Winter warming shapes gut microbiome composition and directional dysbiosis in a temperate lizard

This study demonstrates that winter warming induces directional dysbiosis and shifts in gut microbiome composition in temperate lizards, with constant mild temperatures promoting potentially harmful taxa while fluctuating temperatures better preserve microbial stability.

The Gist

The Big Picture: A Lizard's Winter Nap and Its Gut Friends

Imagine a lizard as a small, scaly house. Inside this house lives a bustling community of tiny tenants: bacteria. These bacteria are the "gut microbiome." They help the lizard digest food, fight off sickness, and stay healthy. Usually, we think about how weather affects these tenants during the summer when the lizard is active and eating.

But this study asked a different question: What happens to these bacterial tenants when the lizard is sleeping through the winter, and what if the winter gets warmer than usual?

The researchers found that even a small change in winter temperature can rearrange the furniture in the lizard's gut, potentially making the house less stable and more prone to trouble, even if the lizard looks fine on the outside.

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The Experiment: Three Different Winters

The scientists took 39 common wall lizards and put them in a lab to simulate winter. They split the lizards into three groups, each experiencing a different "winter scenario":

1. The "Classic Winter" Group (Cold): Kept at a chilly 4°C (39°F). This is what the lizards are used to.
2. The "Mild Winter" Group (Constant Warmth): Kept at a steady 8°C (46°F). Imagine a winter that never gets truly cold, just a bit chilly.
3. The "Flip-Flop Winter" Group (Fluctuating): This group experienced a mix. Five days of cold, then two days of mild warmth, repeating. This mimics a winter with unpredictable warm spells.

After about three months (the lizards' "hibernation"), the scientists woke them up, let them eat again, and collected their poop to analyze the bacteria inside.

The Findings: It's Not About How Many, It's About Who

Here is what they discovered, broken down simply:

1. The "Party Size" Didn't Change (Alpha Diversity)

The Analogy: Imagine a party. You might expect that if the weather gets weird, fewer people would show up.
The Result: The number of different types of bacteria (the "guest list size") stayed roughly the same in all three groups. Whether the winter was cold, mild, or flip-flopping, the lizards still had a full house of bacteria.
The Catch: Just because the number of guests is the same doesn't mean the guests are the same.

2. The "Guest List" Changed (Composition)

The Analogy: Imagine a dinner party. In the "Classic Winter," you have a bunch of helpful, hard-working guests who bring good food and keep the house clean (beneficial bacteria). In the "Mild Winter," some of those helpful guests left, and were replaced by a few rowdy, opportunistic guests who might start a fight or eat all the snacks (potentially harmful bacteria).
The Result:

• Cold & Flip-Flop Winters: These groups kept a diverse mix of "good" bacteria that help digest food and keep the gut healthy.
• Constant Mild Winter: This group lost some of the "good" bacteria and gained more "opportunistic" bacteria. These are the kind of bacteria that can cause trouble if the host is stressed. It's like the house is still full, but the tenants are less cooperative.

3. The "Stability" Factor (Dysbiosis)

The Analogy: Think of the gut microbiome as a well-organized orchestra.

• Cold Winter: The orchestra plays a steady, harmonious tune.
• Mild Winter: The orchestra is still playing, but the music is slightly off-key. It's not a total disaster, but it's not the perfect harmony it used to be. This is called dysbiosis (an imbalance).
• Flip-Flop Winter: Surprisingly, the "flip-flop" group sounded more like the "Cold" group. The changing temperatures seemed to act as a buffer, keeping the orchestra in tune better than the constant mild warmth did.

4. The "Gender" Difference

The Analogy: Male and female lizards have slightly different "house rules."
The Result: Female lizards generally had a more diverse and stable bacterial community than males. It seems the female lizards were better at keeping their internal tenants organized, regardless of the temperature.

Why Does This Matter?

You might think, "So what? The lizards woke up fine."

