|The Utah prairie dog has the biggest burrows|
Squirrel burrows include a deep nest chamber, in which the animal can sleep, rear its young, or simply hide from predators. Connected to this there will usually be a number of tunnels, and multiple entrances. In the event that, say, a weasel, chases you into your burrow, it is really important to have an escape route, so a single entrance is unlikely to be enough.
Considering that squirrels are relatively small animals, these tunnel systems can be quite impressive. Since they're only using the burrows for shelter, they won't necessarily be as dramatic as those of animals like moles, that also need them to travel in search of food, but they can still be quite long and deep.
How long, and how deep? Well, that depends on a lot of factors. Different species will dig different kinds of burrow, and there's no reason to assume that, even given identical circumstances, all members of the same species will dig identical burrows. In any event, circumstances aren't identical. The nature of the environment, and particularly the soil, will affect how big a burrow it's practical to dig at all, even assuming you wanted to - and, generally speaking, animals won't dig a burrow any bigger than they have to, because it's exhausting work.
If the soil is too loose and sandy, for instance, parts of the burrow are likely to keep collapsing. If you really must dig in that sort of soil, it's best to keep the burrow short so that you don't have to spend all your time repairing it. On the other hand, really hard soil is difficult to excavate in the first place, which, again, might limit the size of your burrow. Another factor might be whether squirrels tend to occupy the same burrow over several generations, expanding it as they go - these burrows will tend to be larger than you might otherwise expect, simply because they've had longer to dig them.
Even taking all of that into account, though, it's plausible to expect that there will be some general rules we can apply to different species. Figuring out what those rules are, and what we can predict about burrows in different areas is of more than purely academic interest - engineers, for example, might be interested to know how likely their levees are to be undermined or if something's going to dig holes in their hazardous waste dumps.
There have, of course, been a number of surveys looking into the size and depth of burrow systems, and such studies stretch back over a hundred years. Relatively few have looked at the big picture, comparing different kinds of animal, although there have certainly been some. One way of doing this, rather than going out and checking a vast number of burrows yourself, is to look at all the individual reports that already exist, and see if any kind of pattern emerges. Conservation workers Dirk Van Vuren and Miguel Ordeñana recently published such a review, comparing twenty two different species of ground squirrel found in North America.
They were able to confirm that, as expected, the nature of the soil does make a difference to the nature of burrows in it. This, and various other factors, mean that it isn't possible to create hard and fast rules to predict how squirrels of any kind will dig in any given situation. But there were at least a number of general guidelines that showed up.
Firstly, bigger squirrels tend to dig bigger burrows. That's obviously going to be true for the actual width of the tunnel, since the squirrel has to get into it, but the burrows of larger squirrels also tend to be both longer and deeper. They're probably able to dig for longer, and just feel the need for more space to be comfortable than smaller species do. That probably makes more sense in deciding the length of the tunnel than its depth, but if you're generally digging downwards, or just into the slope of a hill, the one will tend to imply the other.
Now, most squirrel species are fairly small. Among these squirrels, the above rule, while not perfect, seems to hold pretty well. For instance, four of the six species with average tunnels more than ten metres (thirty feet) in length are prairie dogs, which, at two to three pounds in weight, are noticeably larger than most other squirrels.
But, while prairie dogs are undeniably large for squirrels, they are by no means the largest in North America. Those are the marmots, which are large enough not to even look that much like other squirrels. Nonetheless, that's what they are, and it turns out that they dig really quite short tunnels, maybe only five metres (fifteen feet) long or so on average. That's about what you'd expect for a squirrel seven or eight times smaller than they are, and it must have something to do with the different lifestyles of marmots compared with their smaller kin. But, from this review, at least, it's not really clear what that difference is.
Marmots aside, it isn't the case, incidentally, that squirrels twice the size tend to dig burrows twice as big - it's more like one-and-a-half. There's going to be a tendency not to dig any more than you really have to, and it may also be that that ventilation becomes an issue if you dig too far.
The authors also expected that animals burrowing in colder climates would tend to dig deeper, but that turned out not to be the case. While you'd have to go down far deeper than any squirrel wants to to be completely unaffected by the above-ground temperature, soil is a good insulator, and it may be that once you're down a few feet or so, digging any deeper (and having to carry the excavated soil up to the surface) just isn't worth the effort.
The other factor that did make a big difference, though, was how sociable the squirrels were. Many ground squirrels live on their own, as their tree-dwelling cousins tend to, but others can live in large colonies. Many hands make light work, quite apart from the need for greater space if you want to squeeze more squirrels down the hole. Here, once again, the prairie dogs tend to come out on top. That muddies the waters slightly - if the largest squirrels are also the most sociable, how do we know which is the more important for tunnel length?
That's difficult to answer, although it's worth noting that the two species of marmot in the study aren't particularly sociable, which might have something to do with their shorter burrows. However, it's also true that the Utah prairie dog (Cynomys parvidens) digs tunnels far longer than does the white-tailed prairie dog (Cynomys leucurus) of Wyoming and Colorado. Significantly, the two species are almost exactly the same size, but the Utah one is far more sociable than it's cousin.
Having said that, an illustration of how firm these rules are, and how messy biology is compared with physics, is provided by the California ground squirrel (Otospermophilus beecheyi). It's a moderately large, but not especially sociable species, and, as predicted, it typically digs short to moderate tunnels. But just occasionally, it goes much further, and is apparently responsible for the longest squirrel burrow ever measured: a whopping 266 metres (870 feet). Seemingly, this burrow, with 33 separate entrances, was shared by eleven different individuals. Being anti-social, they stayed out of each other's way within the complex. Presumably, they'd started out digging their own burrows, but were living close enough to one another that they bumped into each other's tunnels while digging, until they created one vast burrow, while still inhabiting different bits of it.
Which, of course, brings us to the question at the head of this post. How deep is a prairie dog's burrow? It depends a lot on the species and on the individual colony, for all the reasons given above. But, from this review, the nest chamber is usually about three to five feet below the ground surface. (This may not necessarily be the deepest point in the burrow, by the way, but it's usually pretty close). The length of all the tunnels combined is, for most species, around thirty to forty feet, although those in Utah are typically at least twice that. But there's great variation, and some will dig for a hundred and twenty feet through the soil.
[Picture by "Chin tin tin" from Wikimedia Commons]