Evolution of physiological performance in the Trinidadian guppy (Poecilia reticulata: Peters)

I examined guppy (Poecilia reticulata) physiological performance in an evolutionary context. Guppies are small live-bearing fish native to Trinidad's mountain streams. Guppy populations differ in their mortality rates and life histories as a function of their predator communities. “High-predation” guppies were hypothesized to have increased burst performance. “Low predation” guppies were hypothesized to have increased aerobic performance. To control for environmental effects on phenotype, I measured aerobic and burst performance in laboratory-reared guppies. I explored the underlying morphological and enzymatic correlates of performance. I also examined the effect of stream predators on guppy microhabitat use in the field.; Because habitat use may impact physiological performance, I measured stream velocity, depth and guppy presence/absence in 13 streams in Trinidad. Guppies preferred the slowest moving microhabitat in all streams. Preferred stream velocity was similar across all streams, but “low predation” guppies used deeper microhabitat. Focal behavioral observations indicated guppies were more active in “low predation” habitats.; V˙O_2max and burst speed were measured in six “common-garden” populations of guppies. Predation did not explain population differences in performance. Aerobic performance was more variable among populations of male guppies, while burst speed was more variable among populations of females. Both performance measures were significantly repeatable, suggesting a genetic basis for the observed variation. Burst performance and V˙O_2max were uncorrelated when corrected for body size.; I measured the mass of gills, heart and muscle, and the activity of three muscle enzymes (CS, LDH, and myofibrillar ATPase) from all guppies used in the performance trials. In both sexes, heart and gill residuals were negatively associated with CS and LDH activity. Heart and gill residuals scaled positively with V˙O_2max in males but not females. The morphological and enzymatic variables did not correlate with burst performance in either sex. Populations differed in their morphology and enzymatic activity, but selection for life-history traits did not explain these differences. These results suggest that stream predators are not driving genetic differences in guppy physiology, even though they have a strong impact on guppy life-history traits.