Lemon sharks hang out with friends
16 September 2009, by Tamera Jones
Baby lemon sharks like spending time with lemon sharks their own size, rather than bigger or smaller sharks. They also prefer hanging out with sharks of their own species.
Lemon sharks (Negaprion brevirostris).
Although scientists know a lot about the social airs and graces of other enigmatic marine creatures like dolphins and whales, relatively little is known about the sociability of sharks.
'Anecdotally, we've known that lemon sharks group together, but it's difficult to get hold of enough animals to actually study this behaviour,' says author of the study, Tristan Guttridge from the University of Leeds.
During his PhD, Guttridge joined shark expert Dr Samuel Gruber at the Bimini Biological Field Station in the Bahamas to study the social behaviour and organisation of lemon sharks, Negaprion brevirostris. Gruber is one of the world's leading shark experts, with over 40 years experience, which Guttridge was keen to tap into.
'It's probable that sharks live in groups like this to minimise the potential of being attacked by predators.'
Tristan Guttridge, University of Leeds
Lemon sharks 'are a model shark species to study', because a lot is known about their behavioural ecology, physiology and life history, they're abundant in Bimini and are relatively unphased when in captivity.
Both lemon sharks and nurse sharks live in the mangrove habitat of Bimini during their early lives, which means the two species are very likely to interact. Guttridge wanted to find out how sociable lemon sharks are with other lemon sharks and with nurse sharks.
Guttridge describes in the journal Animal Behaviour how he built holding pens in shallow sand-bottomed water, which he used to test the sharks' social attraction to each other.
He divided the pens into three compartments: one inner and two outer, which he separated using mesh that allowed the sharks to communicate with each other using sight, smell and taste.
After introducing either one, four or no 'stimulus' lemon sharks into one of the two outer compartments at random, he introduced a test shark to the central compartment. He then tested 42 juvenile lemon sharks' tendencies to spend time with the stimulus sharks.
Mutual attraction
To test how nurse sharks respond to lemon sharks, Guttridge placed two nurse sharks in the outer compartment of the holding pen and put two lemon sharks in the inner compartment and recorded how long they interacted with each other.
Guttridge found that, on average, juvenile lemon sharks spent more time near the compartment that held other lemon sharks, suggesting they were attracted to them for some reason. But lemon sharks didn't spend any length of time with nurse sharks.
While lemon sharks aged two to three years old preferred to group with other lemon sharks the same size and approximate age, younger sharks aged up to a year old did not show this preference.
'It's clear this social preference may be important for the survival and development of juvenile sharks and it's probable that sharks live in groups like this to minimise the potential of being attacked by predators,' says Guttridge.
'Lemon sharks actively associate with each other, so they could be recognising each other and potentially learning from each other,' he adds.
Many other animal researchers have shown a link between complex social behaviour and brain size. Previous research has shown that group-living sharks and schooling species such as hammerheads have large brains compared with other sharks.
'These sharks are a lot more sociable than we anticipated. The next thing we'd like to do is to figure out whether they can learn from each other and how they might communicate with each other and look at their brain size,' adds Guttridge.
Social preferences of juvenile lemon sharks, Negaprion brevirostris
T L Guttridge et al
Animal Behaviour, 78 (2009) 543-548
doi:10.1016/j.anbehav.2009.06.009
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