Home improvement

For animals, home is where you have a better-than-even  chance of surviving

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Even with the rudimentary satellite technology of a quarter of a century ago, they were clearly visible from space—bare patches in the South Australian mallee scrub, some of them a square kilometre in area, dotted with distinct mounds. But before the ufologists could claim them, biologists discovered the cause: the hairy-nosed wombat, or rather, their homes.

Australians will warn you to drive around a wombat every time—they’ve written off Holdens before now. At more than 30 kilograms, these sturdy marsupials are heavy-duty diggers. With powerful legs and tough claws, they can excavate a standard eight-metre-long tunnel, with two or more entrances, in no time flat. At the end, they then gouge out a warren—shelter from predators and the pitiless Australian sun.

The vegetation starts to suffer when wombats cluster those burrows and caverns into labyrinthine complexes. Some of them are vast: more than 80 m of tunnels, with maybe two dozen entrances, leading to underground chambers sometimes 30 m across. All this soil disturbance takes its toll on deep-rooted desert plants, and the damage, it turns out, is plain to see if you get high enough.

So animal architects can change the world. The most obvious mover and shaker is the beaver, which not only chops down the timber it needs to construct ingeniously conceived sub-aquatic shelters, but will create the pond if needs be, by damming draining streams. After humankind, no other creature alters the landscape quite like a beaver: one beaver dam in Wood Buffalo National Park in Alberta, Canada, is 850 m across.

Summer and autumn, beavers are a blur, frantically preparing for the bleak North American winter. Besides building, extending or repairing dams of last season, they
excavate ditches leading to prime food sources. When winter sets in, they can swim along these, safe from wolves and bears beneath a shield of ice. A builder would instantly recognise a beaver lodge’s construction: stout poles bear the weight of the roof, while angled buttresses brace the downstream wall of the lodge against the water pressure created by the dam. The lodge itself has two chambers; one is a ‘drying room’, where wet beavers linger before entering the living room. This is a fortress; during autumn preparations, the beavers plastered fresh mud across the roof, so that now, in the dead of winter, it has frozen solid into armour that will turn even a wolverine’s claws. Nor can anything reach them from below—the entrance is below water level.

European rabbits live in systems of burrows known as warrens. They may be three metres in depth and include nesting chambers, living quarters and a number of entrances for foiling predators—up to 60, and roughly corresponding to the number of rabbits using the system. Rabbits must burrow to breed, behaviour that is intimately connected with the endocrine state of the female, or doe, and may even stimulate breeding activity. As well as a breeding site, the warren also provides insulation from extremes of temperature outside, allowing rabbits to occupy territory that may be well outside the range of conditions they could normally tolerate. One of the earliest methods to control rabbits in New Zealand was simply to dig up the warrens by hand and dispose of the young—a strategy which became unfeasible as numbers reached plague proportions.

The evolutionary wiring for all this industry is powerful: if you play the sound of running water to beavers for long enough, researchers found, they’ll start piling sticks against the speakers. When wildlife managers remove “nuisance” beavers, another family sets up house within weeks. If landowners try to drain a beaver dam, the creatures simply block up the culverts.

But it seems you don’t need a sophisticated mammalian brain to master house construction. In fact, you don’t need a brain at all. The home of Difflugia coronata, a single-cell amoeba with no nervous system, is a triumph of micro-masonry—a perfect globe of tiny particles cemented together, opening to an aperture at the base surrounded by an intricate pleated collar. Near the tip stands a ring of masonic spikes. The entire structure spans 150 thousandths of a millimetre, only slightly bigger than the amoeba that lives inside it.

If you’re wondering how a single brainless cell constructs such a stylish and intricate home, so is everyone else. Nobody knows.

Many species of ant construct vast, elaborate underground complexes—like the wombats—of interconnected chambers as refuges, grain stores and nurseries, using only their mandibles.

Ants have solved all the same design and construction problems that we have—support, access, durability and air conditioning are built for, often in precise relation to the soil depth, which poses some interesting questions. For starters, how do ants build complicated, brilliantly designed structures, across broad expanses, with no plans, and no foremen, in total darkness? What’s more, they presumably have to know precisely where they are, and at what depth, in the absence of physical cues.

