My Vert Zoo students and I were discussing last week the demands placed on fish by the medium they move through, and the degree to which their bodies have been shaped by the resulting selective pressures. I made the point that the 1000-fold difference in density of water in comparison to air make it a much more unforgiving medium. That doesn't mean, of course, that moving through air is a piece of cake. Case in point...

Migrating monarch butterflies are shaped differently from their nonmigratory cousins. New work from University of Georgia ecologists appearing online in Evolution and discussed at Science Daily examines the way in which the monumental migration, over 3,000 miles in some cases, has worked to turn the gaudy lepidopterans into more efficient fliers.

Andy Davis and Sonia Altizer looked at sizes and shapes of migrating monarch populations in the eastern and western U.S. and compared them to a number of nonmigratory populations. The map at right, from www.monarchwatch.org, illustrates the flights taken by the migrants. Davis and Altizer found that the migratory butterflies had larger and longer wings than the stay-at-home monarchs. The researchers also found that the eastern migratory population had larger body size than their western counterparts, possibly an adaptation allowing them to store greater energy reserves for their longer migratory flight. Since eastern monarchs face a number of environmental threats, their uniqueness will add fuel to the drive to protect this population.