In this month’s cover image story, we delve into the lives of juvenile lemon sharks in Bimini Research Station with Félicie Dhellemmes and learn about the complexity of animal personality and behaviour
The concepts of animal personality and individual foraging specialisation describe two seemingly related phenomenon: The fact that individuals within populations consistently differ in their behaviour (for the former) and in their foraging habits (for the later). But somehow, we do not know how or if these concepts go hand in hand as they are usually studied in isolation from one another. One possible reason for this divide is that the methodologies required to study both concepts are different and can seem incompatible: Animal personality relies mostly on repeated captive tests in controlled environments, while foraging specialisations are usually studied on free-ranging animals using molecular methods.
So, in order to understand if animal personality and foraging specialisations are indeed linked, one could try finding a study system where animals live freely (accumulating precious, quantifiable molecules in their tissues from the resources they consume), but can also be captured and kept in captivity for standardised personality tests.
Bimini’s juvenile lemon shark population
In Bimini, populations of juvenile lemon sharks have been the scientific obsessions of the late Dr. Samuel Gruber since 1990, the year he established the Bimini Biological Field Station.
Lemon sharks are born in April and May of each year and inhabit shallow, mangrove fringed so-called “nursery areas” for the first few years of their lives. Two of these nurseries in particular have been under intensive scrutiny since 1993: North Sound and neighbouring Sharkland.
Every year, the Bimini Biological Field Station team has been running a 12-night gillnet fishing effort to monitor the shark populations. Each shark captured receives a passive integrated transmitter (PIT) which carries its unique identity and is then kept in a holding enclosure to avoid re-captures. Every year, we estimate that 99% of the population can be captured using these methods. And so, over the years, a plethora of information on Bimini’s lemon sharks has been published, making it the best researched shark population worldwide. We know the age at which they disperse, their home-ranges, their growth rates, their family tree, the life-history trade-offs that shape these populations, and more.
In 2017, Dr. Nigel Hussey, added an important stone to the edifice, using stable isotope analysis and showing that Bimini’s juvenile lemon sharks consistently specialize in dangerous offshore or safe inshore foraging. From 2012 to 2015 Dr. Jean-Sebastien Finger developed captive personality tests for these sharks as part of his PhD, providing us with the last ingredient for this paper’s “recipe”.
Personality in the ecological context
Standing on the shoulders of all researchers that studied the juvenile lemon sharks of Bimini and most notably of Drs. Finger and Hussey, I have dedicated my PhD to understanding how (and if) personality studied in captivity translates to wild behaviour and is of ecological importance. For instance, I investigated the stability of a behavioural syndrome between exploration personality and sociability across ecological conditions and the link between exploration personality and a life-history trade-off.
In this study, the goal was to use the existing captive personality test in parallel to stable isotope analysis, to see whether and when the foraging specialisations observed by Dr. Hussey (high risk habitat vs. low risk habitat) was linked to consistent individual differences in explorative behaviour.
Upon capture, a little piece of dorsal fin was sampled from every shark, then we ran our personality assay. The novel-open field test captures the sharks’ differences in their willingness to explore a new environment. Low scores are representative of a shark’s low tendency to explore. We expected sharks that explore less to have a low-risk foraging profile, but we also expected key ecological conditions to influence the link between foraging and captive personality.
This is because during personality tests, sharks are fed to satiation, they are in a safe spot (and they have been acclimated to their captive conditions), so we expect a behaviour that is representative of non-stressful conditions. But in the wild, the sharks have to deal with pressures that may change their behaviour: In particular, when there is a lot of intraspecific competition, sharks may be more willing to take risks to acquire precious resources and when there are a lot of predators, sharks may be less willing to do so regardless of their captive personality scores. In other words, the captive personality score may only be representative of a shark’s behaviour in a perfect world.
What did the sharks tell us about foraging specialisation and personality?
We found the link between personality and foraging specialisations to depend on the number of predators in the system (we calculated relative predator abundance using acoustic telemetry), but not on intraspecific competition. When there weren’t a lot of predators, explorative sharks had a risk-prone foraging profile (seagrass specialisation) while less explorative sharks foraged in the safety of the mangroves. But this relationship reversed under high predator abundance and less explorative sharks were surprisingly found to forage in dangerous offshore habitats… This may be due to increases in competition in the safe habitats, or to the less explorative sharks being more naive about the presence of predators… We can not say for sure! But we now know that foraging specialisation is linked to captive personality only sometimes and that future research needs to account for ecological conditions.
Who are we?
I am a behavioural ecologist interested in the cause and consequences of animal personality. I recently graduated from my Ph.D on lemon sharks and got my first post-doc on northern pike behaviour. I am involved in many exciting projects, all on large aquatic predators: Sharks, pike and billfishes! I worked on this project with Matt, Tristan and Nigel, who all collected data for a part of their graduate studies in Bimini. They all provided a piece of the “shark puzzle” and all have spent long times in the remote field station “Sharklab” along their quests. Our last team member is Jens, who is my academic supervisor and also supervised Tristan during his Ph.D., he is a welcome visitor on the island and knows the system very well.