The biogeography of group sizes in humpback dolphins (Sousa spp.)

Abstract

Humpback dolphins (Sousa spp.) are obligate shallow-water and resident species, and they typically live in fissionfusion societies composed of small-sized groups with changeable membership. However, we have scant knowledge of their behavioral ecology, starting with potential factors influencing inter-population variability of their group sizes. Here, we compiled a new global dataset of humpback dolphin group sizes based on 150 published records. Our data indicated an inter-specific consistency of group-living strategy among the 4 species in the Sousa genus, as these species preferred living in small-sized groups with a mean size of mostly no more than 10, a minimum size of single individual or small pairs, and a maximum size of several tens or ≈100. In addition, we clearly showed the geographic variations in group sizes of humpback dolphins at a global scale. We found that the geographic variations in humpback dolphin group sizes were primarily associated with the latitude, sea surface temperature, and abundance. To conclude, our findings provide insights into social dynamics and socioecological trade-offs of humpback dolphins, and help better understand how these resident animals adapted to their shallow-water habitats from the perspectives of biogeography and socioecology.

Group Size of Indo-Pacific Humpback Dolphins (Sousa chinensis): An Examination of Methodological and Biogeographical Variances

“…This study made the first attempt to sample comparable group size of humpback dolphins from different geographic regions by using two methods simultaneously. Our data clearly revealed that dolphin group size across three sampling regions, no matter from observer-based counts or photo-identification, were highly variable, typically including single individual, small pairs, and rarely middle-to-large aggregations of several tens (Parsons, 2004;Würsig et al, 2016;Liu et al, 2021c). Notably, small groups with ≤10 members were the most frequently encountered (80-90%), while only a small proportion (<5%) were large groups with >30 members.…”

“…A group was considered as AAC when G observer was ≤10 individuals, indicating that all or almost all individuals were captured in the group (Tyne et al, 2014;Hupman et al, 2018). A threshold, i.e., 10 was selected because humpback dolphins were often observed in small groups with ≤10 individuals (Parsons, 2004;Würsig et al, 2016;Liu et al, 2021c). For AC groups, we calculated the photo-identification group size G photo by counting the number of D1, D2, and D3 individuals present.…”

“…In dolphin societies, group size, social structures, and dynamics differ among species, which is known as interspecific variability of sociality (Gygax, 2002b;Gowans et al, 2007). Additionally, a specific dolphin species can build different sizes of groups at various spatial and temporal scales (Gygax, 2002a;Liu et al, 2021a,b), which is so-called biogeographical or interpopulation variability of sociality (Liu et al, 2021c). Although both observer-based counts and photo-identification have been widely applied in dolphin sociality studies, little attention, if anything at all, has been paid to compare the performance of these two methods in estimating group size.…”

“…Although both observer-based counts and photo-identification have been widely applied in dolphin sociality studies, little attention, if anything at all, has been paid to compare the performance of these two methods in estimating group size. Intraspecific variability in dolphin group size is often confusing, since variances from methodology and biogeography were mixed in many studies, leading to substantial difficulties in comparing the estimates from different systems (Gygax, 2002a,b;Liu et al, 2020bLiu et al, , 2021c.…”

“…Group size estimates have been widely reported for this species across many known populations, and all studies have used either observer-based counts (Chen et al, 2010;Wang et al, 2015) or photo-identification (Chen et al, 2016;Wang et al, 2016). Humpback dolphins are often observed or photographically captured in groups with variable sizes (Würsig et al, 2016), from a single animal to small groups (mostly about ten or fewer), and sometimes to large aggregations (several tens or low hundred ;Parsons, 2004;Liu et al, 2021c). Reducing errors in estimating group size is crucial to density and abundance estimate for this species (Marsh and Sinclair, 1989;Chen et al, 2010), but there is no a good grasp of how well traditional estimation methods (i.e., observer-based counts, and photo-identification) applied to this species.…”