Physiological mechanisms of migration

Migratory movements of organisms have arisen across diverse phylogenetic lines for millions of years (Alerstam, 1990; Drake and Gatehouse, 1995; Dingle, 1996; Greenberg and Marra, 2005; Newton, 2008). Some movements are only a matter of centimeters over uneven surfaces, as seen in the amoeba slime mold Dictyostelium discoideum (Gilbert et al., 2012). While at the other extreme the annual aerial routings of about 80,000 km achieved by the arctic tern Sterna paradisea (Egevang et al., 2010) are well known as birds travel between breeding grounds in the arcticand southern oceans to overwinter. Whereas many may argue as to what constitutes “a migration” across such a broad swath of organisms, it is the process by which selective pressures act on individuals to move at precise times and in specific orientation that serves as the common denominator across the phyla. Such pressures are exerted by the environment - fluctuations to which organisms must respond by altering their phenotypes in order to match the conditions, survive and reproduce. These alterations are collectively termed phenotypic flexibility allowing organisms to match their physiological and behavioral features to the demands of the changing conditions (Jacobs and Wingfield, 2000; Ramenofsky and Wingfield, 2006, 2007; Piersma and Van Gils, 2011). Environmental conditions are a reflection of seasonality that result in variation in abundance of resources namely food and shelter, climatic fluctuations, predator and parasite presence, etc. All are factors that contribute to selection for organisms to express phenotypic flexibility necessary for achieving movement from one location to another. Preparation for, timing, duration and orientation of these movements vary across the phyla so that the mechanisms regulating these patterns are highly specific. To gain an appreciation for the breadth and dynamics of these processes, a great deal can be learned by taking a comparative approach. For example, the first two papers in this issue find commonality of regulatory mechanisms. Photoperiodic conditions specifically wavelength oflight is shown to separate the spring induced functions of migration and breeding in white-crowned sparrows Zonotrichia l. gambelii (Wang et al). Chapman (et al) suggests that exposure to seasonal photoperiods during larval and pupal development influence long-distance migratory flights versus dispersal movements in silvermoth Autographa gamma. The second set of papers discuss fueling options available to organisms with diverse ecological demands. Suarez takes the enigmatic approach by discussing the “known and unknown” options of carbohydrate and lipid metabolism in migratory hummingbirds. The known aspects of the paper are that during feeding bouts involving short distance flights and hovering, birds rely primarily on carbohydrate for fuel. On the other hand, longer distance movements - migrations - require lipid. Many of the environmental variables and regulatory mechanisms of these interrelated systems represent the unknown aspects of his equation and deserve further investigative studies. Bairlein (et al) present a review of their work on the holearctic breeding and long distance migrant, the northern wheatear Oenanthe oenanthe.