The social system of animals involves a complex interplay between physiology, natural history, and the environment. Long relied upon discrete categorizations of “social” and “solitary” inhibit our capacity to understand species, and their interactions with the world around them. Here, we use a globally distributed camera trapping dataset to test the drivers of aggregating into groups in a species complex (martens and relatives, family Mustelidae, Order Carnivora) assumed to be obligately solitary. We use a simple quantification, the probability of being detected in a group, that was applied across our globally derived camera trap dataset. Using a series of binomial generalized mixed-effects models applied to a dataset of 16,483 independent detections across 17 countries on four continents we test explicit hypotheses about potential drivers of group formation. We observe a wide range of probabilities of being detected in groups within the “solitary” model system, with the probability of aggregating in groups varying by more than an order of magnitude. We demonstrate that a species’ proclivity towards aggregating in groups is underpinned by a range of resource-related factors, primarily the distribution of resources, with increasing patchiness of resources facilitating group formation, as well as interactions between environmental conditions (resource constancy/winter severity) and physiology (energy storage capabilities). Combined these factors explain observed variance in context-dependent tendencies towards grouping. The wide variation in propensities to aggregate with conspecifics observed here highlights how continued failure to recognise complexities in the social behaviours of apparently “solitary” species limits our understanding not only of the individual species, but also the causes and consequences of group formation.

We conducted a literature review of camera trap research in regions across the globe within the expected ranges of any member of the Martes complex during 2000 – 2020. We used search terms related to specific species names as well as generic terms such as “marten”, “camera trap”, “survey”, and “study”. We used these to create a database of correspondence authors from whom we requested data. In addition, we contacted experts, and reviewed the activities of major international non­‑governmental organizations. We conducted snowball sampling, obtaining additional datasets from colleagues recommended by previous contacts. Data gathered included longitude and latitude of camera stations, date, time, species names, number of individuals in each image, and other associated information (e.g. use of bait). Whilst camera deployment methods varied across the collated studies (see Appendix S1 for full details of each locality), the general method involved deploying camera traps either without bait or facing a bait station (bait varied with the focal species and included peanuts, eggs, beaver meat, scent lures, and so forth). Cameras were set to take photos in bursts of 1-10 images or videos of 10 seconds – 1 minute with short interval times (1 – 20s). Camera makes and models can also be found listed in Appendix S1.

Anything that opens .csv files e.g. R