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Ecological drivers of group size variation in sika deer: habitat structure, population density, or both?

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Abstract

Habitat structure and population density have long been considered key factors in determining intraspecific variation in the group size of social ungulates. However, the effect of interaction between habitat structure and population density on group size is poorly understood, and whether the underlying causes of these relationships are biological adaptation, emergent properties, or both is still debated. Therefore, over 10 years, we examined group size of the sika deer (Cervus nippon) at different population densities and habitat types (open habitats vs closed forests). We found that deer group size was larger in open habitats than closed forests and increased with population density, and population density and habitat type had a synergistic effect on group size. There was greater density effect in open habitats, while the density effect was very weak in closed forests. Our findings in open habitat support emergent properties, because both population density and visibility enhanced the frequency of group fusion. Alternatively, our findings in closed forests support biological adaptation; the scarce food supply in closed forests may increase the cost of conspecific feeding competition, which consequently limits group size even under high densities. We suggest that both emergent properties and biological adaptation determine group size variability of sika deer, and emergent properties may be the mechanism that contributes to group size increase, whereas biological adaptation may be the mechanism that contributes to group size restriction.

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Acknowledgements

Takuma Suzuki (Animal Life Solutions Co. Ltd.), Keita Nakamura (Idea Co. Ltd.), Konoka Aiba (Japan Wildlife Research Center), Mitsuko Hiruma (Tokyo University of Agriculture and Technology), and Miki Toda (Hayakawa City, Yamanashi Prefecture) helped with fieldwork. Taisuke Yasuda (Mount Fuji Research Institute Yamanashi Prefecture Government) provided advice on the statistical analysis, and Eri Katsumata (Mount Fuji Research Institute Yamanashi Prefecture Government) helped organize the data. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. We thank Mallory Eckstut, PhD, from Edanz Group (https://en-author-services.edanzgroup.com/) for editing a draft of this manuscript.

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Correspondence to Hayato Takada.

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All investigation was performed in accordance with the Ethics Committee for Animal Experiments, Mount Fuji Research Institute, Yamanashi Prefecture Government (ECAE-01-2013-2019).

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Appendix

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Table 3 Mean relative population density (ind./10 km), proportion of open habitats (%), and mean group size for each forest road and year at Mount Fuji

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Takada, H., Washida, A. Ecological drivers of group size variation in sika deer: habitat structure, population density, or both?. Mamm Biol 100, 445–452 (2020). https://doi.org/10.1007/s42991-020-00041-x

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  • DOI: https://doi.org/10.1007/s42991-020-00041-x

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