Odell, J.P.*; Chappell, M.A.: Variation in aerobic capacity within and among “common-garden” populations of Trinidadian guppies (Poecilia reticulata) from different predation regimes.

We measured maximal oxygen uptake rates (VO_2max) in laboratory-reared F2 and F3 offspring of Trinidadian guppies from four different streams. These streams represent habitats that differ in predation intensity and flow characteristics, and contain guppies that have evolved significantly different life histories. Guppies were exercised in a small closed chamber at increasing speeds until they began sprinting. Control experiments demonstrated that the exercise protocol caused elevated whole-animal lactate levels, and did not cause hypoxic conditions in the chamber. We found that VO_2max in guppies was a repeatable trait when measured over one-week and three month intervals, although the correlation between measurements decreased over time. Within population variation in VO_2max ranged between 10 and 13% when the effect of body mass was removed by regression. Within streams, aerobic performance did not differ between the sexes. Male performance was not significantly different among locales; however, female performance was highly variable among streams. In addition, our data suggest a significant effect of stream predation level on female performance. Elucidating the evolutionary forces for the observed differences in female performance requires further study of more stream pairs. In light of recent studies showing rapid evolution of life-history traits in wild guppies, it is of note that this selection regime does not appear to affect male aerobic performance.