Secondarily diurnal geckos return to cost of locomotion typical of diurnal lizards

Previous studies showed that nocturnal geckos evolved a low energetic cost of locomotion (C-min), which increases maximum aerobic speed and partially offsets the decrease in maximal oxygen consumption caused by activity at low nocturnal temperatures. Because the advantage of a low C-min should apply at high diurnal temperatures as well as at low nocturnal temperatures, I hypothesized that C-min remained low in geckos that have secondarily evolved diurnality. I measured C-min in two secondarily diurnal gecko species, Rhoptropus bradfieldi (4.7 g +/- 0.71 SE) and Phelsuma madagascariensis (23.9 g +/- 3.7 SE), during steady exercise on a treadmill and rejected the hypothesis that secondarily diurnal geckos retain the low C-min of their nocturnal ancestors. The C-min in R. bradfieldi (2.468 mL O-2 g(-1) km(-1) +/- 0.489 SE) and P. madagascariensis (1.389 mt O-2 g(-1) km(-1) +/- 0.119 SE) returned to values typical of ancestrally diurnal lizards. This suggests that there is a trade-off that outweighs the performance advantage of low C-min in a diurnal environment and that may cause an evolutionary association between C-min and activity time (diurnality/nocturnality).