Selective mobilization of fatty acids from adipose tissue in migratory birds

During times of high energy demand, stored fatty acids are mobilized from adipocytes. This mobilization has previously been shown to be a non-random process, with more hydrophilic fatty acids being mobilized most readily. The objectives of this study were to characterize the relative mobilization of fatty acids from adipocytes in two migratory bird species and to investigate possible changes in selective fatty acid mobilization associated with the migratory period. Captive ruffs (Philomachus pugnax) and white-crowned sparrows (Zonotrichia leucophrys) were studied. The sparrows were divided into two treatments: 'winter' (photoperiod 8 h:16 h L:D) and 'migrant' (in which migratory condition was induced with a photoperiodic manipulation of 8 h:16 h L:D, followed by 16 h:8 h L:D). Adipose tissue was removed from ruffs and sparrows and incubated for 90 min after stimulation with epinephrine. The proportions of individual fatty acid species released into the incubation medium were compared with their proportions in the adipocytes to determine relative mobilizations. We found that patterns of relative mobilization in ruffs and sparrows are similar to those of mammals, with shorter chain lengths and more double bonds leading to higher relative mobilization. Seasonal condition in sparrows did not alter this pattern. This pattern of relative mobilization from adipocytes seems to be a general rule amongst birds and mammals and should be considered before inferring functionality about selective retention or mobilization of certain fatty acids. The composition of adipose stores in birds may affect migratory performance; however, our results indicate that patterns of relative mobilization at the adipocytes do not vary with season in migratory birds.