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Feeding and breeding responses of five mice species to overwintering aggregations of the monarch butterfly

Glendinning, J.I.; Brower, L.P.

Journal of Animal Ecology 59(3): 1091-1112

1990


ISSN/ISBN: 0021-8790
DOI: 10.2307/5034
Accession: 007346483

(1) During four consecutive years in Mexico (1985-88), we compared the numerical, breeding and feeding responses of five species of mice (Peromyscus melanotis, P. aztecus, Reithrodontomys sumichrasti, Neotomodon alstoni, and Microtus mexicanus) to overwintering aggregations of monarch butterflies that are both lipid-rich and chemically defended. (2) Large numbers of adult P. melanotis, with a significant bias of females, immigrated to trapping grids inside aggregations, established residency, and bred intensively throughout the winter. The other species also immigrated to the aggregations, but did not establish residency. On grids outside aggregations, individuals of all species established residency and showed comparatively little breeding throughout the winter. (3) We estimated an average of 6.5 live, moribund, and bird-damaged monarchs per m2 per night on the forecast floor inside aggregations. Stomach content analyses indicated that P. melanotis inside aggregations fed exclusively on monarchs. Even though all five species scavenged some butterflies on grids outside aggregations, P. melanotis scavenged significantly more. (4) We tested two hypotheses to explain why only P. melanotis established residency on grids inside aggregations: (i) unique plant species compositions in overwintering sites rendered them suitable only to P. melanotis, and (ii) P. melanotis aggressively excluded the other species. We rejected the first hypothesis because the understorey vegetation on grids inside and outside aggregations did not differ significantly, and rejected the second because P. melanotis did not dominate the other species in experimental agonistic encounters. (5) Our data, along with other findings (Glendinning 1989, 1990), support a third hypothesis: all species except P. melanotis are deterred by the monarch's defensive compounds (cardenolides). (6) Peromyscus melanotis, like two species of avian predators, appears exapted (sensu Gould and Vrba 1982) to exploit the monarch aggregations.

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