Bat species in the monophyletic family Phyllostomidae feed on blood, insect
s, small vertebrates, nectar, fruit and complex omnivorous mixtures. We use
d nitrogen stable isotope ratios to characterize bat diets and adopted a ph
ylogenetically informed approach to investigate the physiological changes t
hat accompany evolutionary diet changes in phyllostomids. We found that nit
rogen stable isotopes separated plant-eating from animal-eating species. Th
e blood of the latter was enriched in N-15. A recent phylogenetic hypothesi
s suggests that with the possible exception of carnivory, which may have ev
olved twice, all diets evolved only once from insectivory. The shift from i
nsectivory to nectarivory and frugivory was accompanied by increased intest
inal sucrase and maltase activity, decreased trehalase activity, and reduce
d relative medullary thickness of kidneys. The shift from insectivory to sa
nguinivory and carnivory resulted in reduced trehalase activity. Vampire ba
ts are the only known vertebrates that do not exhibit intestinal maltase ac
tivity. We argue that these physiological changes are adaptive responses to
evolutionary diet shifts.