As I was learning more about these young sharks, two behaviours in particular caught my interest. Firstly, lemon sharks are highly social animals and their social behaviour is more complex than we thought, as Shark Lab director Dr Tristan Guttridge demonstrated nearly a decade ago. Therefore I wanted to verify whether some lemon sharks are consistently more, or less, social than others.
The second behaviour that caught my attention comes from one of the numerous studies done by Doc Gruber, the founder of Shark Lab, and graduate student John Morrissey, way back in the 1980s. That particular study investigated site attachment in young lemon sharks and showed that they have very limited home ranges and keep to their mangrove nurseries for about three years. What was particularly interesting about Gruber and Morrissey’s findings, however, was that on occasion some little sharks ventured out of their home into dangerous waters. The researchers called these movements ‘excursions’. These jaunts made me wonder whether the sharks were experiencing a kind of wanderlust or desire to explore an unknown environment. ‘What if some of these little guys were more of an explorer than others?’ I mused. And if they were, I wondered whether consistently different exploratory behaviour could be demonstrated over long time periods in these young, social lemon sharks.
Answers to such questions may be quite important for these sharks because their survival depends on everyday decisions. For instance, going out into deeper water, away from the safety of the mangrove nursery, would be quite risky for a little shark. At the same time, however, it might provide more food resources. Similarly, being highly social might be safer, but a social shark may be required to share food or even lose out on food due to competition. Therefore, these young lemon sharks need to find a balance between their resource needs and their safety. I considered that finding personality would indicate that not every individual has the same strategy to cope with the risk–benefit trade-offs, and thus gives the species greater flexibility in surviving the vagaries of a changeable environment.
To assess these ideas we first had to find a method to test personality in juvenile lemon sharks with respect to both exploratory and social behaviours. After watching groups of captive lemon sharks we decided to build different-shaped shark pens in the shallow North Sound nursery. The first one was circular and big enough (10-metre diameter) to support six individual lemon sharks that could swim on their own, away from the others if they wanted to. Next to this pen was a rectangular pen designed to be large enough (12 x 6 metres) that a small lemon shark could explore it. After tweaking the set-up over months of experimentation, we were finally able to start testing our sharks.
It went like this: in the first ‘social’ pen, six sharks of similar size were able to swim together either with or away from their pen partners. By recording them on videos for 20-minute sessions, we were able to obtain a score for each of the six sharks that represented whether it was social (swam with the others) or anti-social (stayed aloof and away from the others). After that, the same sharks were ushered into the ‘exploration’ pen, a place they had never seen before. In this pen we could follow their movement patterns, such as how much exploration that each individual showed, again with video analysis. All the sharks were then released into their natural habitat and were recaptured regularly, after six months to a year, and retested.
Until we perform a rigorous statistical analysis, our results are still preliminary, but we are quite confident that the study will clearly demonstrate strong personality traits among individual lemon sharks, even after several trials months to years apart. We suspect that some sharks are far more social than others, and some are explorers while others stick around home. My general conclusion from this first step is that the behaviour of sharks, like that of humans, cannot be generalised into an average because of individual personalities.
However, my three-year doctoral research is only a first step towards understanding personality in sharks. For instance, we are investigating whether these personalities can be observed directly in the wild and whether differences affect growth rate, survival and the probability of capture by humans. These are important questions to answer if, for example, population and conservation management programmes are to be improved. As time passes we will be able to gather enough observations to resolve such questions. In this regard, as I complete my doctoral research this study will be taken up by a new doctoral student, Felice Dhellemmes, who for the next three years will extend and confirm my findings.
Finally, research that would have been difficult or nearly impossible with large animals such as sharks has benefited greatly from the massive experience in field work and knowledge the Shark Lab has gleaned about lemon sharks over the past quarter century. I gratefully acknowledge the help of all the hard-working staff and volunteers, as well as the generous support from several groups that made this project possible.
