinese white dolphin whistles recorded in this study: (a) linear type, (b) downward sweep type, (c) up sweep type, (d) concave type, (e) convex type, and (f) sinusoidal type
At present, many research reports on the whistles of Chinese white dolphins and other camel dolphin populations have been reported, but the relationship and differences between whistle signal characteristics between different populations are not fully understood. Based on the acoustic characteristics of whistle signals such as frequency and duration, this study uses hierarchical clustering to reveal similarities and differences in whistle characteristics between different geographical populations (Figure 2). Studies have shown that the temporal characteristics of whistles are more similar between populations of camel dolphins that are geographically adjacent. The whistles of different geographical populations of Chinese white dolphins in China's waters are similar to each other, while the Population of Chinese White Dolphins in Malaysia (S. chinensis) with the Australian camel dolphin (S. Chinensis) sahulensis) and the Indian camelback dolphin (S. Plumbea) populations are closer on the whistle. In China's waters, the populations of Chinese white dolphins in the waters of Zhanjiang and Sanniang bay are more similar in terms of whistle characteristics than the populations of Chinese white dolphins in Hainan. It is speculated that the degree of individual and gene exchange between the Hainan Chinese white dolphin population and the other two populations may be lower than that between the Zhanjiang and Sanniangwan populations, so the Hainan Chinese white dolphin population may be a relatively independent population compared with its geographical neighbors. The above work provides important supplements and data support for further study of the genetic differentiation of Chinese white dolphins in the South China Sea and to understand the degree of isolation of different geographical populations of this species.
Figure 2 Hierarchical clustering results of similarity of whistle characteristics in different camel dolphin geographic populations
Differences in whistle-calling characteristics in marine mammals are related to animal population size and behavioral status, as well as to the underwater acoustic environment of habitats. The study found significant differences in underwater noise sound pressure levels (SPLs) in the habitats of Chinese white dolphins in Hainan, Zhanjiang and Sanniang Bay, among which the noise sound pressure level in the southwest waters of Hainan was significantly higher than that in Zhanjiang and Sanniang bays, while there was no significant difference between Zhanjiang and Sanniang bays (Figure 3). Animals can modulate the sound characteristics they emit to effectively improve the efficiency of sound propagation in different underwater acoustic environments. Simpler, less variable whistles avoid masking of ambient noise, while lower-frequency sounds can be transmitted over longer distances. Compared with Zhanjiang and Sanniangwan, the Hainan Chinese white dolphin tends to emit whistles that last longer, are less frequent, and have fewer inflection points to adapt to the noisier local acoustic environment, enabling more effective individual information exchange (Figure 4).
Figure 3 Comparison of nine whistle parameters of Chinese white dolphins in the geographical populations of Hainan, Zhanjiang and Sanniang bay (a), broadband underwater noise SPLs(b), ambient noise power spectral density (PSD) and spectral probability density (SPD) in the range of 50-48,000 Hz (c1, c2, c3)
Figure 4 Linear regression analysis of nine whistle parameters of Chinese white dolphins in the geographical population of Hainan, Zhanjiang and Sanniang Bay and SPLs of broadband underwater noise in local habitats
This study is the first to study the whistle characteristics of Chinese white dolphins in the southwest waters of Hainan. The results of the study are of great significance for assessing the isolation of different geographical populations of Chinese white dolphins in China's waters and the impact of underwater noise on these populations. The diversity of Chinese white dolphin whistle calls may reflect differences in acoustic environments in local habitats, differences between zoogeographic populations, or population isolation. The reason for whistle differentiation between the Population of Chinese White Dolphins in Hainan and adjacent populations may be related to genetic and environmental factors. Future studies of animal habitat acoustic environments and acoustic behaviors at larger geographic scales will help to better understand how underwater noise drives differentiation of whistle characteristics between animal populations at the species and interspecific levels.
Researcher Li Songhai of the Institute of Deep Sea is the corresponding author of the study, and Assistant Researcher Dong Lijun is the first author of the paper.