Experimental analyses of body size, flight and survival in pierid butterflies

To explore the consequences of allocation of body mass for flight and fitne ss in butterflies, we experimentally increased body weight and measured the effects on flight activity and survival in the field for two co-occurring pierid species (Pontia occidentalis and Colias philodice) that differ in pa latability, mass allocation and Bight speed. We predicted that increasing b ody weight would reduce flight activity in both Pontia and Colias, but that reductions in survival would be greater in the more palatable Colias than in the less palatable Pontia. Behavioural observations during three mark-re capture experiments showed that weighted butterflies hew less frequently th an control butterflies in both Pontia and Colias. Adding weights significan tly reduced survival in one study with Pontia, but not in a second study. C ontrary to our predictions, adding weights had no significant effects on su rvival in the more palatable Colias. In combination with previous experimen tal manipulations of wing area, these results support the hypothesis that f light muscle ratio - the ratio of thoracic flight muscle to total body mass - is an important determinant of flight frequency in these species. Howeve r, the results do not indicate that flight muscle ratio is important for pr edator escape by these butterflies at our study sites. Different aspects of body size and shape may affect different components of butterfly flight, a nd seasonal variation in predation at our sites may modulate any relationsh ip between flight and survival.