The first assessment of social organisation of the Indian Ocean humpback dolphin (Sousa plumbea) along the south coast of South Africa

Abstract

The Indian Ocean humpback dolphin (Sousa plumbea (G. Cuvier, 1829)) is the most endangered marine mammal species in South Africa, and the overall decline of its abundance and group size may affect the social organisation of the species, potentially accentuating its vulnerability. Understanding the social organisation is therefore particularly relevant to conservation efforts. From photo-identification surveys along the south coast of South Africa from March 2014 to June 2015, we quantified association patterns and investigated the social organisation of Indian Ocean humpback dolphins using the half-weight index, social cluster, and network analyses. During the 101 surveys conducted and 553 h of survey effort, 98 sightings of dolphins were recorded and 65 individuals identified. Using individuals seen at least three times, this study reveals that the social network is well differentiated, as strong social divisions exist between individuals that seem to be split into four distinctive social clusters. Network strength was low; approximately half of the associations were low to moderate, whereas some individuals were strongly associated, especially between four pairs of individuals. Although this study is based on a limited number of individuals, our study nevertheless suggests that the atypical strong social bonds recorded here could result from behavioural responses following the decline in group size and abundance.

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

“…Most humpback dolphin populations live in fissionfusion societies mainly composed of small-sized groups (about 10 or less individuals), of which the group membership changes often, so that most dyadic interactions are relatively short-lived (Karczmarski 1999;Parra et al 2011;Dungan et al 2012Dungan et al , 2016Bouveroux et al 2019;Hunt et al 2019). There is however variability around this central tendency in social life, as solitary individuals are not uncommon, and large aggregations up to dozens or even 100 individuals have occasionally been recorded in some regions (Parsons 2004;Baldwin et al 2004;Würsig et al 2016).…”

Resources and population traits modulate the association patterns in the common bottlenose dolphin living nearby the Tiber River estuary (Mediterranean Sea)

“…However, individuals not identified as females composed the network of animals mostly seen in association with trawling vessels. Non-random associations are common in many terrestrial and marine mammals that exhibit fissionfusion grouping patterns [e.g., African elephants, Loxodonta africana (Wittemyer et al, 2005); Indian ocean humpback dolphins, Sousa plumbea (Bouveroux et al, 2019); killer whale, Orcinus orca (Ford et al, 2000), particularly in female clusters [e.g., bottlenose dolphins (Connor et al, 2000); grey kangaroos, Macropus giganteus (Best et al, 2014); zebras, Equus grevyi (Sundaresan et al, 2007); and giraffes, Giraffa camelopardalis (Carter et al, 2013)]. Here, non-random associations within different groups may indicate that not all individuals have the same role in this society or play a similar part in the network's cohesion (Lusseau et al, 2003).…”

“…The 'gambit of the group' (GoG) assumption, i.e., each animal in a group or a cluster is associating and interacting with every other animal in that group, was adopted to examine dolphins' association patterns (Whitehead and Dufault, 1999;Whitehead, 2008a;Franks et al, 2010). To reduce biases (Chilvers and Corkeron, 2001;Bouveroux et al, 2019), and to include bottlenose dolphins regularly frequenting the study area, only distinctive individuals encountered on ≥5 occasions in two to four different years were used for the association analysis (all individuals, AI dataset; n = 68). The number of 68 individuals resulted in a powerful sample size considering an estimated abundance of 80 resident bottlenose dolphins in the study area (see Pace et al, 2021, for details on population abundance).…”

“…We used the 70% threshold since about 30% of the sightings occurred when trawling vessels were not operating (every Saturday and Sunday, and during the fishing break period for biological recovery each year). Daily sampling periods were used to remove demographic effects occurring during the study period, such as birth, death, immigration, and emigration (Whitehead, 2008b;Bouveroux et al, 2019).…”

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

“…Our socio-behavioral knowledge on humpback dolphins (Sousa spp.) mainly came from studies on IPHDs in the Chinese waters (Chen et al, 2011;Dungan et al, 2012Dungan et al, , 2016Wang et al, 2015), Australian humpback dolphins (S. sahulensis) in the Australian waters (Parra et al, 2011;Hunt et al, 2019;Hawkins et al, 2020), and Indian Ocean humpback dolphins (S. plumbea) in the South Africa waters (Karczmarski, 1999;Koper et al, 2016;Bouveroux et al, 2019). Some studied populations were documented to display fission-fusion dynamics with some long-lasting social relationships.…”