Here is the metaphor: The "Winter Hangover."
Imagine you go to a party where the music is slightly off-key. You might not get sick immediately, but your immune system is a little tired, and your digestion might be a bit off for a few days after.

The study suggests that a "mild" winter might trick the lizard's body into thinking it's time to wake up and eat, but the gut bacteria aren't ready. This mismatch could make the lizard more susceptible to disease later in the spring, or less efficient at getting energy from food.

The Takeaway

Climate change isn't just about hotter summers; it's about warmer, unpredictable winters.

• Constant mild warmth seems to be the worst for the lizard's gut health, encouraging "bad" bacteria to move in.
• Fluctuating temperatures (warm spells mixed with cold) actually seem to protect the gut better, perhaps because the bacteria are kept on their toes and don't settle into a bad routine.

In short: Even if a winter feels "nice" and mild to us, it might be throwing a wrench into the tiny, invisible ecosystem inside a lizard's stomach, potentially making them less healthy when spring finally arrives.

Technical Summary

1. Problem Statement and Context

• Climate Change Context: Winter temperatures are rising globally, often faster than summer temperatures, leading to increased frequency of warm spells and thermal variability.
• Knowledge Gap: While the effects of warming on gut microbiomes during the active season are well-documented in ectotherms (cold-blooded animals), the impact of winter warming during overwintering (dormancy/brumation) is poorly understood.
• Physiological Context: Overwintering involves profound physiological suppression (reduced metabolism, fasting, torpor). It is unclear if winter warming acts as a stressor capable of inducing dysbiosis (microbiome instability) consistent with the Anna Karenina Principle (AKP), which predicts that stress increases inter-individual variability and community dispersion.
• Research Question: How do contrasting winter warming regimes (constant mild vs. fluctuating temperatures) affect the gut microbiome diversity, composition, and stability of a temperate ectotherm (Podarcis muralis, the common wall lizard) compared to cold control conditions?

2. Methodology

• Study Species: 39 adult common wall lizards (Podarcis muralis) captured in Bournemouth, UK (23 males, 16 females).
• Experimental Design:
  • Duration: 104 days of overwintering.
  • Treatments:
    1. Cold (Control): 4±1°C (approximating historical UK winter mean).
    2. Constant Mild: 8±1°C.
    3. Fluctuating: Alternating 5 days at 4°C and 2 days at 8°C.
  • Acclimation: Lizards underwent a 30-day acclimation period with gradually reduced temperature and photoperiod to induce torpor.
• Sampling:
  • Timing: Faecal sampling began 1 week after feeding resumed post-emergence and continued for 5 weeks.
  • Volume: 80 faecal samples collected (some pooled by terrarium when individual identity was lost); 4 substrate controls.
  • Temporal Aggregation: Samples were grouped into three time points: Week 1, Week 2, and Weeks 3+.
• Molecular Analysis:
  • Sequencing: 16S rRNA gene V4 region sequencing (Illumina).
  • Bioinformatics: DADA2 pipeline for ASV (Amplicon Sequence Variant) inference; SILVA database for taxonomy; removal of contaminants and non-bacterial sequences.
  • Data Processing: Rarefaction to 8,000 sequences for diversity metrics; non-rarefied data for differential abundance.
• Statistical Analysis:
  • Alpha Diversity: Linear Mixed-Effects Models (LMM) for Observed ASVs, Shannon Index, and Faith's Phylogenetic Diversity.
  • Beta Diversity: PERMANOVA (Bray-Curtis and Unweighted UniFrac) to test treatment, sex, and time effects.
  • Dysbiosis Metrics:
    • Dispersion: Distance to group centroid (betadisper).
    • Directional Shift: Euclidean distance to the cold-treatment centroid (dysbiosis score).
    • Differential Abundance: DESeq2 analysis at the ASV level, aggregated to genus/order.

