Effects of population density on life history traits in a moth: in search of proximate and ultimate explanations

Population density often modifies the phenotypes of the members of the population. Such density-dependent phenotypic plasticity can affect basic life history traits of the organisms. In insects, a frequently observed expression of such plasticity is the crowding response (CR), where individuals growing at high densities develop faster and attain lower final sizes compared to those at low densities. This plastic change qualitatively differs from the general stress response where lower final sizes are associated with longer development periods. The adaptive significance of CR, as well as the nature of the cues that trigger CR, remains poorly understood. We performed a series of experiments to identify proximate signals leading to CR in the geometrid moth Hypomecis atomaria, a species in which larvae reared in groups consistently pupate earlier and at lower weights than those reared in isolation. Our findings reveal that CR is also induced in complete darkness, suggesting that visual cues of high population densities do not play a decisive role. CR was triggered when the larvae were separated by a mesh barrier, preventing tactile interaction between them. The presence of heterospecific lepidopteran larvae also triggered CR, though to varying degrees. By contrast, neither the presence of dipteran insects in the rearing environment nor human-inflicted tactile stimulation affected the growth schedules of H. atomaria larvae. We conclude that CR is likely induced either by chemical signals or substrate-borne vibrations caused by other larvae. In any case, CR is not a highly specific response to high densities of conspecifics, nor is it a very general reaction to unspecific disturbances. This allows us to narrow down the set of potential adaptive explanations for the phenomenon.