You've Got a Friend in Me

Some species of sharks are solitary, whilst others live permanently in large groups... Some sharks only form schools or aggregations when they need to find a mate. Group-living can aid in "cooperative hunting" or increases a small shark's chances of survival, but sometimes it seems there is more to how groups of sharks form; something more akin to friendship. Human beings are social animals; we have extensive networks of family, friends, colleagues... so we are often interested to understand if animals also share our habit for forming relationships. It fascinates us, because it allows us to empathise with them on a different level; to feel like we can understand them... So we ask, "Do sharks have friends?"

Reef sharks commonly live in social groups (Image Credit: David Clode / Unsplash)

Recently a group of research scientists sought to answer this very question by studying the group dynamics of grey reef sharks (Carcharhinus amblyrhynchos) at the Palmyra Atoll, in the Pacific Ocean. Far away from any major landmass, the Palmyra Atoll is the tip of a chain of small islands, which have been a US Federal Wildlife refuge since 2001. As a result of protection and its seclusion, the region has flourishing, pristine wildlife, including a high density of sharks; as many as 21 sharks per square kilometre (it sounds like my dream holiday destination!) (Papastamatiou et al, 2020).

Location of the Palmyra Atoll

The researchers implanted special transmitters into the body cavity of 41 sharks and then used receivers, which were situated around the reef, to track their movements. They also attached cameras to some sharks to see how they moved around their habitat. This allowed them to determine which individuals commonly occupied the same areas.

They discovered that the movements of grey reef sharks were anything but random... On the contrary, they had limited movement patterns and each individual occupied very specific areas of the reef, where they persistently associated with certain individuals. In fact, there were five distinct communities occupying different areas around the island (Papastamatiou et al, 2020).

Locations of 5 distinct shark communities (different colours) (Papastamatiou et al, 2020)

This elaborate community structure was consistent across several years of sampling - showing the social structure of grey reef sharks is very stable throughout both space and time. This kind of complexity is as developed as that found in other social animals, like mammals, which are commonly considered more 'intelligent' or 'emotionally advanced' (Papastamatiou et al, 2020).

"Sharks can display a degree of social complexity traditionally associated more with mammals and birds"

- Papastamatiou et al, 2020

Group of grey reef sharks in the Palmyra Atoll (Image source: www.phys.org)

The researchers also took the opportunity to determine whether the sharks socialised at particular times of the day. They found that the receivers pinged more commonly during the day, meaning that more sharks were present in the core study areas during daylight. The shark gangs grew to as many as 16 individuals around 3 in the afternoon. This is known as a "fission-fusion dynamic", meaning that the group size changes in a consistent and predictable manner (Papastamatiou et al, 2020).

Fission = 'the process of division'

Fusion = 'when two or more things are combined'

Dynamic = 'the forces that control relationships and how these change'

(Cambridge Dictionary)

So why do these sharks behave in this way? What is the benefit to them? Surely spending a lot of time around other sharks just increases the competition for food, right?

Apparently not! The scientists found that the sharks which looked for food alone (known as "lone foragers"), had significantly lower hunting success compared to those which spent time in groups (known as "social foragers"). They hypothesised that the sharks were using "social information sharing" to find good patches of food - basically they were getting recommendations from friends about where to go eat!

This research shows us that grey reef sharks are not found in groups by chance, but they have a complex and stable social structure within their communities. In fact, they are socially advanced and capable of detailed communication. This social information sharing will allow individuals to acquire more food, have better health and, as a result, be able to have a higher number of offspring throughout the course of their lives. This is known as having a higher "fitness" (Papastamatiou et al, 2020).

This is not dissimilar to how human beings have evolved; where forming groups made survival easier, until eventually a complicated social construct became crucial to daily life. So, maybe we have more in common with sharks than we have previously thought...

References

Papastamatiou YP, Bodey TW, Caselle JE, Bradley D, Freeman R, Friedlander AM & Jacoby DMP (2020). Multiyear social stability and social information use in reef sharks with diel fission–fusion dynamics. Proceedings of the Royal Society B: Biological Sciences, 287:1932. Access online.

By Sophie A. Maycock for SharkSpeak.