ORIGINAL PAPER Intra-sexual variability in feeding behaviour of a mountain ungulate: size matters Francesca Brivio & Stefano Grignolio & Alice Brambilla & Marco Apollonio Received: 18 February 2014 /Revised: 2 July 2014 /Accepted: 4 July 2014 /Published online: 5 August 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Many studies comparing the behaviour of individ- uals 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 vigi- lance 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 mis- leading effects resulting from the comparison between differ- ent 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 ac- count 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 for- age 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. Keywords Alpine ibex . Body mass . Diet selection . Forage quality . Jarman-Bell principle . Vigilance behaviour Introduction Foraging behaviour is a highly influential process in many respects of herbivore ecology. It motivates animal move- ments, controls energy and nutrients intake and therefore affects the individual’ s body condition, reproductive success and survival (Senft et al. 1987; Festa-Bianchet 1988; Spalinger and Hobbs 1992). The study of the factors driving foraging behaviour may provide valuable insights into the mechanisms of resource selection and may thus improve our understanding of animal distribution and population dynamics (Bailey et al. 1996; Wilmshurst et al. 1999; Fryxell et al. 2004). For ruminant ungulates, the primary constraints on foraging behaviour result from the interplay between the individual morphological and physiological limitations and the characteristics of the plant resource under exploi- tation (Arnold 1975). Digestive efficiency and, conse- quently, the feeding niche selected (i.e., the minimum quality and quantity of food necessary for survival) have been shown to be strongly related to body mass (Illius and Gordon 1992). Whereas specific metabolic rate decreases with increasing body weight, gut capacity remains a con- stant fraction of the body weight. Hence, larger ungulates should be able to tolerate a lower minimum dietary qual- ity than smaller ones (Jarman-Bell principle Geist 1974). Communicated by K. E. Ruckstuhl Electronic supplementary material The online version of this article (doi:10.1007/s00265-014-1773-0) contains supplementary material, which is available to authorized users. F. Brivio (*) : S. Grignolio : M. Apollonio Department of Science for Nature and Environmental Resources, University of Sassari, via Muroni 25, 07100 Sassari, Italy e-mail: fbrivio@uniss.it A. Brambilla Department of Earth and Environmental Science, University of Pavia, via Ferrata 9, 27100 Pavia, Italy Behav Ecol Sociobiol (2014) 68:1649–1660 DOI 10.1007/s00265-014-1773-0