Inferring social structure and its drivers from refuge use in the desert tortoise, a relatively solitary species
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For several species, refuges (such as burrows, dens, roosts, nests) are an essential resource for protection from predators and extreme environmental conditions. Refuges also serve as focal sites for social interactions, including mating, courtship, and aggression. Knowledge of refuge use patterns can therefore provide information about social structure, mating, and foraging success, as well as the robustness and health of wildlife populations, especially for species considered to be relatively solitary. In this study, we construct networks of burrow use to infer social associations in a threatened wildlife species typically considered solitary—the desert tortoise. We show that tortoise social networks are significantly different than null networks of random associations, and have moderate spatial constraints. We next use statistical models to identify major mechanisms behind individual-level variation in tortoise burrow use, popularity of burrows in desert tortoise habitat, and test for stressor-driven changes in refuge use patterns. We show that seasonal variation has a strong impact on tortoise burrow switching behavior. On the other hand, burrow age and topographical condition influence the number of tortoises visiting a burrow in desert tortoise habitat. Of three major population stressors affecting this species (translocation, drought, disease), translocation alters tortoise burrow switching behavior, with translocated animals visiting fewer unique burrows than residents. In a species that is not social, our study highlights the importance of leveraging refuge use behavior to study the presence of and mechanisms behind non-random social structure and individual-level variation. Our analysis of the impact of stressors on refuge-based social structure further emphasizes the potential of this method to detect environmental or anthropogenic disturbances.
Adaptive and social behavior that affects fitness is now being increasingly incorporated in the conservation and management of wildlife species. However, direct observations of social interactions in species considered to be solitary are difficult, and therefore integration of behavior in conservation and management decisions in such species has been infrequent. For such species, we propose quantifying refuge use behavior as it can provide insights towards their (hidden) social structure, establish relevant contact patterns of infectious disease spread, and provide early warning signals of population stressors. Our study highlights this approach in a long-lived and threatened species, the desert tortoise. We provide evidence toward the presence of and identify mechanisms behind the social structure in desert tortoises formed by their burrow use preferences. We also show how individuals burrow use behavior responds to the presence of population stressors.
KeywordsBehavioral stress response Bipartite networks Gopherus agassizii Generalized linear mixed models Modularity Mycoplasma agassizii
We thank Phil Medica for feedback on a previous version of this manuscript. We thank Ian T. Carroll and anonymous reviewers for helpful comments to improve the quality of the paper. We thank Clarence Everly, Department of Defense, Ft. Irwin National Training Center for the support in acquiring the data analyzed in this paper. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S.Government.
Compliance with ethical standards
This work was funded by the National Science Foundation Ecology of Infections Diseases grant 1216054 Invasion and Infection: Translocation and Transmission: An Experimental Study with Mycoplasma in Desert Tortoises. This work was also partially funded by a grant from the Department of Defense, Ft. Irwin National Training Center, and by the Ecosystems Mission Area of the U.S. Geological Survey.
Conflict of interests
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. This article does not contain any studies with human participants performed by any of the authors.
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