The Architecture of Burrows

A reprint from this blog of April 22, 2005.

The list of animals that hibernate in underground burrows, live in burrows or nest in burrows is astounding: fox, badger, turtles, some owls and parrots, ground squirrels and marmots, some penguins and puffins, European rabbits, bears, wolves and dogs, otter, some wild pigs, some wild cats, many snakes and lizards, possums, wombats, frogs and toads, moles, mice and rats, nutria, crayfish, armadillos, skunks, muskrats, meerkats, some large spiders, and several types of bees.

Many of these animals spend a fair amount of time constructing, maintaining, and modifying their den systems which, in turn, are structured in response to changing external circumstances and changing internal needs.

Anatomy, behavior, and distribution of wild animals are all greatly affected by the burrow environment, with temperature, humidity, oxygen, living space, availability of food, and protection from flooding and predation all being influenced by burrow architecture.

Despite this, the burrow structure of almost all subterranean mammals is barely known, and data and descriptions of den structures as they relate to soil composition, drainage, external defenses (rock, wire fences, farm buildings), proximity to food and water sources, is almost entirely non-existent.

All of this make understanding burrow architecture as an adaptive tool a difficult task, but it nonetheless clear, among humans digging to their terriers, that burrows are designed with a reason and that certain "tricks" keep popping up. A few I have noticed in the construction of groundhog and fox dens:

• Water and the phreatic zone: Dens of fox and groundhogs are designed to keep the animals dry, yet fox in particular like to locate their dens very near water. Shallow dens are sometimes found carved into the sides of small humps or rises in marshy areas, but these are rarely natal dens. The deeper natal dens of fox are generally found on the rises above the flood plains -- on well-drained hillsides near a creek or pond or water-filled ditch. The ideal spot for a pipe is deep enough to be dry from surface water, but not so deep as to be encountering the water table -- the so-called "phreatic zone".
  
  
• The "plumbers pipe" opening: In these settes the pipes run down for two or three feet before rising again and then dropping again. This creates an earthen "water stop" not so very different from the plumbers p-pipe you will find under your kitchen sink. This is a common feature in many animal burrows, from rat to fox.
  
  
• Depth is determined by soil: This is near-truth. The softer the earth and the easier it drains, the deeper the den is likely to be. One of the chief functions of a den is to keep the animal dry, and in soils that are very light, dryness tends to be achieved with some depth. You may cut through very soft stuff for three or four feet, but below that will be the harder stuff -- a layer of hard sand below the soft, or a layer of hard dry clay below much softer friable soil. The pipe is likely to be in softer soil that lies below the vein of hard stuff. The hard vein works to structurally support the pipe and to keep out the last bits of water. The one exception that I would note is where a ridge is made almost entirely of small cherts, flints or flaked slate. Here digging by a fox or groundhog is so hard, and the drainage of water is so rapid, that the den pipe may be very shallow since water runs through the den as fast as it falls. These shallow and breezy loose-rock dens will not be used in winter, but are likely spots in summer.
  
  
• Rocks, roots and barbed wire at the entrance: This is so common that it is clearly a planned design feature. It is not uncommon to find dens exiting inside a stump or hollow tree, or to have a strand or two of barbed wire running along the lip. Groundhog dens frequently start on one side of a barbed wire fence and exit out another. If there is an abandoned vehicle on the edge of a farm, a groundhog will invariably makes its home under the chassis -- a nice shelter from the rain, but also from predators who will have to slow down to a crawl to avoid beaning themselves on the I-beam and suspension. Hard structures not only make digging out more difficult, it makes a mad dash at the den hole a dangerous and maladaptive strategy for large predators. In addition to hard structures at den entrances and exits, you will frequently find "soft structures" such as thickets of multi-flora rose, bramble, poison ivy, and thick wild grape vines serving the same function.
  