3. Key Results

• Alpha Diversity (Richness/Evenness):
  • No Effect of Temperature: Winter warming treatments (mild or fluctuating) did not significantly alter alpha diversity (Observed ASVs, Shannon, or Faith's PD) compared to the cold control.
  • Sex Effect: Females exhibited significantly higher ASV richness and phylogenetic diversity than males.
• Beta Diversity (Composition):
  • Treatment Effect: Overwintering temperature significantly shifted community composition, though the effect size was modest ($R^2 \approx 0.036–0.048$).
  • Sex and Time: Sex and sampling time explained comparable or greater variance than temperature.
  • Differential Abundance:
    • Cold & Fluctuating: Enriched in fermentative taxa (e.g., Roseburia, Intestinimonas, Rikenella, Bacteroides) associated with Short-Chain Fatty Acid (SCFA) production and gut homeostasis.
    • Constant Mild: Enriched in fewer taxa, specifically Corynebacterium and Brevibacterium (genera containing opportunistic/pathogenic species) and Odoribacter.
    • Temporal Dynamics: Early post-emergence (Weeks 1–2) showed enrichment of opportunistic taxa (Hafnia-Obesumbacterium, Corynebacterium), while later weeks (3+) showed a shift toward fermentative taxa.
• Dysbiosis and Stability:
  • Dispersion: Winter warming did not increase community dispersion (within-group heterogeneity) or inter-individual variability. This contradicts the Anna Karenina Principle prediction of stochastic destabilization.
  • Directional Dysbiosis: Both warming treatments (mild and fluctuating) resulted in significantly higher directional dysbiosis scores (distance from the cold centroid) compared to the cold control.
  • Interpretation: The microbiome shifts were structured and deterministic rather than chaotic. The constant mild temperature induced a specific shift toward potentially pathogenic taxa, whereas the fluctuating regime preserved a broader suite of beneficial fermenters.

4. Key Contributions

1. Novel Temporal Focus: First experimental assessment of how contrasting winter-warming regimes specifically shape gut microbiomes during the overwintering period in reptiles, a critical but overlooked physiological window.
2. Decoupling of Diversity and Composition: Demonstrates that winter warming can alter community composition and induce directional dysbiosis without reducing alpha diversity. This suggests that richness metrics alone may mask functional disruptions during dormancy.
3. Nature of Dysbiosis: Challenges the universality of the Anna Karenina Principle in this context. The study shows that thermal stress during overwintering induces predictable, deterministic shifts (anti-AKP) rather than stochastic destabilization, likely due to the constrained physiological state (torpor) of the host.
4. Role of Thermal Variability: Highlights that fluctuating temperatures (mimicking natural warm spells) may buffer negative effects better than constant mild warming, which enriched potentially opportunistic pathogens.
5. Sex-Specific Responses: Confirms strong sex-specific differences in microbiome structure and response to overwintering conditions, with females maintaining higher diversity and fermentative taxa.

5. Significance and Implications

• Host Health: The enrichment of opportunistic/pathogenic genera (Corynebacterium, Brevibacterium) under constant mild warming suggests a potential risk for disease susceptibility upon emergence, even if diversity metrics appear stable.
• Climate Resilience: The findings suggest that ectotherms may be vulnerable to specific patterns of winter warming (constant vs. fluctuating). Constant warming may disrupt the maintenance of beneficial fermentative bacteria essential for post-hibernation recovery.
• Recovery Dynamics: The microbiome does not immediately return to a "pre-winter" state upon emergence; it undergoes a gradual recovery over several weeks, during which the host is potentially more vulnerable to opportunistic infections.
• Future Directions: The study underscores the need to integrate functional, physiological, and fitness metrics to determine if these subtle, structured microbiome shifts translate into long-term population-level consequences for ectotherms in a warming world.

Conclusion: Winter warming acts as a subtle but significant driver of gut microbiome restructuring in temperate lizards. It induces directional dysbiosis and alters the functional potential of the community (shifting from fermentative to potentially pathogenic taxa), particularly under constant mild temperatures, without necessarily reducing overall microbial diversity. This highlights overwintering as a critical, sensitive period for host-microbiome resilience under climate change.