Effects of group composition on the grazing behaviour of herbivores L.A. Smith a, * , K.L. Wells a,1 , G. Marion b, 2 , D.L. Swain c, 3 , M.R. Hutchings a a Animal Health, Scottish Agricultural College b Biomathematics & Statistics Scotland c Centre for Environmental Management, CQUniversity article info Article history: Received 14 September 2009 Initial acceptance 23 November 2009 Final acceptance 14 June 2010 Available online 20 July 2010 MS. number: 09-00595R Keywords: grazing group composition herbivore individual behaviour physiological state Animal behaviour is often a function of the animal’s physiological state. Groups of animals will often contain individuals with a range of physiological states and the grazing behaviour of herbivores is affected by their physiological state. This study compared the grazing decisions of animals in groups of single and mixed physiological states. Using a grazing model that simulated individual herbivore behaviour in relation to environmental distributions of forage resource (grass) and parasites (faeces), we tested the hypothesis that an animal’s level of parasite exposure via the faecaleoral route is affected by the composition of physiological states in the group. Four physiological states were considered: parasite- naïve, parasitized, lactating and parasite-immune animals. Baseline parasite exposure levels for each state were generated by simulating single-state groups and were compared to simulations of each of the six two-state combinations. In single-state groups parasitized animals had the least and lactating animals had the greatest levels of parasite exposure. When co-grazing with lactating animals, parasitized, immune and naïve animals increased their parasite exposure, relative to single-state groups. When co- grazing with parasitized animals, lactating, immune and naïve animals reduced their parasite exposure, relative to single-state groups. There was no difference in parasite exposure of the immune or naïve animals co-grazing together when compared to the single-state groups. These results highlight the need to recognize the impact of the individual when studying group-living animals. Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. The foraging behaviour of group-living animals is often studied at the population level. However each group of animals is made up of individual members whose behaviour will impact on the dynamics of the whole group. Foraging individuals within a group can vary considerably, for example having different phenotypes (Hoare et al. 2000) or physiological states (Hutchings et al. 2002b). Individuals within a group can also adapt their foraging strategy to both their current environment and their physiological state (Croy & Hughes 1991; Cuthill & Houston 1997; Lõhmus & Sundström 2004). Thus, to understand group foraging behaviour there is a need to understand how changes in the individual’s foraging behaviour impacts the foraging decisions of all group members. Individual-based models, which are capable of simulating the state of each individual in the system, provide an opportunity to explore the way in which an individual’s behaviour can affect other group members (Rands et al. 2008). Here we employed an existing spatially explicit individual-based stochastic approach to modelling of herbivore grazing behaviour (Marion et al. 2005; Swain et al. 2007) to investigate the impact of herbivore group composition (e.g. individuals of mixed physiological states and single-state groups) on grazing behaviour. Herbivore grazing abilities and behaviour are well documented through empirical experimentation and observation (Black & Kenney 1984; Arnold 1987; Bazely 1990; Lynch et al. 1992; Phillips 1993; Hutchings et al. 1998; Wallis de Vries et al. 1998). Therefore, it is possible to develop and parameterize mathematical models that capture herbivore grazing behaviour. The model specifically simulates grazing behaviour in relation to the distri- bution of forage resource (grass) and parasites (faeces) in the environment (Marion et al. 2005). In grazing systems the major route of transmission for a number of parasites is via indirect contact, for example, the faecaleoral route (Sykes 1987) Both macroparasites (e.g. parasitic nematodes) and microparasites (e.g. bacterial pathogens) transmitted via this route are found in host animal faeces. During grazing, herbivores are capable of limiting their risk of infection by avoiding contact with parasites and pathogens in the environment (e.g. via faecal avoidance; Ödberg & * Correspondence: L. A. Smith, Animal Health, Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG, U.K. E-mail address: lesley.smith@sac.ac.uk (L.A. Smith). 1 K. L. Wells is now at The Brooke, 30 Farringdon, London EC4A 4HH, U.K. 2 G. Marion is at Biomathematics & Statistics Scotland, JCMB, The King’s Build- ings, Edinburgh EH9 3JZ, U.K. 3 D. L. Swain is at Centre for Environmental Management, CQUniversity, Rock- hampton, QLD, 4701, Australia Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav 0003-3472/$38.00 Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2010.06.016 Animal Behaviour 80 (2010) 527e534