Metabolic similarity despite striking behavioral divergence: aerobic performance in low- and high-density forms of the Mormon cricket.

Mormon crickets, large flightless katydids from western North America, occur in two forms that differ dramatically in population density and daily movement distances. The low-density form is small and cryptic and moves <1 m/d, while the high-density form is large and dark colored and travels up to 1-2 km/d in migratory bands. We determined daytime body temperatures and measured resting metabolic rate (RMR) and maximal aerobic metabolic rate (MMR) in forced exercise across a 10 degrees - 40 degrees C temperature range. Field body temperatures were 15 degrees - 20 degrees C in the morning and 25 degrees - 35 degrees C during most of the day, and they never exceeded 40.6 degrees C in either form. Mass-adjusted RMR and MMR were positively correlated across temperatures (significantly in some comparisons), indicating repeatability. Similarly, RMR was always positively and sometimes significantly correlated with MMR, suggesting a functional linkage between minimal and maximal aerobic performance. Factorial aerobic scopes (MMR/RMR) were highest at 10 degrees C and declined at higher temperatures, but absolute scope (MMR - RMR) was highest between 30 degrees and 40 degrees C. Given the ca. 1,000-fold contrast in daily movement distances, we expected higher MMR and aerobic scope in the migratory high-density form. However, there were no differences between forms in RMR, MMR, aerobic scope, or ventilation patterns. The forms were also similar in metabolic response to temperature (Q(10)) and in the mass scaling of metabolic rate. The absence of metabolic divergence among low- and high-density forms shows that large differences in locomotor behavior may not require concomitant changes in aerobic physiology.