Far from being mindless killing machines, devoid of any type of advanced mental capacities or developed cognitive abilities, sharks are, in fact, very intelligent. Living in complex social groups, communicating, sharing information and even forming friendships, we now know that sharks are as socially evolved as birds and mammals. So what is the benefit of group living? Do sharks form social relationships? And how complex are their communities?
Let's Get Together
Some sharks live solitary lives, others form temporary "aggregations" to feed or mate, where some live permanently in groups, both small and large (Mourier et al, 2017).
Whilst there exists a myriad of subtle different reasons why each species has evolved to live in a group, they all boil down into two broad and very basic categories: food and sex (what other reasons are there to go to a party?). For example, group-living might mean there are more opportunities to reproduce or there is help to raise the young. It might also mean better foraging opportunities. Group living might also offer protection from predators (Mourier et al, 2017).
Safety in Numbers
Juvenile lemon sharks (Negaprion brevirostris) assort themselves based on their size and live in groups with others that are similar proportions to them. This offers them protection from predators, as they are not around much bigger sharks that might eat them and it is also harder for other types of predators to pick an individual out of the fray (Guttridge et al, 2009). Head over to Mellow Yellow to learn more.
Grey reef sharks (Carcharhinus amblyrhynchos) and whitetip reef sharks (Triaenodon obesus) hunt together in large groups, in order to increase their foraging success (Labourgade et al, 2020). Learn more at Every Shark For Themselves.
No Shark is an Island
Blacktip reef sharks (Carcharhinus melanopterus) live in very complex communities, hundreds strong, with stable sub-groups that remain consistent for many years. Scientists have even proven that they form something akin to friendship, as they spend the majority of their time with specific individuals, rather than mixing randomly. There are also some individuals which act as a go-between amongst different sub-groups (Mourier et al, 2017; Mourier & Planes, 2020). To learn more see Blood Is Not Thicker than Water.
Networking
Within their community, individual blacktip reef sharks differed in their "gregariousness" - how social a particular individual shark was. Some sharks had few associations, where others maintained many strong social connections within a group [shown as a dot with multiple lines connecting to several other dots on the diagram below] (Mourier et al, 2017; Mourier & Planes, 2020).
There were also some sharks that were more mobile within the community and flitted between multiple sub-groups. These individuals interconnected the cliques and created cohesion between the community as a whole (Mourier et al, 2017; Mourier & Planes, 2020).
Let's Stick Together
The scientists also learned that even when certain individuals were extracted by fisheries, the blacktip reef shark social network remained in tact and the community structure did not collapse. In fact, the researchers estimated as many as 25% of the sharks could be extracted before the social structure disintegrated (Mourier et al, 2017).
This is a very promising sign, as it means that their natural social behaviour will not be destroyed if a small amount of blacktip reef sharks were to succumb to fishing. That is not to say that a quarter of these sharks should be fished! But it is comforting to know that if fisheries are carefully managed, the impact on the wider community can be kept to a minimum and these sharks can continue to live amongst their buddies in their extended social groups for many years to come (Mourier et al, 2017).
To learn more about shark social relationships, check out You've Got a Friend in Me.
References
Guttridge TL, Gruber SH, Gledhill KS, Croft DP, Sims DW & Krause J (2009). Social preferences of juvenile lemon sharks, Negaprion brevirostris. Animal Behaviour, 78, 543-548. Access online.
Labourgade P, Ballesta L, Huveneers C, Papastamatiou Y, Mourier J. (2020). Heterospecific foraging associations between reef-associated sharks: first evidence of kleptoparasitism in sharks. Ecology, e03117. Access online.
Mourier J & Planes S (2020). Kinship does not predict the structure of a shark social network. bioRxiv. Access online.
Mourier J, Brown C & Planes S (2017) Learning and robustness to catch-and-release fishing in a shark social network. Biology Letters, 13: 20160824. Access online.
By Sophie A. Maycock for SharkSpeak
