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Animal architecture – that is, structures built by animals other than humans – is prevalent in nature. We offer some images here of termite mounds, wasp and bee hives, rodents’ burrow complexes, beaver dams, elaborate bird nests, spider webs and more. Aside from humans, who are the most prolific and versatile builders, building behavior is common in many mammals, birds, insects and arachnids, as well as a few species of fish, reptiles, amphibians, mollusks, urochordates, crustaceans, annelids and some other arthropods. It is virtually absent from all the other animal phyla. As ecosystem engineers, the influence of animal builders is extensive and they generally enhance biodiversity through their niche construction. There are three broad categories of built structures by animals: homes, traps, and courtship displays. Animals primarily build habitat for protection from extreme temperatures and from predation. Constructed structures raise physical problems which are resolved through the architecture, such as humidity control or ventilation, increasing the complexity of the structure. Over time, through evolution, animals have expanded the use of shelters for additional purposes such as reproduction, food storage, etc. Often, these structures incorporate sophisticated features such as ventilation, temperature regulation, structural strength, multiple escape routes, traps, bait, special-purpose chambers and other features. They are created by individuals or by complex societies of social animals with some society members carrying out specialized roles. Constructions may arise from complex behavior of animals such as night-time sleeping nests for chimpanzees, to inbuilt neural responses in the case of birds. The process of building such structures may involve learning and communication, and in some cases, even aesthetics. Tool use may also be involved in building structures by animals. (The image captions contain additional information)
Protection from Predators
Predators are attracted to animal-built structures either by the prey or its offspring, or the stored caches of food. Structures built by animals may provide protection from predators through avoiding detection, by means such as camouflage and concealment, or through prevention of invasion, once predators have located the hideout or prey, or a combination of both. As a last resort, structures may provide means of escape.
Animals use the techniques of crypsis or camouflage, concealment, and masquerade, for avoiding detection.
Ground-nesting birds which rely on crypsis for concealment have nests made from local materials which blend in with the background, the eggs and young too are cryptic; whereas birds which do not use crypsis for hiding their nests may not have cryptic eggs or young.
In a case apparently of masquerade, the Red-faced Spinetail Cranioleuca erythrops places bits of grass and other material loosely streaming both above and below the nest chamber to break the shape of the nest and to cause it to resemble random debris without any underlying structure.
In endothermic animals (organisms that produce heat through internal means, such as muscle shivering or increasing its metabolism such as humans), construction of shelters, coupled with behavioral patterns, reduces the quantity and energy cost of thermoregulation.
In ectothermic animals (organisms that control body temperature through external means such as frogs), moderation of temperature, along with architectural modifications to absorb, trap or dissipate energy, maximizes the rate of development. The primary sources of energy for an animal are the sun and its metabolism. The dynamics of heat in animal shelters is influenced by the construction material which may act as a barrier, as a heat sink or to dissipate heat, such as in the cocoons of insects. Internal architectural devices, such as walls, may block convection or the construction of air flow systems may cool a nest or habitat.
Materials used by animals in building structures need to not only be suitable for the kind of structure to be built but also to be manipulable by the animals. These materials may be organic in nature or mineral. They may also be categorized as "collected material" and "self-secreted material". Some animals collect materials with plastic properties which are used to construct and shape the nest, including resin collected by stingless bees, mud collected by swallows and silk collected by hummingbirds.
Some materials in nature act as ready made "building blocks" to the animals in question, such as feathers and leaf petioles for some birds and animal hair. Other materials need to be "processed". Caddisfly larvae use stone pieces and also cut sections from green leaves for use in construction. The stone pieces are selected as per their size and shape from a large variety. In the case of leaf sections, these are cut and shaped to a required size. Some wasps collect mud and blend it with water to construct free standing nests of mud. Paper wasp queens build with paper pulp which they prepare by rasping wood with their jaws and mixing it with saliva.
An animal builder may also collect a variety of materials and use them in complex ways to form useful habitat. The nest of the Long-tailed Tit is constructed from four materials – lichen, feathers, spider egg cocoons and moss, over 6000 pieces in all for a typical nest where structural stability is provided by a mesh of moss and spider silk. Inside, the nest is lined with more than 2000 feathers to insulate the nest.
Birds collect animal fur and feathers of other species of birds to line their nests, and some birds use spider silk. Flowering plants provide a variety of resources – twigs, leaves, petioles, roots, flowers and seeds. Basal plants, such as fungi, lichens, mosses and ferns also find use in structures built by animals. The leaves of grasses and palms being elongate and parallel-veined are very commonly used for building. These, along-with palm fibers and horse-hair fern are used to build hanging baskets. Wooden twigs form the greater proportion of materials used in the nests of large birds. Plants and trees not only provide resources but also sites. Branches provide support in the form of cantilevered beams while leaves and green twigs provide flexible but strong supports.
Mud is used by a few species of a wide variety of families including wasps and birds. Mud is plastic when wet and provides compressive strength when dried. Amongst birds, five percent of all birds use mud and stones in their nest for toughness and compressive strength.
The majority of self-secreted materials are produced by insects. In some cases, the self-secreted material is directly applied, or it may be processed, as paper wasps do using salivary excretion and blending it with wood pulp. Self-secreted materials may be processed in some cases. In cribellate spiders, silk produced by the spider are reworked in the cribellum to form fine sticky strands used for capturing prey. In other cases, the scale wax, produced on the bodies of honey bees, is gathered and blended with saliva to form comb wax, the building material for interiors of hives.