Date of Award

5-15-2015

Document Type

Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

First Advisor

Daniel C. Abel

Second Advisor

Richard F. Viso

Third Advisor

Tristan Guttridge

Additional Advisors

Sharon Gilman

Abstract

In order to understand what drives group formations and to predict spatial occurrences, investigations on individual preferences within the groups have proved to be very useful. Groups of sharks have been observed for centuries; however, there is a dearth of quantitative analyses on the mechanisms that drive their formation. In this study, we use controlled, semi-captive behavioural experiments to assess the potential role of familiarity in group formation and on social behaviour in a large marine vertebrate. Juvenile lemon sharks, Negaprion brevirostris, (n =23) in Bimini, Bahamas were captured, measured and tagged with external colour codes for individual identification and housed in pens that were exposed to ambient conditions. Sharks were separated into four holding pens according to their size class and given 14 days to familiarize themselves with their cohort. Following familiarization, pairs of sharks were taken from two holding pens and introduced in a social network pen for a behavioural trial (N=27). An overhead video system recorded behaviours for one hour and tracking software transformed the movement patterns into a coordinate system. Multiple algorithms were used to analyze these coordinates and produced a matrix of interactions between familiar and unfamiliar individuals. Social network analysis indicated that juvenile lemon sharks express a significant preference for familiar individuals for the entire trial (P-value = 0.000501) and this preference declines over the one-hour time period. Previous research suggests that the preference for familiars likely occurs to avoid agonistic interactions or to further the advantages of group living. We suggest that preference for familiars is not facilitated by either the ‘Dear Enemy’ effect or the desire to bolster the benefits of group living. Rather, we believe the two benefits are closely linked as the ‘Dear Enemy’ effect results in spatial isolation between unfamiliar individuals, which ultimately results in strengthened bonds between familiar individuals that bolster the effects of group living. The decline in avoidance of unfamiliars is most likely caused by forced interactions in our semi-captive trials, and in the wild, this would likely not occur. This research advances our understanding of the mechanisms driving group formation in lemon sharks, a model species for large marine predators.

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