Luc Lens

Stabilizing selection on blue tit fledgling mass in the presence of sparrowhawks

Like British great tits, Belgian blue tits have a lower winter body mass when sparrowhawks are present. Since body mass affects manoeuvrability in small birds, tits may balance the risks of starvation and the risk of hawk predation by varying the amount of extra fat carried during winter. Predation pressure by sparrowhawks on young and inexperienced fledglings is at least as intense as that on the adults during winter. We therefore expected that tit fledgling body mass could also be reduced in the presence of sparrowhawks. In the years after one pair of sparrowhawks settled in a study plot, the mean body mass of blue tit fledglings was lower compared with that in years when there were no sparrowhawks. Furthermore, the shape of the curve relating juvenile survival to fledging mass changed, because the survival of the heaviest fledglings was reduced, which altered the selection differential of juvenile survival as a function of body mass from directional to stabilizing. Of seven published studies on the fledgling body mass–survival relation in tits, all three of the studies conducted in the absence of sparrowhawks showed the highest survival rates for the heaviest young, whereas in all four studies with sparrowhawks present this was no longer the case.

THE COST OF MELANIZATION: BUTTERFLY WING COLORATION UNDER ENVIRONMENTAL STRESS

Abstract Evolutionary studies typically focus on adaptations to particular environmental conditions, thereby often ignoring the role of possible constraints. Here we focus on the case of variation in dorsal wing melanization in a satyrine butterfly Pararge aegeria. Because melanin is a complex polymer, its synthesis may be constrained if ambient conditions limit the resource budget. This hypothesis was tested by comparing melanization among butterflies that fed as larvae on host grasses experiencing different drought-stress treatments. Treatment differences were validated both at the level of the host plant (nitrogen, carbonate, and water content) and of the butterfly (life-history traits: survival, development time, and size at maturity). Melanization rate was measured as average gray value of the basal dorsal wing area. This area, close to the thorax, is known to be functionally significant for basking in order to thermoregulate. Individuals reared on drought-stressed host plants developed paler wings, and development of darker individuals was slower and less stable as estimated by their level of fluctuating asymmetry. These results provide evidence that melanin is indeed costly to synthesize, and that differences in environmental quality can induce phenotypic variation in wing melanization. Therefore, studies dealing with spatial and/or temporal patterns of variation in wing melanization should not focus on adaptive explanations alone, but rather on a cost-benefit balance under particular sets of environmental conditions.

Larval food stress differentially affects flight morphology in male and female speckled woods (Pararge aegeria)

Abstract 1.u2002Adaptive plasticity in flight morphology can be of great importance for organisms, in order to deal with changing environments. When different demands are imposed to this morphology, different responses to environmental changes can be expected.