Seasonal hypothermia in a large migrating bird: Saving energy for fat deposition?

We have tested the hypothesis that a large (2 kg) migratory bird, such as t he barnacle goose Branta leucopsis, becomes hypothermic before its autumn m igration, when food is not scarce, but when it is necessary to conserve and /or store energy in the form of fat. Abdominal temperature (T-ab) was measu red in wild geese using an implanted data logger, Commencing a few days bef ore and continuing until approximately 20 days after the start of their aut umn migration, mean daily T-ab fell progressively by 4.4 degreesC. Thus, it is suggested that, rather than increasing the rate of pre-migratory fatten ing, the energy saved as a result of this hypothermia reduces the rate at w hich fat is used and thus enables its more rapid replacement following and, possibly during, migration, The energy saved may also be used for the repl acement of nonfat tissues such as the locomotory muscles and gastrointestin al tract. These observations are the first of their kind from birds in thei r natural environment and, together with other data, demonstrate that hypot hermia in endotherms is not necessarily related to extremely low environmen tal temperature, to shortage of food or to the resting phase of the daily c ycle, The data also highlight the relationship between hypothermia and fat deposition over extended periods in relatively large, endothermic animals a nd mag have some relevance to obesity in humans.