Background: Body size is among the most important characteristics of animals and plants. Larger animals are capable of buffering against their environment (think big polar bear vs tiny chihuahua in the snow!) so that they can survive in a wider range of locations, are capable of eating a wider range of prey, and consume more prey than smaller animals leading to a stronger impact on ecosystems. However, we are still trying to understand the factors that influence body size, both ecologically and evolutionarily.
What I did: A number of previous studies have compared body size in particular animals across different locations to see whether or not there are consistent patterns in that variability. I wanted to collect specimens of a single species (the ebony jewelwing damselfly, Calopteryx maculata) for analysis from across its entire range in North America, but the range is so large (Florida to Ontario, and New York to Nebraska) that I wouldn’t have been able to travel to sufficient sites within the one season that I have available. Instead, I asked a lot of local dragonfly enthusiasts to catch and send me specimens from their local sites. I am extremely grateful to all of them for helping, as this could not have been done without their kind volunteering of time and energy. I ended up with a substantial dataset of animals from 49 sites across the range. I showed that there was a general increase in size further north, but that this was not a simple increase. Instead, there was a U-shaped relationship between latitude and size with larger animals in the south and the north with an intermediate size in the middle. When I looked at the drivers of this trend, it appeared that warm temperatures resulted in higher body sizes in the south. In the north, the animals use shortening days as a signal to accelerate their development and so in the most northern regions animals were developing very quickly despite the cold.
Importance: Large scale (across the whole of an animal’s range) measurements of body size are essential to provide an ecologically relevant test of explanations for changing body size. These findings support previous laboratory work which suggested a twinned role for temperature and photoperiod in driving development in damselflies.
This is part of a series of short lay summaries that describe the technical publications I have authored. This paper, entitled “Time stress and temperature explain continental variation in damselfly body size”, was published in the journal Ecography in 2013. You can find this paper at the publisher’s website or for free at Figshare.
Image credit: Kevin Payravi, http://bit.ly/1q7B2Ph, CC BY-SA 3.0