Increased risk of predation as a cost of high growth rate: an experimental test in a butterfly.

1. Life history theoreticians have traditionally assumed that juvenile growth rates are maximized and that variation in this trait is due to the quality of the environment. In contrast to this assumption there is a large body of evidence showing that juvenile growth rates may vary adaptively both within and between populations. This adaptive variation implies that high growth rates may be associated with costs. 2. Here, I explicitly evaluate the often-proposed trade-off between growth rate and predation risk, in a study of the temperate butterfly, Pararge aegeria (L). 3. By rearing larvae with a common genetic background in different photoperiods it was possible to experimentally manipulate larval growth rates, which vary in response to photoperiod. Predation risk was assessed by exposing larvae that were freely moving on their host plants to the predatory heteropteran, Picromerus bidens (L.). 4. The rate of predation was significantly higher in the fast-growing larvae. An approximately four times higher relative growth rate was associated with a 30% higher daily predation risk. 5. The main result demonstrates a trade-off between growth rate and predation risk, and there are reasons to believe that this trade-off is of general significance in free-living animals. The results also suggest that juvenile development of P. aegeria is governed by a strategic decision process within individuals.