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ISRN Ecology  2013 

Factors Influencing Aggression Levels in Root Vole Populations under the Effect of Food Supply and Predation

DOI: 10.1155/2013/948915

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Which factor determines animal aggressivity? Wynn-Edwards proposed the hypothesis that aggressive level increases with population density; Adams and Mesterton-Gibbons proposed the hypothesis that body weight is an indicator of animal aggressivity; however, Smith and Price hypothesized that aggression level varied with external conditions; that is, the population lived in the most unfavorable environment demonstrated the highest average aggression level, and the population that lived under the most favorable external conditions demonstrated the lowest average aggression level. In this paper, we tested these three hypotheses by manipulating enclosed root vole (Microtus oeconomus) populations under different food and predation treatments and observed their aggressive behavior. Aggressive behavior was measured by matching mice in a neutral arena. The experimental results supported Smith and Price’s hypothesis and Adams and Mesterton-Gibbons’s hypothesis; however, they did not support Wynn-Edwards’ hypothesis. Moreover, we found that reproductively active individuals were more aggressive. We concluded that the growth of population density did not cause or otherwise bring about increased aggressive behavior of root voles, but the external factors (predation and food supply) and physical factors (body weight and reproductive condition) were significantly correlated with aggression levels in a root vole population. 1. Introduction Wynn-Edwards’ hypothesis proposed that animals adjust their population density to available resources through social behavior and that aggressive behavior increases with population density [1]. Some subsequent field experiments supported this hypothesis [2–8]. However, Vale et al. demonstrated another aspect of the relationship between agonistic behavior and population density [9]. They showed that with different genetic strains of mice at equal densities, one strain was aggressive and the other was not. Lidicker examined patterns of wounding in population of California vole (Microtus californicus, Peale, 1848) and concluded that season was more important than density in determining levels of aggression [10]. Aggression levels of fluctuating arvicoline populations have been assessed by observation of agonistic behavior during dyadic encounters in neutral arenas for five species: meadow vole Microtus pennsylvanicus (Ord, 1815) [5, 11–13], prairie vole Microtus ochrogaster (Wagner, 1842) [12, 13], beach vole Microtus breweri (Baird, 1858) [14], red-backed vole Clethrionomys gapperi (Vigors, 1830) [15], and Townsend’s vole Microtus


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