Dress for success: climate pressures predict fur insulation and body size in natural and reintroduced populations of a threatened marsupial

Phenotypic variation in functional traits underpins responses to environmental gradients, influencing thermoregulation, energy balance, and long‐term persistence under climate extremes. Climate change is altering these gradients globally, yet in species that have already disappeared from much of their range, adaptive phenotypes may have also been lost, potentially limiting population viability following reintroduction to different climates. The greater bilby Macrotis lagotis is a threatened Australian marsupial that has undergone severe range contraction since European settlement and has been the focus of subsequent conservation translocations. To test hypotheses of climate‐associated morphological divergence, we quantified spatial variation across its historical distribution and in reintroduced populations in traits that are key determinants of heat exchange and thermal buffering: skull size (as a proxy for body size), ear length, and fur morphology (dorsal and ventral hair length, depth, and width). The maximum temperature of the warmest month was the strongest predictor of morphology, with skull length and width declining significantly as temperature increased; individuals from regions with summer temperatures of 40℃ had skulls 14% shorter and 12.5% narrower than those from 25℃ regions. All six fur traits also declined significantly with increasing temperature, further supporting a heat‐dissipation hypothesis for reduced body size and insulation in hotter climates. Bilbies from a shared source population translocated into climatically distinct reserves (arid, hot versus temperate, cool) showed rapid and significant divergence in fur traits, but not body size, approximately 25 years post‐release, consistent with climate‐associated phenotypic differentiation despite historical range contraction. In contrast, ear length was unrelated to climate across the historical range and between translocation sites, and instead scaled with body size. In a warming world, understanding whether phenotypic responsiveness persists after range collapse is crucial for assessing adaptive capacity and guiding climate‐resilient conservation strategies.