Effect of sexual segregation on host–parasite interaction: Model simulation for abomasal parasite dynamics in alpine ibex (Capra ibex) Nicola Ferrari a, * , Roberto Rosà b , Paolo Lanfranchi a , Kathreen E. Ruckstuhl c a Dipartimento di Patologia Animale, Igiene e Sanità Pubblica Veterinaria, Università degli Studi di Milano, Italy b Environment and Natural Resources Area, IASMA, Research and Innovation Centre, Edmund Mach Foundation, San Michele all’Adige (TN), Italy c Department of Biological Sciences, University of Calgary, Alberta, Canada article info Article history: Received 23 December 2009 Received in revised form 8 March 2010 Accepted 23 March 2010 Keywords: Sexual segregation Parasite transmission dynamics Mathematical model Gastrointestinal nematodes Behaviour Capra ibex abstract We investigated whether sexual segregation might affect parasite transmission and host dynamics, hypothesising that if males are the more heavily infected sex and more responsible for the transmission of parasite infections, female avoidance of males and the space they occupy could reduce infection rates. A mathematical model, simulating the interaction between abomasal parasites and a hypothetical alpine ibex (Capra ibex) host population composed of its two sexes, was developed to predict the effect of dif- ferent degrees of sexual segregation on parasite intensity and on host abundance. The results showed that when females tended to be segregated from males, and males were distributed randomly across space, the impact of parasites was the lowest, resulting in the highest host abundance, with each sex hav- ing the lowest parasite intensity. The predicted condition that minimises the impact of parasites in our model was the one closest to that observed in nature where females actively seek out the more segre- gated sites while males are less selective in their ranging behaviour. The overlapping of field observation with the predicted optimal strategy lends support to our idea that there might be a connection between parasite transmission and sexual segregation. Our simulations provide the biological boundaries of host– parasite interaction needed to determine a parasite-mediated effect on sexual segregation and a forma- lised null hypothesis against which to test future field experiments. Ó 2010 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Host sex is recognised to affect different aspects of parasite dis- tribution with male hosts generally carrying higher parasite loads than females (Poulin, 1996; Zuk and McKean, 1996; Schalk and Forbes, 1997). This pattern has been shown to be the result of gen- der differences in body size, immuno-competence, behaviour and hormone levels (Folstad and Karter, 1982; Zuk and McKean, 1996; Moore and Wilson, 2002; Stoehr and Kokko, 2006). Even if the variation in parasite load might be related to differences in sur- vival, the epidemiological consequence of sexual differences in parasite loads has been less well investigated. Recently it has been shown that host males can be responsible for the transmission and maintenance of infection in the entire host population (Perkins et al., 2003; Ferrari et al., 2004). The mechanisms of male-biased transmission may be related to differences in parasite loads (Skör- ping and Jensen, 2004), but other mechanisms such as differences in immune-response, different habitat use (Ferrari et al., 2007) or the effect of social structure on the contact rate between males and females have been proposed as an important factor shaping the pattern of transmission (Perkins et al., 2008). Sexual segregation is a behaviour that may influence the con- tact rate between males and females. Two types of sexual segre- gation have been most studied: (i) habitat segregation is a behaviour characterised by the different use of space by the two sexes (Bowyer, 2004). This behaviour has been observed in many mammals, with ungulates being most investigated, and dif- ferent patterns of occurrence have been described (Ruckstuhl and Neuhaus, 2005). While habitat segregation is usually expressed when sexes use distinct territories, (ii) social segregation occurs when the sexes live within the same territories but exhibit a dif- ferent temporal use of space and aggregate in sexually segre- gated groups (Conradt, 1998). To explain sexual segregation and the different form this may take, different hypotheses have been proposed, ranging from sexual difference in predation risk, forage selection, weather sensitivity, activity budget and social factors (Ruckstuhl, 2007). Interestingly, investigations into the possible links between sexual segregation and parasite transmis- sion are lacking. Indeed if males are responsible for a higher transmission of parasite infections, then segregation of females from males could result in lower acquisition of new infections in females. 0020-7519/$36.00 Ó 2010 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpara.2010.03.015 * Corresponding author. Tel.: +39 02 503 18097; fax: +39 02 503 18095. E-mail address: nicolaferrari@tiscali.it (N. Ferrari). International Journal for Parasitology 40 (2010) 1285–1293 Contents lists available at ScienceDirect International Journal for Parasitology journal homepage: www.elsevier.com/locate/ijpara