There are many species of sharks that live completely solitary lives. At the other end of the spectrum, there are also many species that form schools or live in large groups. Some sharks regularly hunt in packs, some only aggregate to mate, where others only live in groups when they are juveniles. But are sharks social? Do they form relationships with specific individuals? Basically... do sharks have friends?
The Legacy of Doc
For many years, researchers have been studying the social behaviour of sharks. A laboratorty that specialises in this work is the Shark Lab at the Bimini Biological Field Station in the Bahamas. This lab is the legacy of the iconic and irreplacable Sam "Doc" Gruber, who studied sharks for decades. At bimini they are able to study lemon sharks (Negaprion brevirostris) both in the wild, in nearby mangrove habitats, and also in semi wild conditions in their beach pens (I know... it is THE COOLEST place in the world!).
Lemon sharks are ideal for studies on sociality because they live in groups, in relatively shallow coastal conditions, where they are quite easy to observe. They are also quite mellow sharks and not overtly aggressive, so scientists can equip them with tags or interact with them without threat (Finger et al 2018).
Lemon Sharks Stay in Size-Assorted Groups
In one Bimini study, the researchers investigated whether juvenile lemon sharks preferred to spend time in a group or remain solitary, by monitoring their behaviour in beach pens (Guttridge et al, 2009).
The sharks were subjected to "binary choice experiments"; they were placed into a pen with multiple divisions and the researchers monitored whether they spent more time in compartments alongside other sharks, or whether they hung out more in an empty area. They also analysed whether the size of the other sharks affected who an individual shark chose to spend time with (Guttridge et al, 2009).
This study found that juvenile lemon sharks definitely favour spending time in groups and they also assort themselves by size. Very young sharks (less than 1 year of age) were not picky about the size of their pals, but sharks over 2 years of age, preferred to form groups with individuals that were of a similar size to them (Guttridge et al, 2009).
Lemon Sharks Hang-Out with Particular Friends
Subsequently, other studies have been performed at the Bimini lab, to understand whether lemon sharks choose to socialise with specific sharks. Do they just like to group for safety, or are they actually more attracted to certain individuals?
First, researchers used acoustic tags to identify which individual sharks often associated together. This method involves attaching a tag to the dorsal fin of the shark. The tag emits a signal which can be detected by tags on other sharks within 4 metres. This allows scientists to assess which individuals spend time near to each other (Guttidge et al, 2010).
The researchers found that wild juvenile lemon sharks have particular individuals which they are repeatedly stay near to. One individual what they studied 128 times, was only ever observed near to nine other identified sharks, never with anyone else! This suggests that lemon sharks have certain individuals they prefer to form a group with (Guttidge et al, 2010).
True Friends Stick Together
To advance from this, in another study the researchers observed wild juvenile lemon sharks in the mangrove area. Over short-term (4 - 18 days) and long-term (4 months) timeframes they recorded which sharks were seen in which groups and how many social interactions each individual performed.
They found that lemon sharks do have certain individuals which they prefer to group with and their social groups remain consistent over the long-term (Guttidge et al, 2010).
We are only just beginning to break the surface to understand shark behaviour... there is so much more that we need to know and so many more burning questions we would like to answer. But what the Bimini Shark Lab has shown us already, is that lemon sharks definitely have very complex social lives.
If you would like to stay up-to-date with their research you can follow @BiminiSharkLab on Twitter or Bimini Biological Field Station - Sharklab on facebook or subscribe to the Bimini Shark Lab YouTube channel.
To learn more about shark social relationships, check out The Social Network.
References
Finger JS, Guttridge TL, Wilson ADM, Gruber SH & Krause J. (2018). Are some sharks more social than others? Short- and long-term consistencies in the social behaviour of juvenile lemon sharks. Behavioural Ecology and Sociobiology, 72:17. Access online.
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.
Guttridge TL, Gruber SH, Krause J & Sims DW (2010). Novel acoustic technology for studying free-ranging shark social behaviour by recording individuals’ interactions. PLoS One, 5:2, e9324. Access online.
By Sophie A. Maycock for SharkSpeak
