Size–based dominance and resource use in an inland American alligator population

Background/Question/Methods

Dominance rank may influence resource selection and space use, which in turn impacts an individual’s fitness. In crocodilians, studies of captive populations suggest social behavior is characterized by size–dependent absolute hierarchies (independent of time and location), where large, aggressive males control access to resources. Our goal was to test this suggestion in the field and determine whether resource selection of an inland American alligator (Alligator mississippiensis) population was influenced by size-based dominance. We collected various body measurements and 1502 locations on 20 radio-tagged adult male alligators (total length:  mean = 2.79 m, range = 1.80 - 3.71 m) in an inland riverine system of central Mississippi from spring 2012 to fall 2013. Satellite imagery and water depth grids were used to create ecologically relevant habitat classifications to quantify alligator resource selection at multiple spatial scales. Using multivariate k–select analysis, we tested for differential resource selectivity as a function of conspecific body size. If a dominance hierarchy was present in our sample, we would expect larger, dominant alligators to control access to high quality resources and exhibit different resource selectivity than smaller, subordinate animals.

Results/Conclusions

The first principal component (PC1) of the k–select analysis explained 45.41% of variation in the original data and PC1 and PC2 combined described 66.78% of the variation. A randomization test indicated the first eigenvalue was larger than expected under the random use hypothesis (λ₁ = 0.412, P < 0.01), suggesting the k–select analysis was pertinent. We found no support for differential selectivity of individuals by body size as determined by the direction and magnitude of the marginality vectors (proportional to habitat selectivity). Therefore, for our sample, body size–based despotism does not seem to be a factor in constraining or promoting resource use. Our results may be understood in the context that inland riverine populations may be less crowded and more spatially heterogeneous than both coastal estuarine systems and captive situations. Further, our results may suggest that the formation of a dominance hierarchy in wild alligator populations may be density-dependent. Finally, our research emphasizes the need to address fundamental ecological and behavioral aspects throughout a species’ entire range.