Red deer on a Scottish island are evolving to give birth earlier as temperatures rise due to climate change, research has shown.

Genetic changes in the red deer population on Rum have advanced their calving dates by around 12 days over the last 40 years.

The findings, by a team of scientists from the universities of Edinburgh, St Andrews and Cambridge and the Australian National University, are thought to be some of the first evidence that wildlife is evolving in response to changing environmental conditions.

The discovery of the rapid shift in birth dates in recent years was made using field and genetic data collected from the Isle of Rum National Nature Reserve over a 45-year period.

Dr Timothée Bonnet, of the Australian National University, who led the study, said: “This is one of the few cases where we have documented evolution in action, showing that it may help populations adapt to climate warming.”

Previous studies have shown that the female deer, or hinds, have been giving birth earlier since the 1980s, at a rate of around three days per decade, partly due to the effects of warmer temperatures on the deer’s behaviour and physiology.

Mean maximum temperatures have increased at the rate of around 0.23 degrees per decade in the Hebrides, according to the research.

Professor Josephine Pemberton, of the University of Edinburgh’s school of biological sciences, said: “Long-term studies of individual lifetimes are one of the few ways to understand how populations respond to environmental change and how to manage its effects.”

The team notes what it terms “plastic” changes within the life of individual hinds. They give birth to a single calf each year, and those that reproduce earlier in the year have more offspring over their lifetime.

This is partly because of an association between the genes that causes the hinds to give birth earlier and higher overall reproductive success. 

As a result, genes for breeding earlier have become more common in the Rum red deer over time.

Professor Pemberton said: “We are seeing hinds respond differently during their lifetimes. A female that has a calf on a certain date in a cold year will have a calf on an earlier date following a warm summer. Something is changing that the animals are responding to and it seems to be the summer temperatures before the rut which is promoting grass growth.”

After a warm summer, the animals are coming into oestrus earlier, mating, conceiving and birthing earlier but after cold summers the findings were reversed, said Professor Pemberton.

She said: “Some of the changes we’re seeing is because things are getting warmer and the animals are responding to that in their lifetimes, which we’re calling plastic change.”

Traditionally the calving season has been from mid June but due to the altered environmental conditions, researchers have noted the season beginning as early as late May.

The study also shows that the rutting season, antler casting and cleaning are all shifting – and by a very similar time.

The findings indicate that red deer could continue to thrive under changing weather: “In principle, if the population can evolve, which is very strongly suggested by the data, then it will allow it to do well under changed environmental conditions.”

But more research on the impact of winter weather would need to be carried out to understand the final consequences of climate change.

The study is unique in that it is teasing out the various different causes of the calving changes, although researchers across the world are investigating how organisms are adapting to environmental change.

Professor Pemberton said: “There has been a lot of discussion on whether animals are going to be able to change genetically and fast enough so this would be an indication that perhaps they will be able to.”

The research, published in the PLOS Biology journal, was funded by the Natural Environment Research Council.

Sally Thomas, Scottish Natural Heritage’s director of people and nature, said: “These findings are a fascinating example of the impact climate change may be having on wildlife.

“More and more research is demonstrating climate change is influencing species across the UK and the world. We are seeing growing evidence of the effects on Scotland’s nature. 

“Some species are finding it difficult to deal with changes, while others are moving northwards as conditions begin to change.”