Wednesday, January 26, 2011
Tuesday, January 04, 2011
From sea beasts that disembowel themselves to lizards that make knives out of their own broken bones, you don't want to mess with these kick-ass critters
IN The Restaurant at the End of the Universe, Douglas Adams wrote about the "dish of the day": an animal that wants to be eaten and can say so loudly and clearly, thus circumventing many of the ethical problems associated with meat.
Most creatures do not want to be eaten and many go to extraordinary lengths to avoid it. Some defence strategies are well known: fish and birds gather in shoals for safety, possums play dead, bees sting, and skunks spray a liquid so repulsive that humans caught in the blast are almost always reduced to vomiting.
Yet all of these are just the tip of a very big, very ingenious and very dangerous iceberg. There is a host of other even more peculiar tricks that animals use to convince their persecutors to pick on someone else.
Taking the point
Stabbing a predator with an offensive weapon is a tried and tested defence strategy: thousands of animals and plants are covered with horns, thorns or spines. Nobody who has tried to eat the fruit of the prickly pear - or a hedgehog for that matter - can be under any illusion that they make an easy meal.
But what if you don't have any obvious means of stabbing? At first glance, the 30-centimetre-long sharp-ribbed newt (Pleurodeles waltl) looks innocuous enough, but it carries a secret weapon. Its ribs have sharp tips like spears, and when it is under attack it arches its back and punches them out through its skin. Hey presto: spines.
These spines are doubly dangerous, because like many of its relatives the sharp-ribbed newt also releases a toxic milky liquid onto its skin. As the spines break through the skin they become coated with venom.
Josef Weisgram at the University of Vienna in Austria has studied the newts for several years, and earlier this year he and his team confirmed that the ribs really do break through the skin afresh every time it performs its horrible routine. There are no pores or holes for the ribs to slide smoothly out of (Journal of Zoology, vol 280, p 156).
A similar trick is pulled by the hairy frog (Trichobatrachus robustus), a native of Cameroon. It has retractable claws on its back feet rather like a cat's. The claws are made of bone and are attached to the tip of the frog's toes by a small piece of collagen.
Under normal circumstances the claws are stashed safely inside the frog's foot - until it comes under attack, at which point it snaps the claws off the toe and thrusts them out through the skin with a strong muscular contraction. If the predator persists, the frog kicks and struggles violently, slashing at its enemy with its improvised claws.
Eat my tail
If a lion had its jaws clamped around your arm, you might willingly sacrifice the limb to escape with your life. It's a strategy not uncommon in the natural world. Many animals actively shed limbs or tails to save the rest of their skins, a process called autotomy. In many cases, the animal regrows the lost body part afterwards. Lizards and snakes are the best known autotomists but are by no means the most extreme.
For some snakes it is almost routine. Spanish ladder snakes (Rhinechis scalaris) can break off their tails by vigorously thrashing and rotating their bodies. A survey published this year in the Journal of Zoology (vol 113, p 269) found that up to 20 per cent of adult ladder snakes were missing a tail.
Some lizards take the idea a stage further. They sport brightly coloured tails which continue to thrash around after they have been detached, presumably to distract a predator. The five-lined skink (Plestiodon fasciatus) is a good example, with an electric blue tail that contrasts vividly with its yellow-and-black striped body.
Octopuses, crabs and spiders can also shed limbs. But the true champions of self-amputation are sea cucumbers, marrow-shaped relatives of starfish that live on the sea floor. When startled they eviscerate themselves by shooting the tubes that make up their respiratory system out of their rear ends (The Journal of Experimental Biology, vol 204, p 849). They may even go the whole hog and expel their digestive tract as well. The tubes are sticky and entangle any would-be predators. It sounds suicidal but the sea cucumber grows them back within weeks.The true champions of self-amputation are sea cucumbers, which can eviscerate themselves
Chemical warfare is frowned upon in most human societies, but to animals it's routine. Many species follow the example set by South America's poison dart frogs and simply load their bodies with deadly poisons. They advertise their lethality with bright colours - yellow and black being a favourite - so that predators know to steer clear.
Better still if you can deter predators from a distance, but for that you need ballistics. Four groups of beetle, known collectively as bombardier beetles, have honed this to an art.
The beetles have glands in their abdomens which can shoot rapid-fire pulses of hot caustic liquid (pictured, right). Each gland has two chambers, one containing a mixture of hydroquinones and hydrogen peroxide, the other various enzymes. When the two are mixed, a series of explosive reactions occur, including the conversion of hydrogen peroxide into oxygen and water. These reactions heat the mixture to boiling point and blast it out of the beetle's backside with an audible pop. Among the products of the reaction are quinones, which many species find repellent even when cold.
