Abstract
In large mammalian herbivores, an increase in herd size not only reduces predation risk but also energy intake. As a consequence, the size of the groups made up by herbivores is often assumed to be the outcome of a trade-off depending on local predation risk and food availability. We studied Alpine chamois (Rupicapra rupicapra) group size in a 3660 ha wide area (central-Eastern Alps, Italy) where mouflon (Ovis gmelinii) were also present. We walked seven hiking trails (spanning a total of 33.9 km) in a high-elevation environment every 15 days from May to October in 2007–2009, and located all the groups of free-ranging ungulates within our study site. We analyzed the size variation in female groups with kids of Alpine chamois in relation to (i) forage availability (as estimated using the normalized difference vegetation index); (ii) distance from safe areas; (iii) presence of potential competitor species (i.e., mouflon); and (iv) environmental variables (elevation, slope exposure). We found that forage availability was one of the most important drivers of group size patterns. Larger groups were found in meadows with higher productivity, whereas smaller groups were located in lower quality meadows in sheltered areas, indicating how the female group size may be the outcome of both resource availability and predation risk avoidance. Interestingly, proximity of a mouflon group was able to modify the size of Alpine chamois female groups, especially if the nearest mouflon group was concurrently close and inclusive of a higher percentage of rams.
Acknowledgments
We are grateful to the Trento Province and the Adamello Brenta Nature Park. We are indebted to the “Associazione Cacciatori Trentini” (Trento provincial hunting association) and its game keepers who provided logistic support. Finally, we thank all the students and the field assistants for help in the field. Constructive comments from Stéphane Aulagnier, Jean-François Gerard, and an anonymous referee greatly improved this manuscript. G. Falk revised the English version of this article. The study complies with all relevant national, regional and provincial Italian laws and with the ethical standards. MA and RC were supported by the Italian Ministry of Education, University and Research (PRIN 2010–2011, 20108 TZKHC).
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