Forage quality shapes physiological and gut microbial responses in moose (Alces alces) of Isle Royale National Park.

Plant secondary compounds (PSCs) impose physiological and nutritional constraints on herbivores, yet many species continue to rely on PSC-rich forage during critical periods of the year. Moose (Alces alces) on Isle Royale National Park depend heavily on balsam fir (Abies balsamea) during winter, exposing them to a chemically defended and nutritionally limited diet. To investigate how this foraging strategy shapes both physiological responses and gut microbial communities, we integrated fecal diet composition, urinary markers of detoxification and nutritional status, 16S rRNA gene sequencing, and shotgun metagenomic profiling from free ranging moose of two separated populations from the western and eastern region of the island. Balsam fir consumption varied strongly by region and was positively associated with glucuronic acid to creatinine (GA:C) and urea nitrogen to creatinine (UN:C) ratios, indicating increased detoxification activity and reduced nutritional condition. Microbial alpha diversity declined with higher fir intake in nutritionally limited individuals, while beta diversity differed by region, balsam fir consumption, and UN:C. Several bacterial genera responded to PSC exposure, including increases in the butyrate-producing genus Roseburia and shifts in network prominence of Phascolarctobacterium. Metagenomic data revealed pathways involved in the degradation of aromatic and terpenoid PSCs, although pathway abundances did not differ significantly with balsam fir consumption after multiple testing correction. These results show that winter foraging on balsam fir produces coordinated dietary, physiological, and microbial patterns, with both host and gut microbial detoxification capacities interacting to accommodate the chemical and nutritional challenges of a PSC-rich winter diet.