Why marmots love global warming

Climate change has sparked something of a baby boom for yellow-bellied marmots in Colorado and the animals are growing bigger and living longer

GLOBAL warming is causing unprecedented environmental change, but if you were a yellow-bellied marmot you would be happy about it.

The animal is enjoying something of a baby boom because of the gradually lengthening summers caused by climate change.

Scientists already know that the alterations to climate experienced so far are affecting many species, mostly for the worse. Plants and animals are being driven out of their natural ranges or being pushed towards extinction.

The environmental pressures are also bringing about physical changes in some species and sometimes for the better. The challenge for scientists however is to prove cause and effect to ensure there actually is a direct link between environmental change and species change.

This is what a group of UK and US scientists have managed to do for one species, the yellow-bellied marmot ( Marmota flaviventris), publishing their findings this morning in the journal Nature.

They were greatly assisted in this work by having access to an exceptional 33-year-long study of marmots living in the Upper East River Valley in the Colorado Rockies. This study looked at individual animals, recording hard data about them and their offspring from 1976-2008.

Marmots are a type of ground squirrel common throughout mountainous regions of the northern hemisphere. They form burrows in the ground and – importantly in terms of climate change – they hibernate through the winter.

Dr Arpat Ozgul of Imperial College London and colleagues used the data set and applied new research methods to understand what was happening to change the marmot population.

They were able to show how longer summers caused by a warming climate suited the marmot lifestyle, allowing them to put on more weight before hibernation and then emerge fitter and better able to cope as the spring returned.

The team looked at data on females exclusively, in part because the males tended to leave the group after a couple of years but also because the maternal links to a given pup were known conclusively while the paternal links could not be absolutely confirmed. The researchers looked in particular at body mass “because marmot life history, particularly survival during hibernation and reproduction on emergence, is heavily dependent on this trait,” Ozgul and colleagues write.

The earlier arrival of spring is a great benefit to the marmots, the authors say. “Marmots have been emerging earlier from hibernation and giving birth earlier in the season, which allows individuals more time to grow until hibernation,” they say.

They could actually measure this directly because marmot weights were taken repeatedly over the 33-year study period of the earlier research. Ozgul found that while adult marmots in the earlier years weighed on average about 3.1kg, body mass had increased to about 3.4kg in more recent times. A heavier marmot is a fitter marmot and this extra weight helped more adults to survive to the next year, the authors say. The population of the Upper East River group remained balanced up until 2001, “followed by a steady increase over the last seven years”.

The authors’ statistical analysis showed that while population size increased by about 0.56 marmots a year from 1976 through 2001, population rose by 14.2 marmots per year subsequently, “indicating a major shift in the population dynamics”.

This significant change in the physical characteristics of the marmots but also in their overall population can be tracked back to its environmental source, the slightly longer summers which began in the later 1990s.

What is unknown and not part of this research is what happens next given a more vigorous and successful marmot population. Will the extra marmots improve the fitness and health of marmot predators? Or will marmot burrowing or feeding preferences damage the local ecosystem?

One way or another, environmental change will prompt still more change as species adjust in response to an altered environment.