One of the perks of being a biologist is the opportunity to travel to far-flung places to study animals in their exotic homes. So when Charles Darveau, Raul Suarez and Kenneth Welch found themselves in the depths of a jungle in Panama collecting reams of information on orchid bees, it was all in a day's work. Their mission, Darveau recounts, was to `explain the variation in metabolic rate' seen in these brightly coloured tropical insects (p. 3581 and p. 3593). The team's interest was piqued by reports indicating that insect body size, metabolic rate and wingbeat frequency are all linked. They decided to investigate how insect metabolic rates scale with body size across species. Selecting the multitude of closely related orchid bee species as their subjects, they flew to the Smithsonian Tropical Research Institute in Panama, where David Roubik joined the team. With Roubik's field expertise, the team soon tracked down around 30 orchid bee species. They measured the bees' wingbeat frequencies, metabolic rates, body sizes, wing sizes and wing loading, a measure of the body mass supported by a given wing area. Collecting DNA samples from 32 orchid bee species, Darveau constructed a molecular phylogeny so the team could account for the bees' shared evolutionary past. When they examined the relationships between all these variables, they discovered that wing loading explains most of the variation they saw in wingbeat frequency, and this in turn is highly correlated with metabolic rate. In other words, wing morphology explains more of the differences in metabolic rate among these bee species than body size alone.
The team then turned its attention to energetics on a much smaller scale, delving into the effect of body mass on the design of energy production pathways. They had already found that orchid bees fuel their flight entirely with glucose, so they were keen to examine enzymes involved in glucose oxidation. To see how biochemical pathways have evolved in relation to metabolic rate, they plotted flight muscle enzyme activity (a measure of the amount of enzyme) against metabolic rate for 28 orchid bee species. To their surprise, they discovered that only one enzyme is highly correlated with metabolic rate: hexokinase, which catalyzes the reaction allowing glucose to enter the glycolytic pathway. Suarez explains that the control of flux (the rate at which a metabolic pathway runs) is thought to be distributed among many enzyme-catalyzed reactions, and not restricted to just one `rate-limiting' step near the start of a pathway. `So why has evolution targeted hexokinase as the enzyme that varies across species in the same manner as metabolic rate?', Suarez wonders.
More studies are required to fully answer this question, but the team's results suggest that the evolution of orchid bee body size, flight metabolic rate and hexokinase activity are all closely correlated. For example, they saw that tiny bees need to beat their wings 250 times per second in order to hover, but big bees only have to beat their wings 80 times per second to perform the same aerodynamic trick. Surprisingly, the team spotted that hexokinase activity follows exactly the same scaling pattern as the one for wingbeat frequency. Darveau concludes that `examining the connection between wing design and the design of biochemical pathways might help us understand the evolution of insect flight energetics.'
- © The Company of Biologists Limited 2005