Aposematic Coloration of Moths Decreases Strongly along an Elevational Gradient in the Andes

Abstract

On tropical mountains, predation pressure decreases with elevation. Accordingly, one expects an elevational decay in the prevalence of costly defensive traits such as aposematic coloration. Using light-trap catches of Arctiinae moths (353 species, 4466 individuals), assembled along a forested gradient in the megadiverse tropical Andes of southern Ecuador, we show that the incidence of aposematic coloration decreases strongly between 1040 and 2670 m asl. While over 60% of Arctiinae moths were warningly colored at lowest sites, this fraction decreased to less than 20% in montane forest, yet increased slightly again at the highest sites in the very open Purdiaea nutans forest. In parallel, the incidence of hymenopteran mimics and of species that mimic chemically defended beetles decreased with elevation. Hymenopteran mimics accounted for less than 5% of Arctiinae moths at sites above 2100 m, and beetle mimics were essentially lacking at high elevations. These patterns coincide with a change in gross taxonomic composition of Arctiinae ensembles and with an increase in average body size towards higher elevations. Representatives of Euchromiina and Ctenuchina became scarce with altitude, whereas the prevalence of Lithosiinae increased. Our findings suggest that the variable selective pressures along the elevational gradient favor warning coloration primarily at lower sites, whereas cryptic appearance of adult moths dominates in the tropical upper montane forest.

Larger insects in a colder environment? Elevational and seasonal intraspecific differences in tropical moth sizes on Mount Cameroon

“…Our results are consistent with results affirming Bergmann's Clines in moths from other biogeographic areas (Miller 1974(Miller , 1991Sullivan & Miller 2007;Brehm et al 2019;Fiedler & Brehm 2021;Mungee et al 2021), as well as for some other insects (Shelomi 2012). Although some adult moths, such as sphingids, can increase their temperature by vibrating their wing muscles (Heinrich 2013), we do not expect the relationship between body surface area and thermal metabolism as predicted by Bergmann (1847).…”

“…Studies of elevational patterns in insect body sizes have predominantly focused on interspecific patterns in communities, with both positive and negative relationships between body size and elevation detected (Shelomi 2012). Body size and elevation have been positively correlated in geometrid and arctiine moths in the Costa Rican mountains (Brehm et al 2019), arctiine moths in the Ecuadorian Andes (Fiedler & Brehm 2021), and macromoths in the Swiss Alps (Beck et al 2016). However, no significant relationship between elevation and geometrid body size was found in the Ecuadorian Andes (Brehm & Fiedler 2004).…”

Clearwing butterflies challenge the thermal melanism hypothesis

“…Thus, transparency may incur a cost on aposematism, with a trade-off between crypsis and aposematism. A possible explanation for the unexpected altitudinal distribution of transparency in Ithomiini species may be different predation pressures at different altitudes in tropical mountains (44), which may favor crypsis over aposematism at higher altitudes, as observed in Andean arctiine moths or which conspicuousness decreases with height (45).…”