Explaining global body size variation in dragonflies and damselflies: temperature or predators?

Global body size distributions in dragonflies and damselflies are shaped by temperature and predators

Above: A model replica of a fossil dragonfly (Urogomphus giganteus) in Museum für Naturkunde (Berlin) that lived about 140 million years ago. Dragonflies and damselflies have an unusually rich fossil record, compared to other insect groups. Photo: Erik Svensson.

Dragonflies and damselflies (Odonata) is an old and fascinating insect order, comprising about 6400 species globally. Although not one of the most diverse insect orders, these insects are wellknown to layperson and increasingly popular among amateur naturalists and photographers, due to their interesting behaviours and rich diversity in size, shape and colouration. It is also an insect group with a dramatic macroevolutionary history and a rich fossil record. The ancestors  of Odonata that existed about 300 million years before present, included the now extinct genus Meganeura, where some individuals had a wing span of about 70 cm, the largest flying insects that ever have existed on Earth (Clapham and Karr 2012; Waller and Svensson 2017). The large sizes of these now extinct insects have been explained as being a result of the higher atmospheric oxygen levels during the Carboniferous period, which were over 30 %, compared to the situation today (about 20 %), as flying insects are dependent on high oxygen levels as they breathe through trachea, small openings in the cuticle (Grimaldi and Engel 2005).

Cover article: (open access)
Svensson, E. I., Gómez-Llano, M., & Waller, J. T. (2023). Out of the tropics: Macroevolutionary size trends in an old insect order are shaped by temperature and predators. Journal of Biogeography, 50, 489–502. https://doi.org/10.1111/jbi.14544 

However, even among today’s surviving species of dragonflies and damselflies there is considerable variation in size that remains to be explained. Consider, for instance, the large  Helicopter Damselflies (Megaloprepus caerulatus) in Central and South America with a wing span of about  190 mm vs. the smallest Wisp damselflies of the genus Agriocnemis with wing spans  less than 20 mm. How can we explain this large variation in size today across the globe and which ecological and environmental factors were important in shaping today’s geographic variation in size in Odonata?

A Helicopter Damselfly (Megaloprepus caerulatus), photographed during a night walk in the rainforest reserve La Selva in Costa Rica in February 2020. This is the world’s largest damselfly, where some males can have wing spans of 190 mm. Photo: Erik Svensson.

To answer this question, we compiled size data of dragonflies and damselflies from field guides, scientific articles and internet sources with the aim to create a global database of phenotypic traits of this enigmatic insect order. The first result of this work – to which we owe a great debt to the many students who helped us out – was published in the journal Scientific Data (Waller et al. 2019) and as an open trait database: The Odonate Phenotypic Database (http://www.odonatephenotypicdatabase.org/shiny/odonates/?).

Armed with this database and size information for 775 species of Odonata ranging from the tropics to the temperate zone, we first investigated if there was any latitudinal size gradient. In other words; do dragonflies and damselflies become consistently smaller or larger in size as we moved away from the species-rich tropics where most lineages have their evolutionary origin? It turned out that there was indeed a systematic pattern: at higher latitudes in the temperate zone, dragonflies (suborder Anisoptera), but notably not damselflies (suborder Zygoptera), are larger than in the tropics. This geographic pattern, which is known as the classical “Bergmann’s Rule”, is wellknown among endothermic animals like birds and mammals, but has also been documented among some ectothermic animal groups, including dragonflies and damselflies, and some other insect groups. How can we explain this latitudinal size gradient, then?

The most obvious explanation is temperature. Insects are known to develop faster and reach a smaller size when ambient temperatures are high, like in the tropics. Indeed, we found a significant effect of temperature when analyzing global size variation in Odonata, taking in to account phylogenetic relatedness and by using modern comparative methods. However, temperature is not the sole answer. Two intriguing pieces of data reveal a more complex explanation for these latitudinal size gradients.

