Emerging infectious fungal diseases are responsible for the extinction of myriad species across a range of phyla. As recently shown by the COVID-19 pandemic, social transmission can be key to disease spread, and in this context, humans are not alone in trying to be alone. In group-living species, individuals have been shown to use social behaviour to avoid infection; diseased individuals can isolate from the group, or healthy animals can avoid diseased conspecifics. However, to date, little is known about social behaviour as a mechanism to avoid fungal infection. In this study, we investigate the extent to which wild urban eastern water dragons (Intellagama lesueurii), a gregarious reptile, modify their social behaviour as a response to infection with a recently emerged infectious fungal disease, caused by the pathogen Nannizziopsis barbatae. Using individual data from a long-term study population inhabiting Roma Street Parkland in Brisbane’s Central Business District (QLD, Australia) and focal sampling, we test whether dragons exhibit self-isolation and social distancing behaviours in the context of dyadic social approach events. Statistical analyses of data collected from 633 observations of 123 unique individuals (78 non-diseased, 45 diseased) during 2020 – 2021 suggested that while the presence of the fungal disease had no effect on individuals’ social behaviour, the severity of the fungal disease did significantly affect individuals’ social behaviour. Specifically, we found that (i) diseased individuals were no less social than their non-diseased conspecifics, (ii) non-diseased individuals did not avoid or spend less time with diseased conspecifics, and (iii) models considering the severity of skin lesions caused by Nannizziopsis barbatae, instead of their presence or absence, suggested that individuals avoided more severely diseased conspecifics regardless of their own disease status.  These findings warrant further investigation, given the evidence that many species have displayed behavioural adaptations to disease, and the increasing risk posed by emerging infectious fungal diseases.

Our study was conducted at Roma Street Parkland (-27°27’46’S, 153°1’11’E) in the Central Business District of Brisbane, Queensland’s capital city. The parkland spans approximately 16 hectares and contains a combination of simulated native vegetation and arranged garden beds, differing in their structure and access to water. The park is nestled within Brisbane’s largest transit centre and is surrounded by roads and urban infrastructure (Gardiner et al., 2014).

­­­­The study focused on an urban population of dragons inhabiting the parkland. Eastern water dragons are a large semi-aquatic agamid lizard, native to the east coast of Australia. They display male-biased sexual dimorphism, with males being largerJ than females (Thompson, 1993), along with other distinct morphological differences (Baird et al., 2012). This species is long-lived, with a lifespan of up to 15 years and a generation time of four years (Thompson, 1993), and exhibits a defined social structure that is similar in complexity to those identified in some long-lived mammals (Sah et al., 2018); they are highly gregarious, show both preferences and avoidances for certain conspecifics (Piza-Roca et al., 2018; Strickland & Frere, 2019), and exhibit sex differences in social strategies (Strickland et al., 2014; Strickland & Frere, 2017). 

Roma Street Parkland’s population of dragons has an estimated population size of over 300 adult individuals at a 1:1 sex ratio (Strickland et al., 2014), that have been the subject of longitudinal behavioural, morphological and genetic studies since 2010, and are habituated to human presence (Strickland & Frere, 2017). The parkland’s urban boundaries prevent immigration and emigration with other geographically close populations (Littleford‐Colquhoun et al., 2017). It is estimated that 34.5% of the population currently exhibit clinical signs of N. barbatae infection (Peterson et al., 2020) and are thus considered ‘diseased’.

Focal surveys

To investigate the effects of N. barbatae infection on the social behaviour of dragons, we conducted repeated behavioural focal follows during morning or afternoon surveys. Focal follows were performed by following a transect line through the study area from 8:00am to 3:00pm three days per week from September 2020 to January 2021. This period represents the breeding season, during which dragons are known to be more social (Strickland & Frere, 2019). We defined a social approach as one individual approaching another within two metres (Strickland et al., 2014). This spatial proximity distance is used as a proxy for social tolerance given that eastern water dragons will otherwise react aggressively toward one another (Strickland & Frere, 2017). We also excluded any mating attempts or antagonistic interactions given that we seek to capture affiliative behaviour which are defined as non-aggressive social behaviours and can range from social tolerance of conspecifics to group-living and cooperation (Ward and Webster, 2016).

For each focal follow, a focal individual was selected upon encounter (randomly with respect to its sex, age, or disease status) if it was alone but sufficiently close (between 2m and 10m) to other individuals (potential partners) to increase the likelihood of observing a social approach. For both the focal and partner, right or left facial profile images (Gardiner et al., 2014) were captured using a Canon (Tokyo, Japan) EOS 600 digital camera to allow later identification, and fine-scale behavioural observations pertaining to a social approach were recorded.

We recorded the identity of the individual initiating and the individual receiving the social approach. A social approach was considered successful if the initiator approached the receiver within two metres without displacing the receiver, or unsuccessful if the receiver immediately moved more than two metres away following a social approach (Figure 1). Following a successful approach, the duration of time that two individuals spent within two metres of one another was recorded as their ‘time in proximity’. Time in proximity was recorded until the social approach ended, or for up to five minutes. If the focal individual was not involved in a social approach or moved out of the observer’s sight, the focal follow stopped after 10 minutes. Successive focal follows were not performed within the same immediate area, to avoid observing the same individuals several times (either as focals or partners) within the same survey day.