Social groups in animals are remarkably complex... They exist thanks to numerous individual acts of cooperation, in a delicate balance of benefits weighted against the costs of group living. Sociality in sharks is surprisingly complex; with complicated networks of associations forming over time within their community and certain individuals even having specific, favoured friends. It is also very common for sharks to segregate by size and by sex into sub-groups of similar individuals within their community. So why exactly do these social groups exist? What benefits are there for individual sharks? And how do these societies remain stable?
Do You Wanna Be in My Gang?
In the past, many people thought of sharks as mindless, soulless, eating-machines, but in recent years we are becoming increasingly aware that sharks are very cognitively advanced, and capable of complex and dynamic social relationships. We are only just beginning to scratch the surface of understanding how their remarkable and complicated social lives work...
Sharks form social groups for many different reasons. For some species, like the great white (Carcharodon carcharias), groups only form transiently, when solitary sharks happen upon each other at a rare event, like scavenging on a whale carcass. For other species, like the scalloped hammerhead (Sphyrna lewini), non-permanent groups form for the purposes of mating. On the other hand, there are many species of sharks which live permanently in social groups. For some, the basking shark (Cetorhinus maximus) grey reef sharks (Carcharhinus amblyrinchos) for instance, do so because it offers them improved foraging opportunities (to learn more, check out You've Got a Friend in Me). Others, like the lesser spotted catshark (Scyliorhinus canicula), group because it provides them protection from predators (Springer, 1967; Jacoby et al, 2012).
Social groups are able to function, as each individual benefits from membership in the gang, yet there are always also costs associated with group living, like the increased competition for food resources, space and mates. This is known as a "trade-off" (Jacoby et al, 2010; Jacoby et al, 2012).
Pick on Someone Your Own Size!
Many species of sharks also separate into sub-groups based on their age or sex (known as "size-" and "sexual segregation" respectively). They do so because even other members of their own species can be a threat! Smaller sharks live in different habitats to the larger, older animals, so they do not have such high competition for food and to avoid becoming food themselves! (Jacoby et al, 2012).
Within the older individuals, females also often separate themselves from males to avoid "sexual harassment". The cost of reproduction is different between males and females; male sharks have limited investment in their offspring, whilst the female must devote significant resources to breeding. Whatsmore, shark sex can be quite violent, with much biting and thrashing, which can cause injury to the female (to learn more you can read Fifty Shades of White). Therefore, whilst it benefits a male shark to mate many times, with as many different partners as possible, the female strategy is to only mate at the right time with the right male (Jacoby et al, 2010; Jacoby et al, 2012).).
Lesser spotted catsharks live in same sex groups - with sexually mature females all grouping together in labyrinthine caves around the coast, whilst the males live in deeper offshore waters. The drive for the females to avoid the males is so strong, that they will often even "refuge" in areas which are outside of their optimal thermal tolerance. This means that the females are in sub-optimal conditions and potentially even under stress from cold or heat, just to avoid mating (Jacoby et al, 2010).
I feel like many of you ladies out there could empathise with that feeling!
In order to ensure a female makes the most of any potentially risky mating sessions she does partake in, these catsharks are able to store sperm and delay internal fertilisation until the optimum moment! This maximises their individual "fitness" - avoiding injury, whilst also ensuring successful reproduction (Jacoby et al, 2010). Pretty savvy ladies!
The Social Network
In order to better understand how the complex societies of the lesser spotted catshark are able to remain stable, scientists have studied the bonds between individual sharks. They have discovered that catsharks form consistent, long-term social bonds with specific individuals in their group (on the diagram below, each individual shark is shown as a letter on and their interactions between each other are given by the lines; darker lines means there was a stronger interaction between these two sharks). Different social networks of catsharks (represented by each circle in the diagram below) were able to remain stable throughout time, due to consistent interactions between certain individuals (Jacoby et al, 2010).
These findings tell us that these sharks are not only able to recognise other individuals, but they prefer certain individuals over others... Basically they have friends.
Girls Have Got to Stick Together!
So the scientists decided to experiment. What happened to these social networks when there was a "social perturbation" - like the addition of a male shark to group of females?
There were two different reactions: within a group of females that had very strong social associations, the male (shown as a black spot with letter on the diagram below) was able to assimilate into the group without any sexual harassment occurring (see extreme left and extreme right on the diagram). Whatsmore, the females on the peripheral of the group became more integrated as well (see how the spotted lines on the diagram above become darker on the diagrams below). Comparatively, within groups where females had weaker associations with each other, the addition of the male caused the females to scatter (Jacoby et al, 2010).
The researchers found that there were "key individuals" which held the group together after the introduction of a male; maintaining their social connections and creating social cohesion. It was the lack of these key individuals which caused the group to dissolve after the introduction of a male into the asocial group (Jacoby et al, 2010).
The scientists concluded that it is the unique social connections between individual lesser spotted catsharks allow female groups to stay consistent and stable throughout time. They suggested this has evolved because each individual in the group is seeking to maximise their own "fitness: - maintaining strong social bonds with specific individuals meant females were able to avoid sexual harassment from males. Girl power! In the long-run these friendships allowed these sharks to have a better chance of staying alive, staying healthy and breeding successfully (Jacoby et al, 2010).
Even in the shark world life is easier and everything is better when you have friends to look after you.
Jacoby DMP, Croft DP & Sims DW (2012). Social behaviour in sharks and rays: analysis, patterns and implications for conservation. Fish and Fisheries 13:4, 399-417. Access online.
Springer S (1967). Social organization of shark populations. In: Gilbert PW, Mathewson RF & Rall DP (Eds.). Sharks, Skates and Rays. Baltimore: Johns Hopkins University Press, pp. 149-174.