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

Abstract

Bergmann’s Rule describes an increase in the body size of endothermic animals with decreasing environmental temperatures. However, in ectothermic insects including moths, some of the few existing studies investigating size patterns along temperature gradients do not follow the Bergmann’s Cline. Intraspecific differences in moth sizes along spatiotemporal temperature gradients are unknown from the Palaeotropics, hindering general conclusions and understanding of the mechanism responsible. We measured intraspecific forewing size differences in 28 Afrotropical moth species sampled in 3 seasons along an elevational gradient on Mount Cameroon, West/Central Africa. Size increased significantly with elevation in 14 species but decreased significantly in 5 species. Additionally, we found significant inter-seasonal size differences in 21 species. Most of these variable species had longer forewings in the transition from the wet to dry season, which had caterpillars developing during the coldest part of the year. We conclude that environmental temperature affects the size of many Afrotropical moths, predominantly following prevailingly following Bergmann’s Cline. Nevertheless, the sizes of one-third of the species demonstrated a significant interaction between elevation and season. The responsible mechanisms can thus be assumed to be more complex than a simple response to ambient temperature.

The thermal breadth of temperate and tropical freshwater insects supports the climate variability hypothesis

“…This relationship could be a consequence of cooler temperatures at the temperate location (Figure 2) leading to phenotypic plasticity, as described by the Temperature‐Size Rule (Kingsolver & Huey, 2008) and documented in Ephemeroptera (Atkinson, 1994; Sweeney et al., 2018). Additionally, natural selection might cause populations and species to be larger in the cooler temperate location than the warmer tropical location, consistent with Bergmann's Rule (although with many counter‐examples, Kingsolver & Huey, 2008; Papandreou et al., 2023). Thus, whether a large size in the temperate location is a cause of wide T br or an effect of climate that may then alter thermal tolerance in an exposure‐duration‐dependent manner (Peralta‐Maraver & Rezende, 2021), will require further investigation.…”

“…Additionally, natural selection might cause populations and species to be larger in the cooler temperate location than the warmer tropical location, consistent with Bergmann's Rule (although with many counter-examples, Kingsolver & Huey, 2008;Papandreou et al, 2023). Thus, whether a large size in the temperate location is a cause of wide T br or an effect of climate that may then alter thermal tolerance in an exposure-duration-dependent manner (Peralta-Maraver & Rezende, 2021), will require further investigation.…”