Out of the tropics: Macroevolutionary size trends in an old insect order are shaped by temperature and predators

AimGlobal interspecific body size distributions have been suggested to be shaped by selection pressures arising from biotic and abiotic factors such as temperature, predation and parasitism. Here, we investigated the ecological and evolutionary drivers of global latitudinal size gradients in an old insect order. LocationGlobal. TaxonOdonata (dragonflies and damselflies). MethodsWe compiled data on interspecific variation in extant and extinct body sizes of Odonata, using an already existing database (The Odonate Phenotypic Database) and fossil data (The Paleobiology Database). We combined such body size data with latitudinal information and data on biotic and abiotic environmental variables across the globe to investigate and quantify interspecific latitudinal size-gradients ("Bergmann's Rule") and their environmental determinants. We used phylogenetic comparative methods and a global published phylogeny of Odonata to address these questions. ResultsPhylogenetic comparative analyses revealed that global size variation of extant Odonata taxa is negatively influenced by both regional avian diversity and temperature, with larger-bodied species in the suborder Anisoptera (dragonflies) showing a steeper size-latitude relationship than smaller-bodied species in the suborder Zygoptera (damselflies). Interestingly, fossil data show that the relationship between wing size and latitude has shifted: latitudinal size trends had initially negative slopes but became shallower or positive following the evolutionary emergence and radiation of birds. Main ConclusionsThe changing size-latitude trends over geological and macroevolutionary time were likely driven by a combination of predation from birds and maybe pterosaurs and high dispersal ability of large dragonflies. Our study reveals that a simple version of Bergmann's Rule based on temperature alone is not sufficient to explain interspecific size-latitude trends in Odonata. Our results instead suggest that latitudinal size gradients were shaped not only by temperature but also by avian predators, potentially driving the dispersal of large-sized clades out of the tropics and into the temperate zone.