A Yellow Social Butterfly
Updated: Dec 3, 2021
There are many species of sharks which live solitary lives. On the other hand, there are also many species which form schools or live in groups. Some sharks hunt in packs, some only school to mate. Others aggregate for foraging purposes and 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?
Researchers at the Bimini Biological Field Station's Shark Lab in the Bahamas have been working for years to better understand the group structure of lemon sharks (Negaprion brevirostris). This lab is the legacy of Sam "Doc" Gruber, who studied sharks for decades. They are able to study lemon sharks 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!).
In one 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. 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 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).
Subsequently, other studies have been performed at this lab, to understand whether the sharks were choosing to socialise with specific sharks. Do they just like to group for safety, or are they actually more attracted to certain sharks?
First, researchers used acoustic tags to identify which individual sharks often associated together. This method involved attaching a tag to the dorsal fin of the shark, which emitted a signal that was detected by tags on other sharks within 4 m. The researchers found that wild juvenile lemon sharks have particular individuals which they are repeatedly very close to- for example, 1 individual shark they had tagged was in close proximity with just 9 other sharks on 128 different occasions. This suggests that they have certain individuals they prefer to form a group with (Guttidge et al, 2010).
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. Whatsmore, they also noted that how much each shark socialised varied between individuals; some sharks were more social than others. This suggests that these sharks have individual differences in their social behaviour, which we can call "personality" (Finger et al, 2018).
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.
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, DOI: 10.1007/s00265-017-2431-0.
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.