Moth body size increases with elevation along a complete tropical elevational gradient for two hyperdiverse clades

Abstract

The body size of an animal is probably its most important functional trait. For arthropods, environmental drivers of body size variation are still poorly documented and understood, especially in tropical regions. We use a unique dataset for two speciesrich, phylogenetically independent moth taxa (Lepidoptera: Geometridae; Arctiinae), collected along an extensive tropical elevational gradient in Costa Rica, to investigate the correlates and possible causes of body-size variation. We studied 15 047 specimens (794 species) of Geometridae and 4167 specimens (308 species) of Arctiinae to test the following hypotheses: 1) body size increases with decreasing ambient temperature, as predicted by the temperature-size rule; 2) body size increases with increasing rainfall and primary productivity, as predicted from considerations of starvation resistance; and 3) body size scales allometrically with wing area, as elevation increases, such that wing loading (the ratio of body size to wing area) decreases with increasing elevation to compensate for lower air density.To test these hypotheses, we examined forewing length as a proxy for body size in relation to ambient temperature, rainfall, vegetation index and elevation as explanatory variables in linear and polynomial spatial regression models. We analysed our data separately for males and females using two principal approaches: mean forewing length of species at each site, and mean forewing length of complete local assemblages, weighted by abundance.Body size consistently increased with elevation in both taxa, both approaches, both sexes, and also within species. Temperature was the best predictor for this pattern (-0.98 < r < -0.74), whereas body size was uncorrelated or weakly correlated with rainfall and enhanced vegetation index. Wing loading increased with elevation. Our results support the temperature-size rule as an important mechanism for body size variation in arthropods along tropical elevational gradients, whereas starvation resistance and optimization of flight mechanics seem to be of minor importance.

How has the environment shaped geographical patterns of insect body sizes? A test of hypotheses using sphingid moths

“…This is because randomly distributed errors will even out, and correlation analyses will provide reliable results. We confirmed this theoretical expectation with subsampling simulations based on moth body size data from Brehm et al (; J. Beck & G. Brehm, unpublished data). Furthermore, we assessed the size variability in eight abundant sphingid species, where the body lengths of 208 specimens (14–34 per species) were measured from scans of collection drawers (Johnson, Mantle, Gardner, & Backwell, ; Trueman & Yeates, ; data in ES1).…”

“…Our results are, at least in part, in conflict with some recent, more localized studies on the body sizes of Lepidoptera and other taxa. Brehm et al () reported a size increase with elevation in Costa Rica moths, which was best explained by a negative effect of temperature (cf. Bergmann, TSR) but not by an effect of productivity.…”

“…tribes Smerinthini, Ambulycini, where males tend to be smaller than females; Kitching & Cadiou, ), we measured and compared only males (the more common sex in collections) for consistency. In other Lepidoptera taxa, size measures for males and females are highly correlated (data from Brehm et al, , see there also for further discussion). Furthermore, we would note that any resulting size underestimates within those clades' size would be controlled for by fitted‐random effects models for phylogenetic association (in mixed models, see below).…”

Moth assemblages in Costa Rica rain forest mirror small‐scale topographic heterogeneity

“…Many Arctiinae species are chemically well defended and embedded in complex mimicry rings (Conner, ). Finally, the Geometridae represent another highly speciose clade of moths, but they are mostly more slender and small‐ to medium‐sized insects (Brehm, Zeuss & Colwell, ), whose larvae never have a dense hair cover (Minet & Scoble, ). In contrast to Arctiinae, chemical defense, aposematism, and mimicry are rare among geometrids.…”

Size‐related life‐history traits in geometrid moths: a comparison of a temperate and a tropical community

“…The strong overrepresentation of the Larentiinae in the temperate region, combined with the small size of these moths, strongly contributed to this pattern. Moreover, a recent study (Brehm et al, 2018) showed that altitudinal differences may cause body sizes to vary considerably at relatively small geographical scales. For Ennominae taken separately, the difference was 1.05 (Fig.…”