Acoustic characterization of sperm whale ( Physeter macrocephalus ) population structure and social organization in the South China Sea

Detailed ecological information on sperm whales (Physeter macrocephalus) in the South China Sea (SCS) remains scarce despite its potential as a key nursing ground. Based on passive acoustic monitoring data obtained during vessel-based surveys (2019–2025), this study provides the characterization of the population's coda patterns, which reflect cultural social structure. Using a machine learning-based algorithm to analyze inter-pulse intervals from 2163 regular clicks, estimated total lengths ranged from 7.74 to 12.48 m, with 90% of individuals under 10 m. This result confirms the presence of females and immature individuals in the region, forming social units typical of nursing areas. Unsupervised clustering of 641 codas identified 20–21 discrete types, primarily consisting of 4–7 clicks. The predominance of two “4 + 1 ++ 1” patterns identifies the SCS population as part of the widespread Pacific “four-plus” vocal clan, which is found in the Eastern Pacific (e.g., Galápagos, Peru, and Chile) as well as the Western Pacific (e.g., Tonga and Papua New Guinea). These findings provide critical baseline acoustic data for sperm whales in the SCS and highlight the potential importance of long-term monitoring of this region for the species conservation.