Associate Professor Butler University Indianapolis, Indiana, United States
Increased melanism is common among ectothermic animals as means to increase body temperature under low-temperature, but sunny, conditions. Seasonal melanism, a form of phenotypic plasticity, has been particularly well studied in temperate butterflies in the family Pieridae. In these pierids, the ventral hindwing surface is more heavily melanized in spring and autumn butterflies, which raises body temperature and therefore allows flight at relatively low ambient temperatures. However, wing traits on the distal dorsal surfaces are often less melanized in spring and fall, a pattern that is somewhat puzzling under a thermoregulation hypothesis. My objective was to test an alternative hypothesis, namely that the reduced melanism of these distal dorsal wing traits is due to resource-based tradeoffs. Theory predicts that traits involved in resource-based tradeoffs will be positively correlated when variation among individuals in resource acquisition is greater than variation among individuals in resource allocation strategies, and negatively correlated when variation in allocation is greater than variation in acquisition. Using both wild-caught Pieris rapae butterflies and experimental manipulations of rearing temperature and dietary tyrosine (a melanin precursor) in lab animals, I tested this hypothesis by analyzing correlations among and across groups that should differ in relative resource acquisition and allocation strategies.
In wild-caught butterflies, ventral hindwing melanism was negatively correlated with distal dorsal melanism when analyzed among all seasons (March through November), as would be predicted because of the high variation in allocation of melanin to the hindwing across seasons. However, within any given month these traits were usually positively correlated or uncorrelated thus also supporting the tradeoff hypothesis because within a given month temperatures vary less – and therefore variation in allocation to hindwing melanism also varies less. As with wild-caught butterflies, traits were negatively correlated in lab animals when analyzed across all temperature and dietary tyrosine levels. Traits were also negatively correlated when analyzed across temperature treatments (inducing high variation in melanism allocation) within a given tyrosine treatment (low resource acquisition variation), as expected. Within a given temperature treatment but across dietary resource treatments, however, the traits were positively or uncorrelated. This, too, is predicted because rearing under a common temperature reduces variation in allocation to melanism but manipulation of dietary tyrosine increases variation in resource acquisition among butterflies. These results may help explain counter-intuitive forms of seasonal plasticity in butterflies or in other organisms where plastic traits compete for common resources.