What Is a Den? The Difference Between a Home and a Nursery

The functions vary and frequently combine. Shelter from weather is the most general, since a cavity buffers temperature and keeps rain and wind off, which matters most for young and for animals in extreme climates. Protection from predators is central for species that leave young unattended while foraging, since a defensible entrance converts a vulnerable litter into a difficult target. A stable microclimate permits hibernation and torpor, which depend on conditions staying within a narrow range for months. Storage occurs in several species, with food cached in or near the den. Social function matters for group-living animals, since a shared den is where a group assembles and where young from several mothers may be kept together. And territorial signalling attaches to den sites, which are marked and defended. Those functions pull in different directions, so a den suited to hibernation may be poor for raising young, and several species maintain more than one for different purposes.

Animals acquire dens by several routes and the distinction matters ecologically:

• •Excavation, where the animal digs its own, which requires suitable soil and considerable energy
• •Occupation of natural cavities including caves, rock crevices, hollow trees and root spaces, which are limited in number and frequently contested
• •Taking over structures built by others, which is extremely common, with foxes using badger setts and a long list of species using burrows they did not dig
• •Construction from gathered material, as beavers do with lodges and as several rodents do with above-ground nests
• •Human structures, which many species have adopted readily, including buildings, culverts, bridges and abandoned machinery
• •Inheritance and reuse across generations, with some sett and burrow systems occupied continuously for many decades and expanded by successive occupants

Overwintering makes the sharpest demands on a shelter, and the requirements are specific enough to limit where species can live. A hibernating animal's body temperature falls close to ambient in many species, so the den must stay above freezing, which generally means below ground or under an insulating cover, and snow itself is an excellent insulator, which is why species in snowy regions can use shallower sites than the winter air temperature would suggest. Humidity must be sufficient to prevent drying out over months without drinking. Disturbance is costly, since arousing from torpor consumes a substantial share of an animal's stored energy and repeated disturbance can be fatal, which is why bat roosts and bear dens are legally protected in many jurisdictions and why casual disturbance is more serious than it appears. Bears are a partial exception, since their temperature drops only modestly and they can wake readily, and bear dens are correspondingly less demanding than those of true deep hibernators.

Where suitable dens are scarce, they rather than food determine how many animals a landscape supports, which has direct management consequences. Cavity-nesting and denning species decline when old trees are removed, since a hollow suitable for occupation takes a century or more to form and cannot be replaced quickly, and forestry practices that remove veteran and dead trees eliminate the supply. Artificial provision works, with bat boxes, owl boxes, artificial setts and purpose-built structures adopted readily by several species, though uptake varies and poorly designed boxes can be worse than nothing by overheating. Conversely, abundant artificial shelter has allowed some species to expand into places they could not otherwise occupy, which is part of why foxes, raccoons and several others thrive in cities. Legal protection frequently attaches to the structure rather than the animal, so a badger sett or a bat roost is protected whether occupied at that moment or not, which reflects that the site is the scarce resource.

Shelter, predator protection, a stable microclimate for hibernation, storage and social assembly pull in different directions, so many species keep more than one site. A great many dens are occupied rather than dug, which makes availability the limiting resource where old trees or suitable ground are scarce. Arousing a hibernating animal costs a large share of its stored energy, which is why disturbance is legally restricted.