  
• The hard turn after the den entrance: This is a frequent feature of groundhog den entrances and serves two purposes: 1) it makes it difficult for predators to enter, and; 2) water entering the entrance will tend to follow the straight line of gravity and soak into the ground at the end of the pipe rather than run sideways with the turn of the pipe. I have encountered some rather incredible animals that took this "hard turn at the entrance" technique to absurd levels as they never stopped turning -- they corkscrewed their dens in full circles all the way down in a spiral that went four or five feet deep.
  
  
• The pocket room close to the entrance: This is a common feature of groundhog dens, and I believe these serve as "loafing parlors" where the animal can tuck in out of danger between feeding forays. If you are quiet, and the dog is too, you can often bottle a groundhog in one of these little rooms before it goes deeper into the sette.
  
  
• Right angle turns inside the den pipe: These are very frequent features of groundhog and fox dens and seem to serve two purposes. The first purpose is that they help keep the den dry, as water running down a pipe will tend to follow gravity rather than take a sudden turn to the left or right. The second purpose is clearly defensive; a groundhog or fox can take a stand at a corner and slash out at anything coming down the pipe, inflicting considerable damage if the corner is tight, as it invariably is. Often a sette will have several right-angle turns, each providing additional elements of security, like doors on a hallway. A human digging on a dog will often break into a pipe where the dog and quarry are at a standoff at one of these turns, at which point all hell will break loose as the animals grab each other, or else the fox or groundhog will manage to retreat to the next hard corner in the pipe system -- a kind of running battle that can be very frustrating to the digger!
  
  
• Side pipes and stop-end branches: These are common feature of both groundhog pipes and fox dens. Some of these represent unfinished den digging, but for a fox they also provide a place into which one animal can scoot in order to let another pass by, where one animal can lay up without getting overheated from the warmth of its mate and where it can get a better supply of air. Earth dens are all about keeping dry, maintaining the right temperature, keeping the air flowing, and staying secure. Side passages increase the options on all counts.
  
  
• Field settes are generally deeper and larger than hedgerow settes, and often harder for the human to dig up as a consequence: I believe this is due to a conflux of opportunity and necessity. The opportunity for larger and deeper settes is due to the lack of obstructions like roots and rocks. The necessity is that plowed fields with friable soils require deeper dens to remain dry, while larger more complex settes are an element of security, allowing a groundhog or fox to escape danger in one part of the sette by going to another -- or even bolting out of one entrance into another nearby sette.
  
  
• Large "race track" settes are fairly common in fields. These large settes circle back on themselves several times and may have several layers of pipe. More commonly are simple four-, five-, or six-eyed settes with connecting pipes. A groundhog can run around in these large settes, shoveling dirt behind it to block the dog, and effectively preventing the dog from bottling it in a stop end. The solution I employ is to earth stop the den at two or three spots (generally at exits where the pipe goes left and right). If needed I will also allow two experienced dogs that know each other well to enter opposite sides of the settes and work the animal to the middle. This generally prevents the groundhog from digging in and gets the animal pinned against one of the newly-minted stop-ends in short order. Putting two dogs to ground should be left to people who know their dogs well and who are experienced diggers. It is better to lose quarry than a dog. Two very hard dogs to ground, or two dogs in a den occupied by a skunk, could be a disaster! Never enter two dogs on a fox -- it is simply not necessary, as the fox cannot dig away.
  
  
• Winter "fox porches": A fox will often dig out the first three or four feet of a groundhog den and create a kind of "loafing porch" where it can tuck in out of the wind but still not be fully underground. Fox can easily overheat inside the earth, and this "porch" area allows for easy temperature regulation -- go a little deeper if it is very cold, stay right at the entrance if it is warm. The breezeway allows the fox two excellent avenues of escape if danger approaches -- into the nearby woods or thicket, or deeper underground.
  
  
• An observation on what does not dig: Raccoons and possums do not dig earth dens -- they occupy holes made by other animals or find hollow trees, brush piles, rock crevices, hay bales, barns or crawlspaces. Neither animal is equipped to move dirt beyond a little scratching for worms and beetle grubs. Neither animal will modify a den in a significant way (though both will pull plastic bags and other debris into a den in order to improve insulation and provide bedding).