Intra-sexual variability in feeding behaviour of a mountain ungulate: size matters
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Many studies comparing the behaviour of individuals of different genders or species showed that animal body mass and forage quality/quantity are key elements of the foraging ecology of herbivores. Since body mass could also influence the animal’s sensitivity to predation risk, its vigilance behaviour should consequently be affected. Alpine ibex (Capra ibex) is characterised by a strong dimorphism among males of different ages, thus representing an ideal case study for testing the Jarman-Bell principle, avoiding possibly misleading effects resulting from the comparison between different species or genders. We analysed the fine-scale foraging behaviour of male ibex in order to assess the effect of body mass and the effects of vegetation quality/quantity on both foraging and vigilance behaviour. Our results showed that smaller males were more selective than larger ones, on account of their lower capability of digesting plant. Smaller males scanned the environment more frequently than larger ones. Male ibex grazed more selectively in sites with high quality forage and their bite rate increased as forage biomass decreased. Vigilance frequency increased with increasing forage biomass as, under these circumstances, ibex are able to prolong anti-predator vigilance while chewing bites that have already been cropped. Our findings highlight the effects of body mass per se on both foraging and anti-predator behaviours in herbivores, thus supporting the Jarman-Bell principle. Foraging can arguably be considered a very flexible behaviour with high evolutionary relevance as it enables herbivores to optimally adjust their total energy intake under varying conditions of food resources.
KeywordsAlpine ibex Body mass Diet selection Forage quality Jarman-Bell principle Vigilance behaviour
We wish to thank the Gran Paradiso National Park for their economic and logistical support. We are grateful to the rangers of the GPNP for the capture of animals and for their contribution in data collection. A special thank is due to B. Bassano and A. von Hardenberg for their scientific input during all steps of this research. We thank C. Fracastoro, V. Gerbo, A. Melotto and E. Piano for their invaluable help in the fieldwork. Finally, we thank the Regione Autonoma della Valle d’Aosta for meteorological data. FB post-doc grant and this project were also supported by the Italian Ministry of Education, University and Research (PRIN 2010–2011, 20108 TZKHC, J81J12000790001). SG had the support of the fund “P.O.R. F.S.E. 2007–2013—Obiettivo competitività regionale e occupazione. Asse IV Capitale umano—Linea di attività 1.3.1”. The English version was edited by G. Falceri.
Conflict of interest
The authors declare that they have no conflict of interest.
This study complies with all national and regional laws dealing with ethics and animal welfare. The capture and handling protocol was approved by the Italian Ministry of Environment (protoc. n. 25114/04).
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