Dietary niche specialization sustains winter survival without fat accumulation in morpho-acoustically adapted bats

Traditional mammalian overwintering strategies emphasize energy storage via fat accumulation coupled with hibernation or migration. However, niche exploitation may enable alternative strategies that circumvent reliance on lipid reserves. Here we report a novel overwintering paradigm in the Rhinolophus macrotis group, a clade of auditory-specialized bats characterized by low-frequency echolocation and enlarged external ears (pinnae). Unlike sympatric species exhibiting autumnal fattening, these bats showed no significant pre-winter weight gain. Their equivalent or higher mass-specific oxygen consumption rates compared to sympatric congeners indicate no dependence on reduced basal metabolic rates for energy balance. Instead, the R. macrotis group sustained high winter foraging activity, particularly during warm-temperature intervals. Dietary analyses confirmed their predominant reliance on tympanate insects—an acoustically evasive food resource relatively abundant even during seasonal resource lows—a niche scarcely exploited by sympatric bats. Aerodynamic modeling revealed that their low body mass reduces flight power demands, compensating for increased energetic costs of slow flight imposed by enlarged ears while enhancing maneuverability for hunting evasive targets. These findings provide robust empirical evidence for the viability of a "resource-specialization" pathway to winter survival, wherein morpho-acoustical and behavioral adaptations underpin access to underutilized food resource, enabling an alternative overwintering strategy dispensing with autumnal fattening.