Breaking Bioclimatic Boundaries: Behavioral Plasticity, Not Physiological Adaptation, Enables the Invasion of a Tropical Ant into Temperate China

Biological invasions often challenge our understanding of climatic niches. Recently, the tropical ant Pheidole megacephala was discovered establishing overwintering populations in Beijing (39°N), significantly exceeding its predicted northern limit (28°N). We tested whether this range expansion is driven by rapid physiological adaptation or behavioral buffering. We integrated genetic analysis, physiological assays (CTmin), and microcosm experiments simulating nest excavation under cooling stress with artificial heat sources incorporated to mimic anthropogenic thermal refugia. Results showed that the Beijing populations originated from two distinct introduction events rather than a single cold-adapted lineage. Contrary to the adaptation hypothesis, we found no significant difference in CTmin between the Beijing (temperate) and Guangdong (subtropical) populations. Instead, field surveys linked nests closely to anthropogenic infrastructures, suggesting exploitation of artificial thermal refugia. Laboratory simulations confirmed that under cooling stress, ants actively utilized micro-heat sources. Under moderate cooling, attraction to micro-heat sources inhibited deep excavation, anchoring colonies in vulnerable shallow layers and potentially functioning as an ecological trap. However, during extreme cold, these same heat sources provided a vital survival buffer by maintaining motor function where ants in unheated environments perished. Our findings suggest that behavioral plasticity, specifically the exploitation of anthropogenic microclimates, acts as the primary driver enabling this tropical invader to break macro-climatic barriers. These results emphasize that invasion risk assessments must account for behavioral factors and urban micro-niches.