Energy metabolism of male and female tarantulas (Aphonopelma anax) during locomotion.

We examined aerobic performance traits in male and female tarantulas (Aphonopelma anax). Reproductive fitness in these males relies heavily on locomotory searching to locate receptive females, which are fossorial and sedentary. Because of this dimorphism in life history, we predicted that selection in males would enhance their ability to sustain high levels of aerobic metabolism (compared with females) to support increased locomotory activity during the mating season. Rates of carbon dioxide production were measured in an enclosed variable-speed treadmill. Steady-state rates of carbon dioxide production increased linearly within the range of sustainable aerobic speeds for both males and females. Although there was substantial variation in physiological performance traits among individuals, there were no detectable intersexual differences in maximal rates of carbon dioxide production, maximal aerobic speed, minimum transport or factorial scope.