Obese super athletes: fat-fueled migration in birds and bats

Abstract

Migratory birds are physiologically specialized to accumulate massive fat stores (up to 50-60% of body mass), and to transport and oxidize fatty acids at very high rates to sustain flight for many hours or days. Target gene, protein and enzyme analyses and recent -omic studies of bird flight muscles confirm that high capacities for fatty acid uptake, cytosolic transport, and oxidation are consistent features that make fat-fueled migration possible. Augmented circulatory transport by lipoproteins is suggested by field data but has not been experimentally verified. Migratory bats have high aerobic capacity and fatty acid oxidation potential; however, endurance flight fueled by adipose-stored fat has not been demonstrated. Patterns of fattening and expression of muscle fatty acid transporters are inconsistent, and bats may partially fuel migratory flight with ingested nutrients. Changes in energy intake, digestive capacity, liver lipid metabolism and body temperature regulation may contribute to migratory fattening. Although control of appetite is similar in birds and mammals, neuroendocrine mechanisms regulating seasonal changes in fuel store set-points in migrants remain poorly understood. Triacylglycerol of birds and bats contains mostly 16 and 18 carbon fatty acids with variable amounts of 18:2n-6 and 18:3n-3 depending on diet. Unsaturation of fat converges near 70% during migration, and unsaturated fatty acids are preferentially mobilized and oxidized, making them good fuel. Twenty and 22 carbon n-3 and n-6 polyunsaturated fatty acids (PUFA) may affect membrane function and peroxisome proliferator-activated receptor signaling. However, evidence for dietary PUFA as doping agents in migratory birds is equivocal and requires further study.

Metabolic plasticity mediates differential responses to spring and autumn migrations: Evidence from gene expression patterns in migratory buntings

“…How, at the molecular level, the liver, muscles and adipose tissues meet the differential energy requirements of different seasonal states in migrants is not well understood. This is despite the fact that striking changes in the metabolism and energy homeostasis associated with nocturnal flight have long been known in migratory songbirds (Agatsuma & Ramenofsky, ; Guglielmo, ; Ramenofsky et al., ). Recently, however, we reported molecular differences in the hypothalamic regulation in parallel with differences in the seasonal migration phenotype ( Zugunruhe was higher in spring than in the autumn migratory state) in night‐migratory red‐headed buntings (Sharma et al., ).…”

The role of humidity and metabolic status on lean mass catabolism in migratory Swainson's thrushes (Catharus ustulatus)

“…Birds are capable of amazing feats of endurance exercise, with some species completing non-stop flights covering up to 11 000 km over 9 days [1]. Endurance exercise in birds is primarily powered by fat metabolism, given the high energy density and storage capacity of fat versus other fuels, and birds have adapted a suite of seasonal physiological adjustments to provide fat at a sufficiently high rate to match the metabolic demands of flight [2][3][4][5]. However, there is substantial evidence that birds also catabolize non-fat components before and during migratory flight, even while they maintain adequate fat loads [6][7][8][9][10][11][12][13].…”

Animal migration research takes wing

“…Completing such a journey requires unusual capacities for energy storage, mobilization, transport and utilization. Indeed, many long-distance migrants are supreme endurance athletes; some birds, for example, are able to sustain exercise at 10-14 times their basal metabolic rate for more than a week [21].…”

“…Fatty acids are ideal for storing energy, yet diffi cult to transport and metabolize. Recently, biochemical adaptations have been unraveled that enable fatty acids to fuel migration [21]. A related question is whether some migrants engage in 'natural doping' -preferentially selecting favorable foods that enhance migratory performance [22].…”