Longer summers are causing large mountain rodents called marmots to grow larger and get better at surviving, according to a 33-year study published Wednesday in Nature.

The research, carried out by scientists at University of Kansas and Imperial College London in the United Kingdom, looked at a population of yellow-bellied marmots (Marmota flaviventris), which are large ground-dwelling "squirrels" that live at around 3,000 meters in the Rocky Mountains in Colorado.

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.

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.

Today's 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.

To reach their conclusions, the researchers analyzed 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.

The results show that the average mass of adult marmots increased from 3,094 grams in the first half of the study to 3,433 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.

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 Ozgul.