Four butterflies ranging from bright yellow to pale yellow pinned on a blue board.

The brimstone butterfly is one of the species assessed using the new computer vision pipeline Mothra ©The Trustees of the Natural History Museum, London

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Climate change has caused Britain's butterflies to get bigger

In response to a warming world, many species are physically changing their body sizes.

While for some this means getting smaller to cope with extreme temperatures, for insects the response is more varied.

New research using computer vision to analyse tens of thousands of butterfly specimens has found that some British butterflies are steadily getting bigger in response to climate change. 

Butterflies across the British countryside have been steadily increasing in size over the past few decades.

As the average temperature of the planet has increased as a result of the climate crisis, it has caused butterflies with late-stage larvae to grow bigger.

The research, carried out by a team of scientists from the Museum, the University of Southampton and the University of California, has used a computer vision pipeline which allows the rapid analysis of thousands of specimens, vastly increasing the sample size for projects such as this.  

Dr Phillip B Fenberg, from the University of Southampton and co-author of the paper, says, 'Our paper is among the first to show that computer vision can be applied to these digital images for testing hypotheses on how animals may respond to climate change.

'This is accelerating our potential to understand how the biosphere will react to climate change.'

The study, published in Methods in Ecology and Evolution also managed to confirm that females are the larger sex for most species of British butterflies. While this had long been generally assumed, this is the largest study to have actually tested it.

A butterfly with bright yellow wings and purple lining their edges.

The clouded yellow butterfly is a species which often visits the UK, occasionally staying to breed on the south coast ©The Trustees of the Natural History Museum, London

The changing size of animals

As the global temperature steadily warms, it is impacting the natural world in a variety of ways.

Some species are shifting their ranges, moving north or south as their usual habitats get more inhospitable, others are changing their migratory patterns, while at least one species is confirmed to have gone extinct as the rising sea levels overwhelmed the only island it was known to live on.

But climate change is also having some more surprising impacts on animals.

One study, for example, found that in response to the warming climate birds are shrinking and so consequently their wingspans growing. It is thought that this might be to help keep their bodies cooler, as smaller animals lose body heat quicker due to a larger-surface-areas-to-volume ratio.    

This new study has shown that for British butterflies the opposite seems to be happening, as warmer temperatures are leading to larger butterflies.

The team of researchers were able to use the Museum's collection of British butterflies, analysing some 125,000 specimens. They were then able to pair the monthly temperature records experienced by the immature stages of 24 different British butterfly species and looked for patterns in the relationship between size and temperature.

They found that for 17 of these species, the increase in the temperature when the butterflies' larvae were developing caused a corresponding increase in adult butterfly size. 

A butterfly specimen being analysed by the computer programme, digitially drawing lines to measure the wings.

The computer vision pipeline system Mothra allows scientists to analyse thousands of specimens much faster than by hand ©Wilson et al. 2022

Helped by Mothra

While the changes in size of butterflies is important and corroborates previous studies which have found similar results, one of the biggest breakthroughs in this project is the use of computer vision to analyse the specimens themselves.

The researchers at the Berkley Institute for Data Science developed a computer vision pipeline system called Mothra which allowed the scientists to analyse over 180,000 photographs of specimens. The system automatically measures the orientation of the specimen, the length of the wings, and well as the sex of the butterfly.

The system was first tested against hand measured and sexed specimens, and found to be incredibly accurate. Mothra now allows researchers to rapidly analyse thousands of butterflies in a fraction of time it would ordinarily take researchers doing the same work by hand.

Stephen J Brooks is an entomology researcher at the Museum and co-author of the paper.

'Natural history collections contain information on how the natural world responds to change through time,' explains Stephen. 'But the sheer size of these collections often makes it difficult to extract this information.

'Our study has shown the value and power of digitisation and machine learning to rapidly release this wealth of evidence, which can be used to conserve species in a changing world.'

The work is part of a huge push globally to digitise natural history collections which should, in theory, allow more people to have access to these collections and expand the knowledge that can be learned from them.