ADAPTIVE VARIATION IN GROWTH-RATE - LIFE-HISTORY COSTS AND CONSEQUENCES IN THE SPECKLED WOOD BUTTERFLY, PARARGE AEGERIA

An important assumption made in most life-history theory is that there is a trade-off between age and size at reproduction. This trade-off m ay, however, disappear if growth rate varies adaptively. The fact that individuals do not always grow at the maximum rate can only be unders tood if high growth rates carry a cost. This study investigates the pr esence and nature of such costs in Pararge aegeria. Five females from two populations with known differences in life history (south Sweden a nd Madeira) were allowed to oviposit in the laboratory and their offsp ring were reared in environmental chambers under conditions leading to direct development. We measured several aspects of life history, incl uding development times, pupal and adult weights, growth rate, female fecundity, longevity and larval starvation endurance. In both populati ons there seemed to be genetic variation in growth rate. There was no evidence for a trade-off between age and size at pupation. As predicte d, larvae with high growth rates also lost weight at a relatively high er rate during starvation. High weight-loss rates were furthermore ass ociated with a lower probability of surviving when food became availab le again. This is apparently the first physiological trade-off with gr owth rate that has been experimentally demonstrated. In both populatio ns there were significant differences in growth rate between the sexes , but the populations differed in which sex was growing at the highest rate. In Sweden males had higher growth rates than females, whereas t he reverse was true for Madeira. These patterns most likely reflect di fferences in selection for protandry, in turn caused by differences in seasonality between Sweden and Madeira. Together with the finding tha t males had shorter average longevity than females in the Swedish, but not in the Madeiran, population, this indicates that a lower adult qu ality also may be a cost of high growth rate. We argue that for the un derstanding of life history variation it is necessary to consider not only the two dimensions of age and size, but also to take into full ac count the triangular nature of the relationship between size, time and growth rate.