Intraspecific scaling and early life history determine the cost of free-flight in a large beetle (Batocera rufomaculata)

The scaling of the energetic cost of locomotion with body mass is well documented at the interspecific level. However, methodological restrictions limit our understanding of the scaling of flight metabolic rate (MR) in free-flying insects. This is particularly true at the intraspecific level, where variation in body mass and flight energetics may have direct consequences for the fitness of an individual. We applied a ¹³C stable isotope method to investigate the scaling of MR with body mass during free-flight in the beetle Batocera rufomaculata. This species exhibits large intraspecific variation in adult body mass as a consequence of the environmental conditions during larval growth. We show that the flight-MR scales with body mass to the power of 0.57, with smaller conspecifics possessing up to 2.3 fold higher mass-specific flight MR than larger ones. Whereas the scaling exponent of free-flight MR was found to be like that determined for tethered-flight, the energy expenditure during free-flight was more than 2.7 fold higher than for tethered-flight. The metabolic cost of flight should therefore be studied under free-flight conditions, a requirement now enabled by the ¹³C technique described herein for insect flight.