A tiny mouse living nearly 7,000 metres above sea level in the Andes is helping scientists rethink the limits of life on Earth.
The leaf-eared mouse uses a combination of adaptations to survive in conditions once thought uninhabitable, a new international study co-authored by McMaster University researchers reveals.
Researchers had believed mammals could survive at only up to about 5,500 metres — roughly the altitude of the highest permanent human settlements. The discovery of these mice living more than a kilometre higher challenged that assumption.
“It was completely unexpected. People did not think mammals could survive at these altitudes, but they’re there,” says Graham Scott, a professor in the Department of Biology who co-authored the study.
The extreme environment, often compared to conditions on Mars, is defined by freezing temperatures, extremely low oxygen levels, a lack of water and little to no plant life. That combination makes it difficult for most animals to survive.
To understand how the mice endure such extremes, Scott and colleague Grant McClelland joined an international team to study the animals in Chile, comparing animals from these high elevations with those from lower ones.
Their findings, published today in Science, suggest that survival in these conditions is not driven by a single adaptation, but by a combination of changes across the body.
“Evolution is a complex process,” says McClelland, a co-author of the study and a professor in the Department of Biology. “When animals encounter really challenging environments, there are a lot of different things they need to cope with, not just the obvious ones.”
High-altitude mice were better able to maintain body heat and sustain oxygen use in cold, low oxygen conditions than their lowland counterparts.
At a cellular level, their muscle tissue resembles that of endurance athletes.
“They’re more like a marathon runner than a sprinter,” explains Scott. “Their muscle cells are packed with mitochondria that allow them to sustain heat-producing activity for longer periods.”
This enhanced capacity lets the mice maintain high rates of oxygen consumption, which is critical for generating heat in freezing temperatures.
The animals also rely more heavily on fats as a fuel source, an effective way to provide energy to both shivering muscle and specialized heat-producing tissues.
But some of the most surprising findings had little to do with oxygen or temperature.
At such extreme elevations, food is scarce. The mice survive on unusual and unpredictable food sources, including lichens, which are symbiotic organisms that grow on rocks, and possibly seeds or insects carried up by the wind.
Genetic analyses revealed that the high-altitude mice have evolved changes in genes involved in metabolizing food and can detoxify plant compounds that would normally be harmful.
“We were initially focused on the most obvious environmental challenges, things like low oxygen and cold, but there were important factors we didn’t expect, including how these animals deal with what they’re eating,” says Scott.
The findings paint a picture of evolution as a multi-layered process, where multiple systems shift to meet complex environmental challenges.
It also highlights how much there still is to learn about the limits of life.
“Sometimes our assumptions about the most extreme environments animals can live in can be questioned,” says McClelland. “Evolution has a lot of room to experiment.”
That insight may have broader implications as species face a rapidly changing climate.
“We tend to focus on temperatures as the big challenge,” says Scott. “But animals are dealing with many pressures at once and evolution may push them in ways we don’t always anticipate.”