Setting the pace of life: membrane composition of flight muscle varies with metabolic rate of hovering orchid bees.

Patterns of metabolic rate variation have been documented extensively in animals, but their functional basis remains elusive. The membrane pacemaker hypothesis proposes that the relative abundance of polyunsaturated fatty acids in membrane phospholipids sets the metabolic rate of organisms. Using species of tropical orchid bees spanning a 16-fold range in body size, we show that the flight muscles of smaller bees have more linoleate (%18 : 3) and stearate (%18 : 0), but less oleate (%18 : 1). More importantly, flight metabolic rate (FlightMR) varies with the relative abundance of 18 : 3 according to the predictions of the membrane pacemaker hypothesis. Although this relationship was found across large differences in metabolic rate, a direct association could not be detected when taking phylogeny and body mass into account. Higher FlightMR, however, was related to lower %16 : 0, independent of phylogeny and body mass. Therefore, this study shows that flight muscle membrane composition plays a significant role in explaining diversity in FlightMR, but that body mass and phylogeny are other factors contributing to their variation. Multiple factors are at play to modulate metabolic capacity, and changing membrane composition can have gradual and stepwise effects to achieve a new range of metabolic rates. Orchid bees illustrate the correlated evolution between membrane composition and metabolic rate, supporting the functional link proposed in the membrane pacemaker hypothesis.