Evolution of juvenile growth rates in female guppies (Poecilia reticulata): predator regime or resource level?

Abstract

Recent theoretical and empirical work argues that growth rate can evolve and be optimized, rather than always being maximized. Chronically low resource availability is predicted to favour the evolution of slow growth, whereas attaining a size-refuge from mortality risk is predicted to favour the evolution of rapid growth. Guppies (Poecilia reticulata) evolve differences in behaviour, morphology and life-history traits in response to predation, thus demonstrating that predators are potent agents of selection. Predators in low-predation environments prey preferentially on small guppies, but those in high-predation environments appear to be non-selective. Because guppies can outgrow their main predator in low- but not high-predation localities, we predict that predation will select for higher growth rates in the low-predation environments.However, low-predation localities also tend to have lower productivity than high-predation localities, yield-ing the prediction that guppies from these sites should have slower growth rates. Here we compare the growth rates of the second laboratory-born generation of guppies from paired high- and low-predation localities from four different drainages. In two out of four comparisons, guppies from high-predation sites grew significantly faster than their low-predation counterparts. We also compare laboratory born descendants from a field introduction experiment and show that guppies introduced to a low-predation environment evolved slower growth rates after 13 years, although this was evident only at the high food level. The weight of the evidence suggests that resource availability plays a more important role than predation in shaping the evolution of growth rates.

Figure 1

Mean growth rates and standard deviations for (a) south slope and (b) north slope populations. Growth rates are the slope estimated by regression of ln(mass) on age. Symbols represent a given drainage within each comparison. Open symbols are the upstream (low predation) populations, filled symbols are the downstream (high predation) populations. There was an average of seven females in each treatment in the south slope study and 15 females in each treatment in the north slope study.

Figure 2

Mean growth rates and standard deviations in the El Cedro and Aripo introduction experiments. Open bars are the upstream (low-predation) introduced population, filled bars are the downstream (high-predation) source population. Numbers in bars are samples sizes. (a) Results for El Cedro experiment at 4 and 13 years after introduction, where guppies were tested at two food levels. In both cases, the bars to the left are the lower food level. Downstream guppies grew significantly faster at high food levels in the 13 year assay. (b) Results for El Cedro 7 years and Aripo 11 years after introduction. In these assays, guppies were tested at a single food level. No differences among populations were detected.