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Behavior

Birds of a Feather: Social Dynamics of Juvenile Lemon Sharks

Wilson, Alexander D.m., et al. “Integrating Network Analysis, Sensor Tags, and Observation to Understand Shark Ecology and Behavior.” Behavioral Ecology, vol. 26, no. 6, 25 July 2015, pp. 1577–1586., doi:10.1093/beheco/arv115.

Illustration of human socialization. Source: Pixabay

Human beings are very social creatures, we form cliques and groups almost immediately. We form long-lasting friendships, have sleepovers, guys trips, girls’ weekends and nights on the town. But what if I told you humans aren’t the only animals that socialize. There are even some species of sharks that form social groups. Dr. Alexander Wilson and his colleagues did a study on the social dynamics of juvenile lemon sharks and found that, like people, they interact with other individuals based on personality traits and, also like humans, their level of socialization varied from individual to individual as a result of differences in personality.

Example of a social network map. Source: Wikipedia

When researchers study social dynamics, they analyze how individuals in a population interact and behave together. It is often hard to observe these group dynamics in animals, particularly in marine animals, because they are hard to see, and the presence of researchers can often disrupt these dynamics. That’s why these researchers used a new piece of technology, biotags. Biotags allow researchers to keep track of the movements of hard to observe, fast moving animals. These tags record data of the individuals such as heart rate, body temperature, speed and other information.

Example of a type biotag called a PSAT. Source: Desert Star Systems

To analyze the data collected from these biotags, the researchers decided to use social network analysis, which allowed them to study complex relationships between individuals as they move through an area and interact with each other. Using social network analysis in combination with biotags to study group dynamics is a new method in the field of animal behavior that is proving very effective.

 

The team chose lemon sharks (Negaprion brevirostris) because they are known to be social as juveniles and have been shown to form social groups based on body size. They were interested in seeing how personality affects interactions between individuals. They also sought to answer several questions including: 1) Are sharks staying in one place or in one group? 2) Can these social dynamics be accurately modeled mathematically, 3) which individuals are interacting the most and is there a pattern in who interacts with whom? 4) Do individuals in a group behave similarly to each other?

Photograph of a lemon shark. Source: Wikipedia courtesy of Terry Goss

Ten lemon sharks were collected from Eleuthra in the Bahamas, acclimated and then released for observation. The personality traits of interest to the researchers included: leadership, or the tendency to be at the front of the group, refuging, or the tendency to hide in the mangroves, and sociability, frequency an individual occurs within one body length of another individual. They observed the sharks one at a time for 100 seconds each, twice a day for 8 days. Several measurements were taken, the amount of time two individuals spent together, proportion of time spent swimming, proportion of time spent swimming fast, number of changes in swimming (moving from swimming to staying still and vice versa). The researchers compared the similarity in traits of individuals interacting with each other and found that they formed groups of similar time spent swimming and time spent swimming fast, and that they often form same-sex groups. Groups were also shown to have differing levels of leadership, which makes sense if you think of the phrase “too many cooks in the kitchen.” It is understandable that it would be difficult to have a group where everyone wanted to be in the front. Another interesting finding was the stark contrast in movements between social males and social females. Social males were shown to spend more time swimming both night and day, but social females were shown to spend less time swimming night and day. There was, however, no significant difference between the sexes among less-social individuals. The team found that, as a whole, the sharks were more likely to swim fast during the night than during the day.

This study is unique in that it shows how social network analysis can be used in combination with biotagging to understand the social behaviors of marine species. It also shows another dimension of attributes that lead to the formation of specific groups among lemon sharks, when previously all that was known about the groups was that they are based on body size. We often don’t think of animals, particularly sharks, as having personalities, so it is interesting to see there are measurable differences between individuals’ level of socialization, desire to lead and boldness to venture out of the safety of the mangroves. In the end, the adage appears to stand, birds (or lemon sharks) of a feather, flock together.

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