B-Social networks in sharks


Recently research about social structure in animal populations has advanced considerably. This has revealed that organisms such as fish, which were at first thought to be socially primitive, have complex and structured social networks. Sharks are often observed in groups, such as grey reef sharks (Carcharhinus amblyrhynchos) that aggregate in groups of female and scalloped hammerheads (Sphyrna lewini) that swim in polarized schools. Their relative brain-body ratios are comparable to those of mammals, suggesting that they might be capable of complex cognitive behaviours. Sharks have already been observed to form dominance hierarchies and have been shown to be capable of learning.

But very little is known about the overall organization, structure, and complexity of the group/population of sharks.

Large solitary marine top-predators such as sharks have been observed to aggregate at specific areas. Such aggregations are almost certainly driven by foraging and behavioural strategies making space for diverse spatial organisations. Reef-associated shark species often show strong patterns of site fidelity that could be viewed as a prerequisite for sociality. However, there is limited empirical evidence that such aggregations are driven by intrinsic social factors in open system. Association data for 133 regularly sighted blacktip reef sharks (Carcharhinus melanopterus) were obtained from photo-identification surveys conducted between 2008 and 2010 in Moorea coral reefs (French Polynesia). Based on association data, we adapted a recently developed social network approach to demonstrate evidence of 4 main communities and 2 sub-communities within the population surveyed. We confronted the resulting structure with candidate explanatory variables. While the influence of sex and length was weak, results show for the first time that sharks formed spatial groups characterized by non-random and long-term associations, despite opportunities for social relationships to develop between communities. We conclude that the observed grouping patterns not only resulted from passive aggregations for specific resources, but rather the communities had been developed from an active choice of individuals as a sign of sociability. This suggests that a stable grouping strategy may confer substantial benefits in this marine predator. Individual preferences and adaptation to local conditions, as well as demographic, ecological and anthropogenic factors, may explain the social variability between units.

Figure above: Photo-identification of blacktip reef sharks. Global view of both sides of a female (A) ‘‘Op19’’ and a male (B) ‘‘V19M’’; note the elongated claspers that extend beyond the pelvic fins in males (B) and their absence in females (A). Photographs of both sides of the dorsal fin of 6 different individual C. melanopterus (C) ‘‘V12M’’; (D) ‘‘Tao25’’; (E) ‘‘Op26’’; (F) ‘‘Op18’’; (G) ‘‘Op27’’ and (H) ‘‘V21M’’ taken between 2008 and 2010; note the margin patterns between black and white parts of the dorsal fin that is unique to each individual. Persistence of patterns over 10 years for (I) “V27F” and (J) “V12F”. All photographs are copyright to Johann Mourier except (I) and (J) on top that are courtesy of Ila Porcher. (From Mourier et al. 2012_Animal Behaviour)

Figure above: The blacktip reef shark social network and the spatial use of its communities. Each individual in the network is represented by a node. Social group structure was identified using the modularity matrix technique and membership in the four communities is displayed by colour. Social groups were composed both by males (squares) and females (circles). The shade of the edges represents the likelihood of individuals belonging to the same group (the darker, the greater). Node size is proportional to the length of individuals. Bottom left_Ranging of members of each community. (Adapted from Mourier et al. 2012_Animal Behaviour).

My objectives was to investigate the organisation of shark aggregations, determining whether social interactions are characterized by non-random partner selection in groups. Since reef sharks are giving birth in nursery area and let juveniles growing autonomously without taking care of them, the question is to investigate if young sharks recruit and join their parents to form family structured groups. By genetically sampling groups of sharks, I used microsatellite markers and relatedness analysis to determine if shark groups are composed of close relatives. While forming non-random and stable social groups, blacktip reef sharks do not particularly associate with close relatives.

Figure above: Blacktip reef shark social behaviours observed during the study: (a) two mature males following; (b) two mature males paralleling; (c) four adult shark milling. (from Mourier et al. 2012_Animal Behaviour)

Sicklefin lemon sharks also form social groups.

Network of lemon shark associations in Moorea (Clua et al. 2010)