Compensatory responses in lepidopteran larvae:: a test of growth rate maximisation

It is often claimed that some organisms maximise individual growth rates (within limits set by physiological and environmental constraints) whereas others do not. Adequate experimental protocol to test for these hypotheses has not been available. We proceeded from the idea that a maximiser should be unable to further improve its growth performance, i.e. there should be no adaptive plasticity in relative growth rates. In the case of an insect larva always maximising its weight gain, we should thus expect no increase in growth rate following a short-term environmental perturbation. Another prediction is that initial and final weights of a larval instar should be highly correlated. We applied this approach to test for the growth rate maximiser status of the geometrid moth Epirrita autumnata. Based on various lines of ecological evidence it had previously been suggested that this spring-feeding species is strongly selected for high growth rates, and may be growing at its maximum physiological potential. Contrary to expectations, the larvae responded to starvation treatments by a subsequent compensatory increase in their relative growth rates. Such a plasticity may indicate that the rates of gaining weight are not maximised even in a 'classical' example a time-constrained organism. This observation supports the idea that endogenous regulation of growth rates is widespread, and it remains to be shown if true maximisation ever exists in nature. In contrast, possibilities to compensate for adverse conditions experienced early in larval development appeared to be limited. This was indicated by the strong correlation between initial weight in the last instar, and pupal weight. Though consistent with our expectations, such an inability to compensate can hardly be ascribed to growth rate maximisation, as indicated by quantitatively similar patterns found in an unrelated species with different ecology, the butterfly Polygonia c-album.