Ain't No Mountain High Enough

Almost anywhere you can go on Earth, you will find life. A field of molten lava might be going a bit far, but otherwise, life seems to have adapted to almost every environment, from ridiculously hot springs at the bottom of the sea bed to barren ice fields to microscopic cracks in apparently solid rock far underground. Much of this life is, of course, far too small to see with the naked eye - bacteria or their cousins the archaea. Mammals are somewhat more limited; we don't find them inland in Antarctica, for instance. But what are the limits beyond which even the best-adapted mammals cannot live comfortably?

One of the easier limits to measure in this way is altitude. This is, after all, a fixed value, whereas factors such as temperature can vary throughout the year. Certainly, it would be useful to know whether a given species can survive as the world warms, but that can be a complex question to answer. When it comes to altitude, we simply have to go and look. For this reason, when describing the habitat of a creature in a conservation catalogue or the like, the altitudinal range of the animal is often described in numeric terms, while the preferred climate is described more vaguely.

For example, for a jaguar (Panthera onca), one might find a listing of "forest, savannah, shrubland, grassland, and wetlands" with a note that they prefer the dense forest. And, given that it would also mention that they live from northern Mexico to southern Brazil you'll also get some idea of their preferred temperature range. But it's unlikely that you'll get much more detail on their limitations than that. However, many such listings will add something to the effect of "sea level to 3,000 metres (9,800 feet)" because that's something it's easier to measure.

For most mammals, the primary limitation on the altitude at which they can live comfortably is the type of vegetation that can be found there. This is obviously going to vary depending on latitude, and to some extent, other geographical factors (such as whether the mountainside is south-facing or not). In the case of the jaguar, for instance, we can note that, at the equator, 3,000 metres is roughly the height of the timberline - the point above which trees are scarce enough that they cannot form a forest with a closed canopy. Given the jaguar's other preferences, that's probably not a coincidence.

Above the timberline, we come to the tree line, beyond which the air is too cold and soil too frozen to allow trees to set down roots and grow. Above this, we have a region of alpine meadow, with vegetation, but lacking trees - essentially it's tundra. That ends at the snow line, and above that, in the case of a small number of the world's very highest mountains, lies the "death line" beyond which the air is too thin for humans to survive without an oxygen supply.

Unsurprisingly, mammals don't live that high up. But how close can they get?

As this botanical paper notes, there is no central database for this sort of thing, but it is possible to glean the information from lists of conservation data, species catalogues and so on. But even these can be prone to error, whether because of unreliable sightings being used as a data point, or, in the case of older reports, because the equipment used by early mountaineers to measure altitude was not very accurate. 

Another example can be provided by the yak (Bos mutus). This is clearly a high-altitude animal, living as it does on the Tibetan Plateau, a place that can be uncomfortable for humans who have not been properly acclimated to the thin air. We know that it feeds on a type of grass that can grow successfully right up to the snow line at around 6,100 metres (20,000 feet) and it is often said to live this high itself. But, in fact, there don't seem to be any reliable sightings of it above about 5,500 metres (18,000 feet)... still impressive, but it's unlikely to be the world record holder.

The snow leopard (Panthera uncia) is another obvious contender, and it, too, is sometimes said to live up to the snowline. But that's really just guesswork, not based on any specific sightings at that altitude. The limit is more likely closer to 5,800 metres (19,000 feet) and could even be a little lower. Foxes and wolves have been reported at similar heights, and even well above them, but these are old reports where the altitude was measured using equipment now known to be unreliable, so perhaps they go no higher than the yak.

A better place to look might well be goats and their relatives. Goats evolved in large part to take advantage of mountainous terrain where nothing else wanted to live, and being grazing animals used to low-quality vegetation, the sort of grass that's found well above the tree line is perfect for them. While it's entirely plausible that such animals as Siberian ibex (Capra sibirica) do indeed live up to the snowline in the Himalayan region, they too, haven't been definitively seen higher up than the yak. However, there are apparently reliable sightings of blue sheep (Pseudois nayaur) - actually a type of goat related to the ibex - as high as 6,000 metres (19,700 feet) which is likely the record for large animals.

Smaller animals, such as rodents, might be a better bet. They don't require so much food to survive, and it's hard to believe that nothing is eating the grass even at very high altitudes. Such records do, however, seem to be rare, perhaps because mountaineers are less likely to spot something small as they are a wayward goat or snow leopard. Until recently, however, the record holder was thought to be the large-eared pika (Ochotona macrotis) often said to live as high as 6,400 metres (21,000 feet). This turns out to be a repetition of a typo in a published manuscript where the subsequent correction had been overlooked. That still puts it at 6,130 metres (20,100 feet), where it was seen during a 1921 expedition to Mount Everest... but once again this leaves us with the question of how accurate the aneroid barometers used at that time to estimate altitude really were.

But regardless of what the true record-holder is, the upper limit seemed to be in the vicinity of 6,000 metres, probably because there isn't much to eat above that height, even when you're relatively close to the equator.

Except, of course, that you will have noted that, just above, I said "until recently".

That's because the record was broken in 2020 and newly published evidence shows that this was not some individual that had somehow got lost, but a part of a self-sustaining population habitually living higher up than any previously recorded mammal.

The record comes, not from the Himalayas, but from the Andes and specifically Mount Llullaillaco, said to be the second-highest active volcano on Earth. The animal in question is the yellow-rumped leaf-eared mouse (Phyllotis xanthopygus), which inhabits mountainous terrain from Peru to southern Chile, as well as the cold lowlands of Patagonia. Like most American mice, it's technically a member of the "hamster" family, but you'd never know that by simply looking at it so "mouse" is as good a term as any.

The new evidence concerns mice seen between 6,100 and 6,200 metres (20,000 and 20,300 feet) elevation over a period of years, confirming their resident status over a number of generations (like most mice, they don't live long) and the presence of microbes only found in mouse droppings in an area that they had been using at this altitude. The actual 2020 record, however, was an individual seen alive at the summit of the volcano, 6,739 metres (22,110 feet) above sea level, a significant increase on even the previous dubious records, let alone the confirmed ones. Indeed, it's higher than the highest confirmed sighting of nesting birds - a pair of alpine choughs (Pyrrhocorax graculus) seen at 6,500 metres on Mount Everest.

An obvious question is what on Earth the mice are eating this high up. It's well above the altitude at which one would find grass, although fumaroles are known to be able to support small patches of moss and associated insects high up on other nearby volcanos. The new study that found the faecal microbes also found DNA from plants and insects in the same area, but none actually looked to be present on the barren, icy slopes. Conceivably, food is blown there on the wind, because clearly, they're getting it from somewhere but that remains speculation. The authors also note that the mice were only seen during the day; they're nocturnal elsewhere, but it may be that the nights are simply too cold to be out up above the snowline.

For all we know, the only reason these mice don't venture even higher than this is because, having reached the summit of the mountain, there's nowhere higher for them to go...

[Photo by Dick Culbert, from Wikimedia Commons.]