Social Distancing Responses to Fungal Disease in an Australian Wild Lizard Population

A five-year study of Eastern Water Dragons in Brisbane reveals that while increasing numbers of diseased conspecifics generally reduce social distance due to crowding, infected individuals paradoxically maintain larger distances from others, indicating that fungal infection constrains density-driven proximity through partial social avoidance.

The Gist

Imagine a busy, sunny park in Brisbane, Australia, filled with hundreds of Eastern Water Dragons. These aren't just any lizards; they are social creatures. They like to hang out, bask in the sun together, and even fight over the best spots. Think of them like a group of friends who love to gather at a popular coffee shop.

But lately, this "coffee shop" has a problem: a nasty, invisible fungus called Nannizziopsis barbatae has moved in. It's like a contagious cold that doesn't just make you sneeze; it leaves ugly, crusted sores on the lizards' skin and can be fatal.

This study is like a five-year reality show where researchers watched these lizards to answer a big question: When your friends get sick, do you still hang out with them, or do you back away?

The Big Discovery: "Social Distancing" in Reptiles

The researchers found that the lizards are actually quite smart about this. They don't just ignore the sickness; they change their behavior in two interesting ways:

1. The "Crowded Room" Effect
Imagine the park gets super crowded. Even if everyone is healthy, when there are too many lizards in one spot, they naturally get closer together because there's no room to move. The study found that when there were many sick lizards in a specific area, the healthy ones got squeezed in closer, too. It's like being in a packed elevator; you can't avoid touching people even if you want to. This is the "crowding effect."

2. The "Sick Friend" Rule
Here is the twist: The lizards did notice the sickness.

• Healthy lizards tried to keep their distance from the sick ones, but only as much as the crowd allowed.
• Sick lizards themselves started acting differently. They kept a wider distance from everyone else, even the healthy ones. It's as if the sick lizard felt too weak to fight for a spot, or perhaps it instinctively knew it was a "walking germ bomb" and tried to stay away to protect its friends.

The "Partial Avoidance" Analogy

The paper calls this "partial avoidance."

Think of it like a party where someone has the flu.

• Total Avoidance would be: "I'm leaving the party immediately and never coming back."
• No Avoidance would be: "I don't care, I'm going to hug everyone."
• Partial Avoidance (What the lizards did) is: "I'm still at the party, but I'm standing on the other side of the room. I'm not hugging the sick person, but I can't leave the party because I need to eat and sleep here."

The lizards couldn't completely avoid the fungus because the fungus lives in the dirt and rocks (the environment), not just on the lizards. So, they couldn't run away entirely. Instead, they adjusted their "personal space bubbles" to minimize the risk.

Why Does This Matter?

This is a big deal for science because we usually think of "social distancing" as something humans do. This study shows that even reptiles, who we often think of as cold and unfeeling, have complex social rules. They can sense danger and change their behavior to survive.

However, there is a limit. As the number of sick lizards grew from about 20% to over 60% of the population, the "social distancing" became harder. When the whole room is full of sick people, you can't really keep your distance anymore. The study suggests that while the lizards are trying to be smart, the sheer number of sick friends might eventually overwhelm their ability to stay safe.

The Takeaway

In simple terms: These lizards are trying to be good neighbors. They are balancing the need to hang out with friends against the danger of catching a deadly disease. They aren't perfect—they get crowded—but they are doing their best to keep a safe distance, proving that even in the wild, "stay away from the sick" is a survival instinct.

Technical Summary

1. Problem Statement and Context

• The Conflict: Social living offers fitness benefits (e.g., predator avoidance, mating) but increases pathogen transmission risk. While social distancing (avoiding diseased conspecifics) is documented in some taxa, its efficacy and expression in wild reptiles facing Emerging Infectious Fungal Diseases (EIFDs) remain poorly understood.
• The Specific Challenge: Fungal pathogens like Nannizziopsis barbatae (Nb) differ from viral/bacterial agents because they persist in the environment (environmental reservoirs). This creates a transmission dynamic where direct host-to-host contact is not the only vector, potentially limiting the effectiveness of behavioral avoidance.
• The Knowledge Gap: It is unclear whether wild reptiles exhibit partial avoidance (modulating spacing without full social cessation) in response to environmental and direct transmission risks, and how this behavior interacts with population density (crowding effects).

