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

Abstract

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.

The challenges of recognising individuals with few distinguishing features: Identifying red foxes Vulpes vulpes from camera-trap photos

“…The number, and changes in numbers, of non-residents we identified on territories was consistent with known seasonal patterns of fox behaviour, and the origins of many of these animals could be established because they were recognised as residents on other territories [70,79].…”

“…Of these, 78 (41%) were identified in more than one season. A visit to a garden was defined as a sequence of photos of the same fox with a time interval of < 15 minutes between any two photos [70]; a time interval of > 15 minutes between two consecutive photos was taken to indicate the end and start of separate visits by the same fox [79]. Individuals were often photographed multiple times during each visit: median photos per visit = 3.1, range 1–17.…”

Spatial Aspects of Gardens Drive Ranging in Urban Foxes (Vulpes vulpes): The Resource Dispersion Hypothesis Revisited

“…Intrinsic factors such as social organization and stage of lifecycle are important determinants of animal spatial ecology (e.g., [48]). Further, spatio-temporal overlap between conspecifics is affected by social network structure and stability in higher density fox populations [30] affecting movement within and between group territories. In our study, dominance status was reliably assigned in only three of 20 foxes, and we had no information on network connections c.f.…”

“…An unbalanced sample of adults and subadults prevented incorporation of age in our analysis, and welfare issues preclude collaring juvenile or younger subadults. However, age is reported to be an important driver of space use [30], and further studies are recommended to determine whether age effects could account for the large proportion of fox-specific variation in core area size. Seasonal and inter-annual variation did not affect core area in the current study, which may in-part be explained by temporal stability of garden-based resources (e.g., supplementary food [28]).…”

“…Additional biometric data were collected during assessment prior to anesthesia or from anesthetized animals, including weight in kg, sex, body condition (poor, fair, good, very good), and breeding status (breeding, non-breeding, bred this year) if apparent (Supplementary Information (SI) Table S1). Approximate age was quantified using date of capture, tooth wear, weight, and breeding status (SI Table S1) as follows: subadult (SA) (6 to 12 months old, following e.g., [30] or adult (>12 months old); Table 2). Social status was inferred from weight, age, and breeding status and from direct observation where possible.…”