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

Nicolas Beerli,Jan Beck,Ian J Kitching,Florian Bärtschi,Liliana Ballesteros‐Mejia

doi:10.1111/jbi.13583

Abstract

AbstractAimWe mapped the geographical pattern of body sizes in sphingid moths and investigated latitudinal clines. We tested hypotheses concerning their possible environmental control, that is, effects of temperature (negative: temperature size rule or Bergmann's rule; positive: converse Bergmann rule), food availability, robustness to starvation during extreme weather and seasonality.LocationOld World and Australia/Pacific region.MethodsBody size data of 950 sphingid species were compiled and related to their distribution maps. Focusing on body length, we mapped the median and maximum size of all species occurring in 100 km grid cells. In a comparative approach, we tested the predictions from explanatory hypotheses by correlating species' size to the average environmental conditions encountered throughout their range, under univariate and multivariate models. We accounted for phylogeny by stepwise inclusion of phylogenetically informed taxonomic classifications into hierarchical random‐intercept mixed models.ResultsMedian body sizes showed a distinctive geographical pattern, with large species in the Middle East and the Asian tropics, and smaller species in temperate regions and the Afrotropics. Absolute latitude explained very little body size variation, but there was a latitudinal cline of maximum size. Species' median size was correlated with net primary productivity, supporting the food availability hypothesis, whereas support for other hypotheses was weak. Environmental correlations contributed much less (i.e. <10%) to explaining overall size variation than phylogeny (inclusion of which led to models explaining >70% of variability).Main conclusionThe intuitive impression of larger species in the tropics is shaped by larger size maxima. Median body sizes are only very weakly related to latitude. Most of the geographical variation in body size in sphingid moths is explained by their phylogenetic past. NPP and forest cover correlate positively with the body size, which supports the idea that food availability allowed the evolution of larger sizes.

Highlights

Tropical insects of impressively large body size compared to their temperate counterparts are a common observation of field naturalists, as well as those admiring museum collections – starting with A. v
It remains unclear whether such observations necessarily imply a larger size for the average tropical insect species – it just may be that their maxima are more extreme in regions of higher species richness, and it is these that capture our attention
Species in the Arabian Peninsula, in South Asia and in the Indo-Australian tropics are distinctively larger than temperate as well as Afrotropical taxa

Summary

Introduction

Tropical insects of impressively large body size compared to their temperate counterparts are a common observation of field naturalists, as well as those admiring museum collections – starting with A. v. Humboldt’s scientific explorations of the Neotropics around 1800, on which he collected insects of appreciable size (Barragán et al, 2009). Reported accidentally shooting hawkmoths so large he mistook them for small birds. It remains unclear whether such observations necessarily imply a larger size for the average tropical insect species – it just may be that their maxima are more extreme in regions of higher species richness, and it is these that capture our attention (cf coloration in tropical birds; Bailey, 1978). In a comparative approach we investigate how environmental conditions and phylogenetic inertia may have shaped these patterns

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