ECOLOGY/EVOLUTION: The Difference a Week Makes -- Sugden 307 (5717): 1843b -- Science:
Not all monarch populations migrate, and parasite prevalence is known to be lower in the migratory monarch populations. Butterflies from migratory populations inoculated with a protozoan parasite showed reductions in flight performance and endurance in experimental cages, probably because the parasite influenced metabolic processes associated with flight (there were no changes in wing morphology associated with the presence of the parasite). The authors estimate that the impairment would lengthen the migratory journey from 9 to 10 weeks. Under these conditions, parasitized butterflies would likely suffer a reduced chance of reaching their destination, thus accounting for the differences in parasite burden between migrant and nonmigrant monarchs.
An evolutionary hypothesis on the evolution of migration in monarchs.