We studied the morphology and diet of Musonycteris harrisoni, an endangered nectar-feeding bat endemic to western Mexico, and explored potential reasons for the cranial specialization of this extraordinarily long-snouted bat. We achieved 28 captures in the wet season and 30 in the dry season, which represents the largest data set in existence for this rare species. We took morphological data and collected pollen from the bats' pelage. Diet analysis revealed the genera Cleome, Pseudobombax, Crataeva, Agave, Helicteres, and the columnar cactus Pachycereus pecten-aboriginum as main food plants. We found no indication for a tight relationship between M. harrisoni and long-tubed flowers that would obligatorily require a long rostrum for exploitation, with the possible exception of the columnar cactus. However, because cacti are common food plants for many less-specialized nectar-feeding bats, the extraordinarily long rostrum might permit M. harrisoni to use the same abundant plant species as larger species while maintaining a smaller body size, therefore having lower absolute energy requirements. This in turn could facilitate a resident lifestyle in a habitat with annual resource bottlenecks. Males have significantly longer snouts (approximately 9%) than females, suggesting that rostrum length also might be influenced by intraspecific interactions or restrictions. Our favored explanation is that the rostrum length of females may be limited by stability issues, because calcium mobilization during pregnancy and lactation can considerably weaken bone structure.