Marmots growing in size and number due to climate change

An adult yellow-bellied marmot (photo by Ben Hulsey)

July 2010. Longer summers are causing large mountain rodents called marmots to grow larger as well as improving their survival rates, according to a 33-year study published today in Nature.

The research, carried out by scientists at Imperial College London and collaborators in the UK and USA, looked at a population of yellow-bellied marmots (Marmota flaviventris), which are large ground-dwelling ‘squirrels' that live at around 3000 metres in the Colorado Rocky Mountains.

Regional changes in the climate have created longer summers and have led to marmots waking up earlier from hibernation, giving them more time to reproduce and gain weight before the next hibernation period. The study shows that the marmots are growing fatter and healthier as a result. Longer summers also mean that individual marmots are reproducing earlier and their offspring are more likely to survive the upcoming winter, so the marmot population is increasing in size.

Two adult yellow-bellied marmots (photo by Ben Hulsey)

Yellow-bellied marmot
Yellow-bellied marmots are adapted to living in environments with a short summer and a long winter by hibernating for seven to eight months of the year. Failure to gain enough weight before the colder months can be life-threatening, as a marmot loses around 40 percent of its body mass during hibernation.

The study, which began in 1962 and focuses on the most comprehensive data collected between 1976 and 2008, is the first study of any species to show that a shift in seasonal timing can cause changes in body mass and population size simultaneously.

Only awake for 4-5 months
Dr Arpat Ozgul, lead author of the study from the Department of Life Sciences at Imperial College London, said: "Marmots are awake for only four to five months of the year. These months are a busy time for them - they have to eat and gain weight, get pregnant, produce offspring and get ready to hibernate again. Since the summers have become longer, marmots have had more time to do all these things and grow before the upcoming winter, so they are more likely to succeed and survive.

"We have observed changes in the body mass of individual marmots over the past 33 years and changes in their population size over the last decade, but we do not know what might happen in the future. Will populations thrive in the changing climate? We suspect that this population increase is a short-term response to the lengthening summers. We hope that by continuing this long-term study we will shed important light on the marmots' future response to climate change," added Dr Ozgul.

Recording body data
To reach their conclusions, the researchers analysed data on body mass, survival and reproduction of female yellow-bellied marmots in the Rocky Mountains in Colorado. Every year, the researchers live-trapped marmots at each colony multiple times during the summer and individually marked them using numbered ear tags. They recorded the sex, mass and reproductive condition of each captured animal.

Large increase in population
The results show that the average mass of adult marmots increased from 3094 grams in the first half of the study to 3433 grams in the second half. The research also shows that population growth increased from 0.56 marmots per year between 1976 and 2001 to 14.2 marmots per year between 2001 and 2008.

Professor Tim Coulson, one of the authors of the study from the Department of Life Sciences at Imperial College London, said: "The marmots have provided yet another example of how climate change can impact the natural world. We have shown how we can model the consequences of environmental change on wild populations. If we can get better at predicting how climate change is likely to influence the natural world, perhaps we can devise ways to help species predicted to be adversely affected by our rapidly changing climate."

Professor Sir Brian Hoskins, Director of the Grantham Institute for Climate Change at Imperial College London, said: "Scientists have carried out numerous studies on animals and plants that can tell us about the impacts of climate change. Today's study shows that marmots are also one of these species, acting as climate change ‘canaries', giving us an early warning about the effects of climate change on our natural environment."

The research was carried out by scientists from Imperial College London, the University of Sheffield, University of Florida, University of Kansas, University of California, Los Angeles, and Stanford University. It was funded by the Natural Environment Research Council (NERC), the Wellcome Trust, National Science Foundation (NSF), the National Institutes of Health (NIH) and the National Institute of Aging (NIA).