First, when taking in to account other environmental factors, such as bird species diversity (a proxy of predation risk on Odonata, as we know that birds are important predators on these large insects), we found that the effect of bird diversity was three times stronger in explaining size variation than the effect of temperature. Moreover, the effect of bird diversity was unlikely to solely be a general diversity effect, unrelated to predation, as mammal diversity (which served as an independent control variable) did not show such a strong effect. This strongly suggests that the diversity of avian predators has a negative effect on the average size of dragonflies and damselflies across the globe.

An additional piece of evidence in this puzzle comes from how the latitudinal size gradient in Odonata has changed over macroevolutionary time, during the last 210 million years.  To investigate this, we used data on extinct species of Odonata from the Paleobiology Database (https://paleobiodb.org/#/). By combining size and age information and information about the paleo-latitude of different-sized fossils, we found that the latitudinal size gradient has changed over macroevolutionary time. The recent latitudinal size gradient where the largest species are found at the highest latitudes is actually of recent origin. In the past the latitudinal size gradient had a different sign, and the largest species were found at low latitudes in the tropics. These findings from the fossil record decisively show that a simple explanation based on temperature cannot fully explain why the largest species are found at the highest latitudes as there is no reason to think that they should respond differently to temperature now than they did in past geological times. Instead, we interpret these changing latitudinal size gradients as a result of the evolutionary radiation of birds, that emerged on the geological scene about 150 million years ago. Birds then diversified rapidly after the most recent mass extinction at the end of the Cretaceous Period, 65 million years ago.

In the tropics, bird predation on Odonata is high, as illustrated by this Rufous-tailed Jacamar (Galbula ruficauda) that has caught a Buenos Aires Darner (Aeshnidae: Rhionaeschna bonariensis) in REGUA wetland reserve in the Atlantic Forest of Brazil in January 2018. Photograph by E.I. Svensson.

As birds are major predators on Odonata, we suggest that as they started to diversify at lower latitudes, their increased presence selected against larger sized species that could, due to their higher dispersal capacity, subsequently escape predation by invading the temperate region with lower predation pressure. Thus, increased pressure from birds in combination with higher dispersal capacity of large-bodied species of Odonata partly shaped the current latitudinal size gradient, alongside with temperature.

Our study therefore illustrate that both temperature and birds were responsible for creating the current latitudinal size gradient and a single explanatory factor is thus insufficient to fully explain today’s global size distribution. We hope that our work will stimulate research on other organismal groups and that researchers will consider both abiotic factors like temperature but also biotic factors like predator pressure when seeking to explain latitudinal size gradients. However, research on both Odonata and other insects is limited by lack of body size and ecological information, particularly for tropical taxa. In the present study, for instance, we only had access to size data for 775 out of the total of 6400 species of Odonata that are known, or about 12 %. This mainly reflects the lack of field guides and data from the tropics. Clearly, our study has only laid the foundation for future work in this area and there is an urgent need for more data from the tropics for this and other insect groups.

Clapham, M. E., and J. A. Karr. 2012. Environmental and biotic controls on the evolutionary history of insect body size. Proceedings of the National Academy of Sciences of the United States of America 109:10927–10930.
Grimaldi, D., and M. S. Engel. 2005. Evolution of the insects. Cambridge University Press, New York.
Waller, J. T., and E. I. Svensson. 2017. Body size evolution in an old insect order: No evidence for Cope’s Rule in spite of fitness benefits of large size. Evolution 71:2178–2193.
Waller, J. T., B. Willink, M. Tschol, and E. I. Svensson. 2019. The odonate phenotypic database, a new open data resource for comparative studies of an old insect order. Sci. Data 6:1–6.

Written by:
Erik Svensson
Professor, Department of Biology, Lund University, SE-223 62 Lund, SWEDEN

Additional information:
Website: https://portal.research.lu.se/en/persons/erik-svensson
Twitter: https://twitter.com/EvolOdonata
Mastodon: https://ecoevo.social/@EvolOdonata

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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