Quantifying group size in the red fox: impacts of definition, season and intrusion by non‐residents

Determining group membership is fundamental to studies of social behaviour and monitoring population changes. However, this can be challenging for ecologically-important solitary-foraging carnivores such as red foxes, which have flexible social systems. We used camera-traps to quantify seasonal changes in rates of territory intrusion by non-residents and compared group definitions based on shared space use (spatial overlap) and social encounters (spatiotemporal overlap). Group sizes based on spatial overlap were overestimated but incorporating a minimum number of sightings (sighting threshold) improved accuracy. Groups defined by spatiotemporal overlap were similar in size to those based on spatial overlap with a sighting threshold but included different individuals, highlighting the challenges of determining group membership. Groups were smallest in spring and summer and largest in autumn and winter because all definitions failed to exclude non-residents during the mating and dispersal seasons. However, non-residents were recorded year-round: over half were known or probable neighbours, and so may be relatives of territory residents. Strangers were most common in winter, when non-residents were more likely to be males in search of extra-group copulations. We conclude that groups of territorial, solitary foragers may be defined more accurately by combining patterns of space use, sighting frequency and social connectivity rather than considering these measures in isolation. When social information is not available, spatial overlap measures should include a sighting threshold. Surveying several adjacent territories concurrently helps identify the origins, and motivations, of non-resident visitors. Introduction Group size is the most obvious feature of sociality and a key measure of social complexity (Whitehead, 2008). It is also a fundamental parameter in many population studies (e.g. Harris, 1981; Iossa et al., 2009). For obligate social species, groups are usually spatially isolated and can be defined using shared space use and/or social behaviour (Gese, Rongstad & Mytton, 1988; Hennessy, 2007; le Roux et al., 2009). While similar techniques have been used to define groups of facultative social species, the meaning of a ‘group’ varies widely. For instance, terms such as ‘spatial group’ have been applied when individuals with overlapping home ranges, and hence shared space use, spend little time socialising (Wagner, Frank & Creel, 2008; Martin et al., 2013). Red foxes Vulpes vulpes are solitary foragers, but form groups of ≥ ten adults at high population densities because there are limited opportunities for dispersers to establish their own territory (Baker et al., 1998; Iossa et al., 2009). This social flexibility makes it difficult to determine the membership of fox social groups, and is further complicated because neighbouring territories overlap as population density increases (Trewhella, Harris & McAllister, 1988; Baker et al., 2000; Potts et al., 2013). However, despite the challenges, it is essential to be able to quantify red fox group size and interand intra-group movements to improve management strategies for such a widespread (Schipper et al., 2008) and economicallyand ecologically-important species (Webbon, Baker & Harris, 2004; Saunders, Gentle & Dickman, 2010; Fleming et al., 2017; Radford et al., 2018). Although social relationships are an obvious consideration, hitherto red fox group size estimates have been based on capture and/or space use data (Harris, 1981; Harris & Rayner, 1986; Poulle, Artois & Roeder, 1994; Baker et al., 1998, 2000; Iossa et al., 2009). These techniques limit the accuracy of group-size estimates because low capture rates and infrequent recaptures make it difficult to identify all the members of a social group, and to monitor the rates of territorial intrusion by non-group members (Baker et al., 2001; Soulsbury et al., 2011), which can lead to population density being overestimated (Sutherland, Elston & Lambin, 2013). While camera traps have great potential to compile individual sighting histories, thus far they have not been used to define red fox group sizes and composition (Sarmento et al., 2009; Bengsen, 2014; Ramsey, Caley & Robley, 2015). To further our understanding of the social structure of red foxes, we used camera traps to compare three commonly-used methods of defining social group size in mammals. In particular, we quantified seasonal variation in fox group size, and patterns of territory intrusion by non-residents. We addressed the following hypotheses: (i) group size is largest when based on space use alone due to non-residents making extraterritorial movements (Baker et al., 2001; Soulsbury et al., 2011); (ii) group size estimates are smallest, and most realistic, when based on social connections because foxes avoid encounters with nongroup members (White & Harris, 1994); (iii) groups are largest in autumn prior to the onset of dispersal (Baker et al., 2007; Soulsbury et al., 2008); (iv) territory intrusion is highest in winter when males seek extra-group copulations (Soulsbury et al., 2011); and (v) nonresident visitors are usually neighbours from surrounding territories as home ranges often overlap in Vulpes species (Baker et al., 2000; Eide, Jepsen & Prestrud, 2004; Kitchen et al., 2006; Darden, Steffensen & Dabelsteen, 2008).