It’s still a mystery, but one hypothesis is that ants can somehow estimate their depth by sensing carbon dioxide levels, which increase as they burrow deeper.

Many animals understand the relative merits and applications of specific materials, but others simply manufacture them themselves—to exacting specifications. Silk production has evolved independently in many different arthropods, from spiders to caddisfly larvae to the master spinner, the silkworm moth larva.

Silk is so versatile that it can be used not only to build homes and temporary shelters, but to snare and store prey, set tripwires or provide a safety tether. The diving bell spider even constructs a waterproof bubble from the stuff.

Aerial web-building spiders can produce at least seven different types of silk, each ideally suited to a specific purpose, by utilising a range of special proteins evolved over 400 million years of R&D. They can control the silk’s dynamic properties through versatile organs called spinnerets and spigots, which themselves are a reflection of the many uses silk has ended up being put to.

When it comes to high-tech building materials, silk takes a lot of beating. In Madagascar, the Darwin’s bark spider slings 25-metre-long silken strands across rivers. Testing has proven them more than ten times tougher than Kevlar. Even run-of-the-mill spider silk has only slightly less tensile strength than steel, but it’s six times less dense (a strand long enough to circle the globe would weigh in at barely half a kilogram), so weight for weight, spider silk is five times stronger.

Wonder materials like this don’t just increase survivorship: they allow organisms to expand existing ranges and occupy new niches, exploit different prey and employ novel predatory strategies.

The shells of molluscs such as snails, clams, oysters and many others are composed of a protein structure coated with calcium carbonate. They are stiff, but grow with the animal by extending continuously in a geometric whorl. High rates of growth result in wide-mouthed forms such as paua, and low rates result in more tightly-coiled shapes. The generating curve—approximately the shape of the aperture—may be round, like the snail shell pictured, or elongated like a cone shell, depending upon the environment. Wave-washed environments are usually inhabited by snails whose shells have a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and sculptured forms are more common in sheltered environments. Burrowing forms are smooth, elongated, and lack elaborate sculpture to decrease resistance when moving through sand.

Other creatures have learned to exploit silk without going to the trouble of making it themselves: the tailorbird of Asia plunders spider webs for silk, which it then uses as thread, stitching large leaves together to make a cradle for its nest.

Perhaps the functional opposite of silk is the rigid shell of calcium carbonate that protects many molluscs. In gastropods, such as snails, whelks and cowries, the shell is secreted from the mantle, practically from embryony. All gastropod shells coil about an axis—each species exudes a shell so distinct that mathematical modellers can replicate it exactly, simply by feeding in the appropriate logarithm. Rapid growth around the axis generates wide-mouthed shells like the paua’s. Slower deposition creates the tightly coiled, narrow tubes of the turret shells, while variations in the generating curve produce the wondrous spikes and whorls of the spiny murex and the lustrous lips of the cowries.

And, just like humans, you get the odd southpaw: while the shells of more than 90 per cent of gastropod species are dextral, or coil right-handed, the remainder are almost always sinistral. A very few species produce roughly even numbers of both.

Not all shelters have to last. That’s when simplest (and energetically cheapest) works best. Each evening, the tropical queen parrotfish settles into a secluded coral nook for the night. But sleeping diurnal fish are vulnerable to attack by the reef’s night shift, like moray eels, which hunt by smell. So the parrotfish blows a bubble of mucus, completely encapsulating itself in a pliable membrane which allows gases to exchange but, it’s thought, blocks the fish’s scent from hunters.

Homebuilding has helped myriad creatures survive the trials of life in environments everywhere. Without this heritable skill, thousands of species wouldn’t have made it this far, and many more would have disappeared with them, because wombats and beavers, ants and amoebae all create micro-habitats—or whole new niches—for others.

The flip side of all this animal industry, however, is when it undermines our own. Introduced to Tierra del Fuego, without predators and into the wrong sort of vegetation, beavers became a plague, flooding thousands of hectares. Rabbits turned much of New Zealand’s South Island high country to dust with their digging.

Some animal structures endure through ages—ecologists say that badgers hand their setts down from generation to generation, and cores from the centre of termite mounds in South Africa have been carbon dated at 4000 and 5500 years old. These represent more than simply serendipitous habitats—they are as much a part of our collective landscape as the pyramids, or the Great Wall.

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