The beetles are good shots. The African bombardier beetle (Stenaptinus insignis) can aim in any direction with great precision. Some species have flanges on their abdomens that act as launch guides.
The bombardier beetles' boiling volleys are fearsome weapons, but they are not the most distasteful. That distinction must surely go to the horned lizards, the only vertebrates that shoot their own blood at predators. The Texas horned lizard (Phrynosoma cornutum) relies for protection mostly on camouflage and its spiny skin, but if push comes to shove it can shoot jets of poison blood out of its eyes (the lizard feeds on poisonous ants and accumulates the prey's toxins in its bloodstream). Formidable as this sounds, it is not entirely effective: the lizards often get eaten by carnivorous mice.
Last but not least, one can only admire the entirely unsubtle Turkey vulture (Cathartes aura), which deals with potential predators by vomiting up semi-digested rotten meat. "It is certainly one of the most horrific smells that I have encountered in nature," says ecologist Lawrence Igl of the US Geological Survey in Jamestown, North Dakota, who says the trick might either deter the predator, reduce the vulture's weight allowing it to fly away, or simply provide the predator with an alternative meal.
Call in the big boys
"I'll set my big brother on you," and "My dad's bigger than your dad." These playground threats have worked for many of us at some point and are surprisingly common in nature. If you can't fight off a predator yourself, it's a good idea to summon something else that can.
Plants, seemingly so passive, do it all the time. When herbivores start munching on their leaves, plants can release a cocktail of chemicals called green leaf volatiles (GLVs) into the surrounding air. Tobacco plants are particularly sly. When attacked by tobacco hornworm caterpillars, they release GLVs which attract insects called big-eyed bugs, which fly in and chow down on the unfortunate caterpillars (Science, vol 329, p 1075).
Single-celled marine animals called dinoflagellates have a cunning twist on this idea. When predators approach, they often start to glow - a strategy that on the face of it appears suicidal. In some cases the dinoflagellates flash briefly, startling the predator, but others glow continuously. Either way, the predators often retreat (Journal of Experimental Marine Biology and Ecology, vol 36, p 217). But why? It turns out that the glow attracts other, larger predators which are likely to attack the original predator rather than the dinoflagellate. In effect they pursue a policy of mutually assured destruction: any predator that tries to make a meal out of the dinoflagellate is likely to be eaten itself.
When we think of camouflage, it's usually intricately patterned big cats, or perhaps insects that look like bird droppings. But a squirrel chewing on the sloughed-off skin of a rattlesnake?
Yet that is exactly what female California ground squirrels (Spermophilus beecheyi) do to disguise themselves from their slithering predators. After a good chew on the snakeskin they lick themselves and their pups, anointing all concerned with the rattlesnake's odour - and it seems to work. In 2006 Barbara Clucas at the University of Washington, Seattle, showed that rattlesnakes were less interested in bits of filter paper that smelled of both snake and squirrel than they were in paper that just smelled of squirrel, suggesting that the squirrels' behaviour really did protect them. Some chipmunks anoint themselves too, and similar behaviour with predator scents has been observed in rats and mice, Clucas says.
Clucas and her colleagues have now gone further, trying to work out how this strange behaviour evolved. She points out that many rodents will bite predators, and also groom themselves when stressed. So it seems plausible that an ancestral rodent might have bitten a predator, picking up its scent in the process, and then inadvertently smeared it all over its fur. This could then have evolved into the anointing behaviour seen today.
If things are really desperate, you could always try blowing yourself up. It will do nothing for your own survival prospects - far from it - but it could help your relatives.
Suicidal behaviour is often found in social animals that live in closely related groups. Honeybees are a classic example. They sting intruders to save their hive mates even though this usually results in a fatal loss of their insides.
Ants also live in colonies, in which many thousands of sterile workers support a single breeding queen. Several species of social ants go in for explosive suicide, and the carpenter ant (Camponotus cylindricus) is one of the worst. Workers have massively enlarged glands running down the length of their body. When they are attacked these burst open, spraying opponents with sticky goo.
The gloopy liquid slows the attackers and sticks their mandibles together. It is also laced with irritating and corrosive chemicals (Journal of Chemical Ecology, vol 30, p 1479). The detonated worker usually dies, but the colony has been protected.
Termites have also got in on the act. Globitermes sulphureus lives in south Asia, where its soldiers are known as kamikaze termites. They have large salivary glands which produce a yellow liquid that rapidly congeals and entangles intruders (Insectes Sociaux, vol 44, p 289). If the soldiers are particularly overexcited, they go into violent contractions and burst their body walls, spraying opponents with the fluid.
Michael Marshall protects himself by squirting poison blood from his eyes
Via New Scientist. #apocalypseworld