Intra-Population Variability in Group Size of Indo-Pacific Humpback Dolphins (Sousa chinensis)

Abstract

Group size is a key social trait influencing population dynamics of group-living animals. The Indo-Pacific humpback dolphins (IPHDs), Sousa chinensis, a shallow water delphinid species, display a fission-fusion social system. Yet little is known about how social organization of this species vary with temporal scales and behavioral state. In this study, we sampled group size estimates from the world’s second largest population of humpback dolphins (Sousa spp.), which inhabit the eastern waters of Zhanjiang, China. IPHD group sizes changed seasonally and inter-annually, but not with tidal phases. Group sizes also changed with behavioral state of IPHD groups and with number of mother-calf pairs present. IPHDs formed larger groups in the autumn than in other seasons, which might be related to seasonal changes in food availability and reproductive cycle. Of the groups observed, we recorded the presence of mother-calf pair in 85 groups (i.e., nursery groups: 47 ones with one pair, 25 ones with two pairs, and others with three pairs). Notably, nursery groups were about 2–4 times larger than non-nursery groups. In addition, group sizes greatly increased with the number of mother-calf pairs. Living in relatively large groups, more protection, food, and resources might be available for IPHD mothers and calves, and such social strategy provide higher reproduction efficiency and survival success for this species. During our observations, feeding (45.5%) and traveling (25.2%) represented the majority of IPHD’s behavioral budget, while socializing (8.4%) and resting/milling (6.8%) were not frequently observed. Resting/milling groups were approximately 50% smaller than feeding, traveling, or socializing groups, while the latter three types had a similar mean group size. Large groups when IPHDs foraged, traveled, or socialized, might provide more added group benefits. For the first time, our findings clearly revealed intra-population variability in IPHD group sizes across different behavioral and temporal variables, and provided a better understanding of IPHDs’ adaptations to various biological processes and ecological constraints.

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

“…A matrix heatmap was illustrated to show the number of boat-based survey days and humpback dolphin sightings per month from 2013 to 2019 in three survey regions. Frequency histograms were illustrated to display group size patterns obtained from various methods in different regions (Bouveroux et al, 2018;Liu et al, 2021b). The skewness, kurtosis, and median value of group size data were calculated for each subset (Doane and Seward, 2011).…”

“…Generalized linear mixed models (GLMMs) were built to examine variances in group size of humpback dolphins, including fixed and random effects. In this study, the fixed effects were predicted by method (G observer or G photo ) and region (SWH, SNB, or LZB), and the random effects by year (2013)(2014)(2015)(2016)(2017)(2018)(2019) and season (spring: March-May, summer: June-August; autumn: September-November; or winter: December-February; Liu et al, 2021b). In the R 4.0.5 (R Development Core Team, 2021), the package "lme4" was used (Bates et al, 2015) to construct GLMMs with a Poisson family and logit link function (Vargas-Fonseca et al, 2018;Dorning and Harris, 2019).…”

“…To keep consistency, two primary observers were maintained throughout the survey period and across different sampling regions. Within an encounter, one primary observer would count the number of dolphins and the other would take photos (Liu et al, 2021b). In this study, the term "group" was referred to any aggregation of humpback dolphins (including solitary individual) in the observers' effective field of view, generally either socially (i.e., engaged in similar behaviors) or spatially associated (e.g., within 200 m of each other; Karczmarski, 1999;Jefferson, 2000).…”

Season, age, sex, and location impact the density of tooth rake mark and dorsal fin notch of Indo‐Pacific humpback dolphins (Sousa chinensis) in the northern South China Sea

“…Even though several of these populations have been shown to decline (e.g., Huang et al, 2014; Lin et al, 2022; Zeng et al, 2020), probably because of various human activities (Jefferson et al, 2009; Wang et al, 2007), few effective conservation measures have been taken (Jefferson & Smith, 2016). Despite the large amount of research conducted on this species, many of the basic parameters of studied populations, such as the population size, habitat use, and social structure remain poorly understood, and gaining knowledge about these parameters is crucial for taking adapted conservation measures (Hung & Jefferson, 2014; Liu et al, 2021). Although social behaviors are frequently witnessed in IPHDs (e.g., Liu et al, 2021), they have been studied little.…”

“…Despite the large amount of research conducted on this species, many of the basic parameters of studied populations, such as the population size, habitat use, and social structure remain poorly understood, and gaining knowledge about these parameters is crucial for taking adapted conservation measures (Hung & Jefferson, 2014; Liu et al, 2021). Although social behaviors are frequently witnessed in IPHDs (e.g., Liu et al, 2021), they have been studied little. Investigating social interaction patterns in different populations of IPHDs would aid in understanding these animals' social organization.…”