Guest blog by Dr Andrew Bladon, University of Cambridge, on his new paper published in the Journal of Animal Ecology.
They say that only mad dogs and Englishmen go out in the midday sun. Perhaps we should add lepidopterists – people who study butterflies – to that list. It was early August, in the peak of the 2018 heatwave, as I netted another small white Pieris rapae on the steep slopes of Pegsdon Hills Wildlife Trust reserve in Bedfordshire. The air temperature was 34.8°C in the shade. I knew because the fine temperature probe at my waist told me, and I could feel every degree of it.
I suppose few people have ever looked at a butterfly and thought “I wonder how hot it is”. When I tell people that I spent the summer of 2018 measuring the temperature of butterflies, the first question I get is “Wait, how do you take the temperature of a butterfly?”. The answer is with a fine, 0.25mm thermocouple on the end of a wire, which can be delicately inserted through a hole in a butterfly net and touched gently onto the butterfly’s body. But the more pertinent question is really “Why would you take a butterfly’s body temperature?”.
Taking a butterfly’s temperature with a fine probe © Eleanor Bladon
The answer to that question is that we wanted to investigate how butterflies might respond to climate change. There’s plenty of evidence that, over the last 20–30 years, butterflies have been on the move. In Europe and North America many species are now found further north than they used to be whilst others, particularly upland species, are disappearing from the lowest elevations where they used to be found. But these population changes are caused by the behaviour of individuals; individual butterflies which are responding, on a local scale, to changes in the temperature around them. We know very little about these responses, but understanding them is crucial to understanding the large scale changes we are seeing.
The marbled white Melanargia galathea is a species which is moving north through Europe as the climate changes © Andrew Bladon
At this point, one may consider the fact that butterflies, being insects, are “cold-blooded” (or ectothermic, to use the scientific term). That is, they cannot generate their own body heat, and rely on the temperature of their environment. So does that mean that they simply warm up at the same rate as the air? Not at all, because being ectothermic simply means that animals (including insects, reptiles and amphibians) can’t use their metabolism to warm up or cool down. But what they can do is change their behaviour.
If you’ve spent any time watching butterflies, you’ve probably seen them basking on a cool morning. By angling themselves towards the sun, butterflies can heat themselves up to fly. In warmer weather, they can close their wings and angle away to cool down. This is a form of behavioural temperature control. Being small, butterflies can also make use of sheltered pockets in the environment, caused by the lumps and bumps of a hillside, or the structure of vegetation, which offer warmer or cooler patches within the landscape. This means that, despite being ectotherms, butterflies have a couple of different methods available to them for altering their temperature. We’ll return to that shortly.
Brimstone Gonepteryx rhamni are able to maintain a fairly stable body temperature © Andrew Bladon
Our first question was whether different species of butterfly have different body temperatures. By catching and measuring nearly 4,000 butterflies over two summers, we found that the answer is yes. When compared to the air temperature, butterflies’ body temperatures show a range of responses. Some species, such as the large white Pieris brassicae and brimstone Gonepteryx rhamni, maintain a fairly stable body temperature over a wide range of air temperatures. We term this a good “thermal buffering ability” and hypothesise that this means they are able to cope well with a wide range of air temperatures.
By contrast, species such as the threatened Duke of Burgundy Hamearis lucina and mountain ringlet Erebia epiphron have less control, and their body temperatures change a lot even over narrow ranges of air temperature. There may be good reasons for this: at the low temperatures which both species are likely to experience (the Duke of Burgundy flies in early spring, and the mountain ringlet lives in the uplands), being able to warm up when it’s cold is probably more important than being able to cool down when it’s hot. But this means these species are likely to be more vulnerable to climate change, because they have less control over their body temperature in warmer weather.
The orange-tip Anthocharis cardamines is another species good at keeping its body temperature stable © Andrew Bladon
Second, we wanted to see whether there were any patterns across species – do species’ characteristics predict their buffering ability? It turns out that different groups of butterflies (called Families) tend to have different abilities.
The white butterflies (Pieridae), including the brimstone and large white, as well as the orange-tip Anthocharis cardamines and green-veined white Pieris napi, are pretty good at thermal buffering, while the brush-footed butterflies (Nymphalidae), which includes familiar species such as the peacock Aglais io, red admiral Vanessa atalanta and meadow brown Maniola jurtina, as well as the fritillaries and mountain ringlet, are generally much worse at thermal buffering. And within Families, the larger species tend to be better at buffering than their smaller cousins.
Finally, we wanted to understand more about how different species alter their body temperature, and whether this could tell us anything about their long-term prospects. Remember I said that butterflies had a couple of options for thermoregulation? For 483 of our 3,797 butterflies, we caught them on vegetation: either nectaring on flowers, resting on a leaf, or basking in the sun.
Meadow brown Maniola jurtina are among the species with less control over their body temperature © Eleanor Bladon
In addition to measuring the temperature of the butterfly and the air, we took a third temperature reading from close to the exact spot the butterfly was using. The difference between this “perch temperature” and the air temperature gave us a measure of to what extent butterflies choose habitats which are a different temperature from the air. And the difference between a butterfly’s body temperature and the temperature of its perch gave us a measure of how much the butterfly was able to alter its body temperature (for example by basking) beyond the temperature provided by the habitat.
For each species, we looked at the average difference between these measures and found that this predicts species’ population trends, measured by Butterfly Conservation’s monitoring schemes. Species which are able to alter their body temperature away from the temperature of their perch, such as the comma Polygonia c-album and ringlet Aphantopus hyperantus, have been doing well over the last 40 years, whereas species which rely on choosing habitats of the right temperature, such as the small heath Coenonympha pamphilus and small copper Lycaena phlaeas have suffered the most severe declines.
Can the peacock Aglais io keep its cool in the face of climate change? © Andrew Bladon
The most interesting thing about these findings is that the majority of our study species are considered to be “habitat generalists”. They are found widely across the landscape, in a broad range of habitats, and tend to feed on a broader range of food plants as caterpillars. Yet these species show diverse responses to temperature, and those which are less able to cope with temperature change will be vulnerable to climate change. Alongside our established “habitat specialists”, we must consider these “thermal specialists” in our conservation planning, to ensure that, as the hot summers like 2018 become more common, these butterflies are able to keep their cool.
Paper: In Journal of Animal Ecology "How butterflies keep their cool: Physical and ecological traits influence thermoregulatory ability and population trends"
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The paper draws attention to differences in thermal autoregulation between species. Presumably there may be differences within species, where these are widely distributed.