Premigratory fat metabolism in hummingbirds: A Rumsfeldian approach

Hummingbird migration is a remarkable feat, given the small body sizes of migratory species, their high metabolic rates during flight and the long distances traveled using fat to fuel the effort. Equally remarkable is the ability of premigratory hummingbirds in the wild to accumulate fat, synthesized from sugar, at rates as high as 10% of body mass per day. This paper summarizes, using Rumsfeldian terminology, "known knowns" concerning the energetics of hummingbird migration and premi- gratory fattening. Energy metabolism during hover-feeding on floral nectar is fueled directly by dietary sugar through the path- way recently named the "sugar oxidation cascade". However, flight without feeding for more than a few minutes requires shifting to fat as a fuel. It is proposed that behavior and metabolic fuel choice are coadapted to maximize the rate of fat deposition during premigratory fattening. The hummingbird liver appears to possess extraordinarily high capacities for fatty acid synthesis. The analysis of "known knowns" leads to identification of "known unknowns", e.g., the fates of dietary glucose and fructose, the regulation of fat metabolism and metabolic interactions between liver and adipose tissue. The history of science behooves recog- nition of "unknown unknowns" that, when discovered serendipitously, might shed new light on fundamental mechanisms as well as human pathological conditions (Current Zoology 59 (3): 371-380, 2013).