Social structure in a critically endangered Indo-Pacific humpback dolphin (Sousa chinensis) population

1. Social behaviour can be a vital aspect of population viability when it influences survivorship and reproductive fitness, which is often the case for especially social species, including cetaceans. Understanding this relationship is important in at-risk populations, where knowledge of network structure can provide insight into how they may respond to human impacts. 2. Association patterns in the critically endangered eastern Taiwan Strait (ETS) population of Indo-Pacific humpback dolphins, Sousa chinensis, were investigated using the seasonally collected (2007-2010) photo-identification data of 74 individuals. 3. Social network structure was characterized by high strength and reach, features that are thought to be atypical for the species. 4. Standardized lagged association rates were best fitted by a two-level model: short-term associations that dissipated in less than a day, and long-term associations projected to persist nearly 10 years. These persisting relationships were primarily among calves and their inferred mothers. The short-term associations, which were probably limiting the network's modularity, seemed to occur among more weakly associated individuals temporarily participating in large mother-calf groups. 5. When associations were defined by proximity (within one body-length), relationships involving calves were characterized by a significantly high clustering coefficient, and some calves had high associations with other calves and more than one adult, making maternal relationships ambiguous. This could be evidence for cooperative calf rearing, which may be important for calf survivorship and initiating dyadic relationships among calves. 6. Given their apparent uniqueness among populations of this species, the association patterns observed among the ETS S. chinensis, calves and their inferred mothers in particular, could be a response to conditions (small population size, isolation, confined distribution, and anthropogenic stressors) that make long-term cooperative behaviour advantageous. Thus, minimizing human impacts that disrupt social dynamics, such as interactions with fisheries and habitat fragmentation, should help to improve this population's long-term-viability. Copyright (C) 2015 John Wiley & Sons, Ltd.