The Difference a Week Makes

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Science  25 Mar 2005:
Vol. 307, Issue 5717, pp. 1843
DOI: 10.1126/science.307.5717.1843b

Migration is well established as a mechanism by which animals cope with seasonal variations in food supply. It is has also been suggested as a possible way of reducing the burden of parasitism in a range of hosts, either by weeding out infected individuals or by allowing them to escape from environments in which parasites have accumulated. Bradley and Altizer provide evidence that one of the more spectacular examples of migration—that of the monarch butterfly in the North America—may have evolved at least in part as such a mechanism.

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. Because habitat loss and climate change are expected to affect migrant populations more severely, the prevalence of parasites is likely to increase. — AMS

Ecol. Lett. 8, 290 (2005).

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