What Is a Burrow? Architecture Built Without Hands
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Digging a hole solves several problems at once, supplying shelter from weather, protection from predators, a place to raise young and a stable microclimate. Animals across many groups build them, some to designs elaborate enough that the structure itself does work the animal cannot.
What a burrow provides
Soil is an excellent insulator, so temperatures a metre down vary far less than at the surface, remaining cool in summer heat and above freezing in winter, and that stability alone is sufficient reason to dig in extreme climates. Humidity is higher and more constant underground, which matters enormously for amphibians and invertebrates vulnerable to drying. Predators are excluded, or at least forced to dig, which converts a chase into a siege that most predators decline. Young can be left in a defensible place. Food can be stored in stable conditions, and several rodents maintain larders that support them through winter. And the structure persists, so the investment in digging is amortised across seasons and frequently across generations, with some tortoise and badger systems continuously occupied for decades or longer, which makes them effectively inherited property.
The designs
Burrow architecture varies with what the animal needs and several features recur:
- •Multiple entrances, which permit escape when one is blocked and which improve ventilation
- •Chambers with distinct functions, including nesting, food storage and latrines, which is documented in badgers, rabbits and many rodents
- •Passive ventilation driven by airflow, with mounds of differing height at each entrance creating a pressure difference that draws air through the tunnel without any effort from the occupant, which prairie dogs exploit
- •Plugs and doors, including the hinged lids of trapdoor spiders and the earth plugs rodents use to seal chambers
- •Bolt holes, short unfinished tunnels near the surface used for rapid escape
- •Depth matched to the local frost line or water table, which constrains where a species can live at all
The animals that build them
Burrowing has evolved repeatedly and the diggers are drawn from across the animal kingdom. Mammals include rabbits, badgers, moles, ground squirrels, wombats, aardvarks and numerous rodents, and the adaptations are recognisable across unrelated groups, with shortened forelimbs, enlarged claws, reduced eyes and ears, and cylindrical bodies. Birds include puffins, sand martins, kingfishers and burrowing owls, several of which occupy holes dug by others. Reptiles include tortoises whose burrows in the southeastern United States shelter hundreds of other species and which are consequently treated as a keystone. Invertebrates include earthworms, crabs, shrimp, termites and a vast range of insects, and their collective effect on soil is enormous. Amphibians and fish burrow in mud to survive dry seasons. The convergence on similar body shapes across unrelated diggers is a standard example of how a physical task shapes anatomy.
The cost of digging
Excavation is expensive and the constraints explain a good deal about which animals dig and where. Moving soil costs energy, with measurements indicating that burrowing consumes many times the energy of walking the same distance, which is why burrowers are generally small, since the cost rises faster than the benefit with size. Oxygen is limited underground, since soil exchanges air slowly and a working animal in a sealed tunnel depletes it, and burrowing mammals typically tolerate far higher carbon dioxide concentrations than surface-dwellers and have blood with a higher affinity for oxygen. Soil type constrains everything, since sand collapses, clay is hard when dry and waterlogged when wet, and the presence of a suitable substrate determines where a species can live regardless of what is above ground. Groundwater sets a floor. Predators specialise on burrows, including snakes, weasels and ferrets, which is why multiple exits exist. And an occupied burrow accumulates parasites, so several species maintain more than one and rotate between them.
What they do to the ground
Burrowing animals are substantial geological agents and the term ecosystem engineer was largely developed to describe them. Digging mixes soil layers, brings subsoil to the surface, buries organic matter and creates channels that carry water, air and roots downward, which is why earthworm activity transformed understanding of soil formation after Darwin devoted his final book to it. The effect on drainage and on erosion can be substantial in both directions. Abandoned burrows are occupied by other species, so a digger supports a community far beyond itself, which is why gopher tortoises and prairie dogs are managed as keystone species. In archaeology, burrowing is a significant disturbance process that moves artefacts vertically and mixes deposits, which is a recognised problem in interpreting stratified sites. And fossil burrows are themselves trace fossils, recording behaviour from long before any body fossils of the makers survive.
The takeaway
A metre of soil buffers temperature and humidity, excludes predators, shelters young and stores food, and the structure persists so the digging investment lasts for seasons or generations. Mounds of different heights at each entrance drive airflow through a tunnel without the occupant doing anything. Abandoned burrows shelter whole communities, which is why tortoises and prairie dogs are managed as keystone species.