Mule (Odocoileus hemionus) and white-tailed deer (Odocoileus virginianus) are similar species, distributed across North America in both areas of sympatry and allopatry. Potential differences in their fundamental and realized niches that might be responsible for their distribution patterns remain largely unknown. Therefore, to directly compare the fundamental nutritional niche of mule and white-tailed deer we conducted in vivo digestion and intake tolerance trials using 7 tractable mule and 6 white-tailed deer, raised under similar conditions in captivity, across 6 diets ranging in fiber, protein, and plant secondary metabolite (PSM) content. We found that compared to white-tailed deer, mule deer had higher fiber, energy, and dry matter digestibility, produced glucuronic acid (a byproduct of PSM detoxification) at a slower rate when consuming the monoterpene [alpha]-pinene, and required 50% less digestible protein intake and 20% less digestible energy intake to maintain body mass and nitrogen balance. These differences might provide an advantage to mule deer when sharing landscapes with white-tailed deer containing forages of lower nutritional quality (i.e., higher fiber and PSM content). To compare their realized habitat niches, we determined presence and absence of both species at 312 cameras traps during the summers of 2018-2019 across a range of elevations, habitats, and overstory thinning treatments in the Colville National Forest (CNF) in northeastern Washington. We compared their habitat use with single species occupancy models and found that topography was the strongest indicator of differing habitat use; mule deer were more likely to occupy steep slopes and higher elevations and white-tailed deer the opposite. Using conditional, two species occupancy modeling, we found that mule and white-tailed deer occurred independently of one another (i.e., no dominance) in the CNF in summer. The absence of spatial and temporal segregation between the two species was supported by ~ 90% overlap in daily activity patterns and similar elapsed time between subsequent intra- or interspecies detections at camera sites with both species detected. Our research suggests that mule and white-tailed deer likely minimize potential competitive interactions in part by evolving different abilities to cope with low quality forage and by segregating topographically, thus are largely syntopic
