Mice (Peromyscus maniculatus, P. spicilegus, and Microtus mexicanus) as Predators of Overwintering Monarch Butterflies (Danaus plexippus) in Mexico

Abstract

The discovery of accumulation of monarch butterfly wings on the forest floor beneath a 2.25 hectare aggregation of overwintering monarch butterflies in the Sierra Chincua, Michoacan, Mexico suggested that small mammals prey upon these butterflies. During two trapping sessions in 1979 and 1980, we captured 112 Peromyscus maniculatus labecula, three Peromyscus spicilegus, one Microtus mexicanus salvus, and one Sorex saussurei. Feeding experiments showed that all three mouse species eat monarchs and that captive P. m. labecula killed and at least partially ate an average of 25 butterflies per night. Stomach contents of wild-captured P. m. labecula showed that they prey naturally upon the monarchs. Spectrophotometric and thin layer chromatography analyses of cardenolides in the stomach contents of both experimental and wild individuals indicated that the mice do not select or reject butterflies on the basis of cardenolide content. Rather than being adversely affected by feeding on monarchs, female mice were found to be larger, heavier, and reproductively more active in the butterfly colonies than outside them. The mutual impact between the Peromyscus population and the butterflies may be substantial.