2. Methodology

• Study System:
  • Species: Eastern Water Dragons (Intellagama lesueurii).
  • Location: Roma Street Parklands (RSP), Brisbane, Australia (a semi-enclosed urban park).
  • Pathogen: Nannizziopsis barbatae (Nb), a fatal fungal pathogen causing skin lesions.
• Data Collection (2018–2023):
  • Dataset: 5 years of longitudinal data covering 146 unique adult individuals (65 males, 81 females).
  • Surveys: Visual surveys conducted ~3 times weekly during the active season (Sept–April), recording GPS locations, behaviors, and individual identity (via facial pattern recognition).
  • Disease Assessment: Biannual captures involved visual inspection for lesions and skin swabs for qPCR analysis. Individuals were classified as Diseased (visual lesions + qPCR positive) or Healthy (no lesions + qPCR negative). Asymptomatic qPCR-positive individuals were excluded due to low sample size.
• Spatial Analysis:
  • Core Home Range: Defined as the 50% Kernel Utilization Distribution (UD) for each individual per season.
  • Exposure Metric: DiseasedCount – the number of unique diseased conspecifics overlapping the focal individual's core home range.
  • Response Variable: Social Distance – the minimum distance (m) to the nearest neighbor during a survey.
• Statistical Modeling:
  • Model Type: Bayesian Generalized Linear Mixed Model (GLMM) with a Gamma distribution and log link.
  • Censoring: Addressed left-censoring due to GPS resolution limits (distances <1.64 m were recorded as 1.64 m) using a censored likelihood approach (brms package).
  • Fixed Effects: Focal disease status, sex, sighting frequency, core home range area, DiseasedCount, and the interaction between focal status and DiseasedCount.
  • Random Effects: Individual identity and Individual-Season grouping to account for repeated measures.

3. Key Contributions

• Population-Level Perspective: Unlike previous dyadic or short-term studies, this research provides a long-term, population-level analysis of social spacing dynamics in a wild reptile population.
• Quantifying Partial Avoidance: The study demonstrates that avoidance is not binary (avoid vs. interact) but a continuous modulation of space use ("partial avoidance") that balances social benefits against infection risk.
• Disentangling Density and Disease: The study isolates the effect of disease pressure from general crowding, showing that while density drives proximity, disease status modifies this relationship.
• Environmental Transmission Context: It specifically addresses how behavioral responses function in systems where environmental reservoirs (soil/substrate) facilitate transmission, challenging the assumption that social distancing alone can prevent infection.

4. Key Results

• Crowding Effect: There is a strong negative correlation between the number of diseased conspecifics in a home range and social distance. As DiseasedCount increases, individuals generally move closer together (Est = -0.30), likely due to habitat constraints and high local density.
• Disease Status Effect: Diseased focal individuals maintained significantly larger social distances than healthy individuals (Est = 0.11), regardless of the surrounding disease pressure.
• Interaction Effect (The Core Finding): A significant positive interaction was found between focal disease status and DiseasedCount (Est = 0.12).
  • Interpretation: While healthy individuals reduced their spacing as the number of diseased neighbors increased (following the crowding effect), diseased individuals attenuated this reduction. In high-disease environments, sick lizards maintained greater distances from neighbors compared to healthy lizards in the same environment.
• Robustness: Results remained consistent when the minimum sighting threshold for home range estimation was lowered from 30 to 25 sightings.

5. Significance and Implications

• Behavioral Ecology: The findings suggest that infected individuals may engage in self-isolation (reduced social tolerance or movement limitations) or that healthy individuals actively avoid sick conspecifics. This "partial avoidance" acts as a buffer against transmission, even when complete avoidance is impossible due to environmental reservoirs.
• Epidemiology: The study highlights that disease risk modulates density-driven social behavior. As disease prevalence rises (Nb prevalence in the park rose from 21% to 62% during the study), the ability of individuals to maintain effective spacing diminishes, potentially creating a tipping point for transmission dynamics.
• Conservation: In urban and fragmented ecosystems where wildlife is forced into high-density habitats, understanding these behavioral plasticity limits is crucial. The results suggest that while behavioral avoidance occurs, it may be insufficient to halt transmission of environmentally persistent fungi without additional management interventions.
• Future Directions: The authors call for research into the environmental persistence of Nb and whether behavioral strategies remain plastic under escalating infection pressure, as well as the demographic consequences of these behavioral shifts.