EVOLUTION OF COLOR PATTERN AND PUBESCENCE CHARACTERISTICS IN MALE BUMBLEBEES: AUTOMIMICRY VS. THERMOREGULATION.

Much of the research on the adaptive nature of insect color patterns has been concerned with the importance of crypsis (Kettlewell, 1961; Ricklefs and O'Rourke, 1975), the selective advantages associated with aposematic coloration and associated mimicry (Brower, 1958a, b, c; Wickler, 1968) and the adaptive nature of thermal colors for body temperature regulation under different environmental conditions (Edney, 1971; Hamilton, 1971). In the bumblebees, pubescence color pattern has been studied extensively as color patterns have been used by taxonomists, with variable success, to distinguish closely related species (Franklin, 1912, 1913; Stephen, 1957; Milliron, 1971, 1973a, b). Female bumblebees, as with many other hymenoptera, possess a sting which serves as an effective predator deterrent and bumblebees are considered aposematically colored to warn potential predators of their noxious characteristics. Brower et al. (1960) showed that toads (Bufo terrestris), once having been stung by a bumblebee, would avoid subsequent encounters with bumblebees or their Asilid fly mimics, but that naive toads freely ate bumblebees from which the sting had been removed. The numerous species of flies (many within the family Syrphidae) which exhibit extremely close mimicry of bumblebees in both color pattern and behavior, attest to the effectiveness of the protection afforded these palatable species through Batesian mimicry. The haplo-diploid sex determination (Dzieron's rule) in bumblebees yields males which are genetically similar to their mothers. The males, however, lack the defensive characteristics of the females. There is a potential selective advantage for males to look like the relatively noxious female bumblebees. They lack the defense mechanism of the females but have the genetic potential to look like the females. From these facts one can predict that sexual automimicry, in which the relatively harmless males resemble the relatively harmful females, may be important. Automimicry has been documented for Monarch butterflies (Brower, 1970). Larvae raised on food plants of the genus Asclepias containing cardiac glycosides were unpalatable to birds whereas larvae raised on Asclepias which did not contain cardiac glycosides were highly palatable to birds. This paper was stimulated by the fact that male and female bumblebees are not always similar in the characteristics of their pubescence. This situation could make the males more vulnerable to attack by experienced predators that had already encountered females of the same bumblebee species. The potential that counterselective agents are responsible for these dimorphisms is suggested by the fact that many of the Syrphid-fly mimics of worker bumblebees closely resemble the workers in pubescence characteristics; more than do the male bees of some species (Waldbauer and Sheldon, 1971; pers. observ.). To make predictions about the nature of the selective pressures affecting these dimorphisms, it is important to understand the differences in behavior of male and female bumblebees. Queen and worker bumblebees forage for nectar and pollen during the day and the majority of them return to the colony at night. These colonies are usually located at the surface or underground, often in old mouse nests