Cryptic creatures: the art of camouflage
A FLY DRONES NOISILY in the humid, dripping greenery of a north Queensland rainforest. Faint but getting stronger, the scent of dung draws it down until it is flying in ever smaller circles. It spies its target – a tantalising, white-and-brown bird-dropping on a green leaf – and lands close by.
Not for a moment does it see the attack coming. In an instant, the dung unfurls legs and opens powerful jaws, and the fly finds itself in the grasp of one of nature’s best tricksters, a bird-dropping spider.
It’s a stunning and intricate deception: a spider dressed up as bird poo, complete with a wafting odour of dung and “bird-dropping” webs it has spun on the leaf in a decoy splash pattern. The disguise is so convincing all it has to do is sit back and wait for dinner to arrive.
Camouflage is a do-or-die business in the animal kingdom and most species use it to some extent.
If they come up with a convincing masquerade they can evade predators, find food and survive to pass the advantageous gene on to future generations until the disguise becomes as perfectly refined as that of that bird-dropping spider. Fail and they maybecome extinct.
Given aeons for natural selection and evolution to do their work, animals can develop an astonishing complexity of camouflage and deceptive techniques to fool predator, prey and even mates. Shape, colour, smell and movement are all manipulated. All’s fair in love and survival.
The disguise of that bird-dropping spider works both aggressively and defensively, simultaneously luring its prey and convincing birds and other predators that it isn’t something to eat. It’s a popular ruse that’s also used by frogs, moths and other spiders to hide from predators in plain sight.
“The principle of camouflage is to make it economically unviable for a predator to pursue a particular species of prey,” explains Professor Mark Elgar, an evolutionary biologist at The University of Melbourne. “Camouflage increases the search time and, as a consequence, the predator will simply target another species, either because it doesn’t see the camouflaged individual or it just finds something more obvious to eat. In that context, most of it concerns a predator’s vision.”
Camouflage for survival
The easiest way for an animal to disguise itself is to be cryptic, or invisible, in its surroundings.
To that end, stick and leaf insects and katydids have evolved some of the insect world’s most complex camouflage to hide themselves from predators. Many, such as Macleay’s spectre (also known as the spiny leaf insect), have the texture of sticks, bark or dry leaves, while others, such as Haffner’s snubnosed katydid, imitate living foliage, right down to the veins in a leaf. Some katydids even develop blemishes to match the spots caused by disease or insects’ nibblings.
A convincing appearance only works if its owner also acts the part, so during the day the most effective cryptic creature keeps quite still or sways slightly, like a dead stick or leaf in the breeze. If disturbed, it falls to the ground and stays still.
Entomologist Paul Zborowski, who has spent decades photographing some of Australia’s most inconspicuous creatures, rates the insects of Central Australia’s gibber deserts as the most convincing cryptic creatures he’s seen. “It’s an incredibly old habitat so the creatures have had a long time to adapt,” Paul explains. “Most of them behave like stones and don’t move all day, feeding only at night.”
A tawny frogmouth sitting motionless on the end of a stump, its head raised so its mottled grey-and-brown body looks just like an extension of the wood, also illustrates the importance of pairing a persuasive costume with behaviour. Professor Gisela Kaplan, from the centre for neuroscience and animal behaviour at the University of New England, Armidale, says the frogmouth’s skill at camouflaging is a learned behaviour. While adopting a pose may be a reflex, and can be observed in a hatchling’s first week, the ability to choose a backdrop which matches its colouration and maximises the camouflaging effect does not develop for 4–6 months. When they fledge, she says, they land in any place and are usually highly visible, to the distress of their parents, who often try in vain to motion the youngsters to a safer location.
Fixed camouflage is only good against a relatively unchanging environment, so some animals, such as the cuttlefish, have evolved an adaptable disguise. Like other cephalopods, including squid and octopuses, the cuttlefish can almost instantly change its colour, patterns and texture to match its surroundings, using a sophisticated system of specialised cells and muscles.
On Queensland’s reefs, scientists have been studying another ocean dweller that uses colour change, although not to blend into the back-ground. Dr Karen Cheney, from The University of Queensland’s school of biological sciences, says the bluestriped fangblenny alters its colouration to mimic other species of fish, allowing it to school with them and benefit from the safety that lies in numbers.
The fangblenny’s most impressive impersonation is its black-with-neon-blue-stripe version of the juvenile striped cleaner wrasse, a marine do-gooder that removes parasites from larger fish. Not only does the fangblenny benefit from the reduced predation that comes with the wrasse’s mutually beneficial relationships with other fish, but the disguise also lets it get close to its prey. It darts out from the wrasse’s cleaning station to nip at unsuspecting fish passing by, but doesn’t attack those coming in to be cleaned.
“This is probably so it can maintain the relationship with the cleaners, because they would chase it off otherwise,” Karen says. “The fangblenny is not an exact mimic – it has a different body shape and swims in a different way – but it looks similar enough to prevent detection by other fish, in order for it to get close enough to attack.”
The fangblenny is engaging in aggressive mimicry – a form of deception used by predators to lull their prey into a false sense of security. Some, like the fangblenny, mimic a harmless creature to get close to their target. Others pass themselves off as their target’s prey or food. Anglerfish species employ the latter technique, literally fishing for their meals. The fish has a special lure, or illicium (Latin for “lure”), on its forehead, from which dangles a bait, called the esca.
When trying to attract prey, the anglerfish rests, camouflaged, swinging the lure in front of its mouth and wiggling the esca. When prey comes to investigate, the anglerfish opens its large mouth and sucks the hapless victim in so fast it’s impossible to see without the benefit of a slow-motion replay of the catch.
In many cases, the mimic is perfectly designed to deceive its target, focusing on specific aspects of its chosen prey. In northern Queensland, within the woven-leaf walls of the aggressive green tree ant’s nest, the Cosmophasis bitaeniata (a type of jumping spider) wanders undetected. It doesn’t look anything like the ants swarming around it, yet they pass by as if it’s not there. It reaches the section where worker ants are guarding the larvae – its prey – and gently takes one from the mandibles of its protector.
Imitation is deadly flattery
The ants have poor vision and rely on scent to alert them to potential danger, so the spider has evolved a realistic imitation of the ant’s chemical markers, rather than a pointless physical likeness.
Another species, known as the green tree ant mimicking spider, bears a strong resemblance to the green tree ants, even raising its forelegs over its head to mimic antennae. If the jumping spider can use chemical mimicry to get close enough to eat stray green tree ants, why does this one bother with the physical imitation?
“The answer in that case depends on [who is] being fooled,” says Mark Elgar, who studies the Cosmophasis bitaeniata. “It’s not the green tree ant, because its eyesight is simply not good enough to require that level of visual mimicry. It’s more probable [it’s aimed at birds], which don’t typically like eating the ants and are likely to be a major predator [of spiders].”
While the green-tree-ant mimics focus their deception on one species of prey, the fringed jumping spider has a bag of tricks to use on a variety of victims.
Found in north-east Queensland and rainforests and riverine areas around Darwin, this species hunts other spiders using a number of complex, prey-specific techniques, explains Martyn Robinson, an educational naturalist with the Australian Museum.
For example, when hunting a female jumping spider of the Euryattus genus, it taps on her curled-leaf home in the web like a male interested in mating, then attacks with fast-acting venom when the female comes to investigate. If she recognises it and retreats inside the leaf, it adjusts its approach, waiting at the doorway of the nest for her to become complacent and re-emerge (it’s been observed waiting for nearly two hours).
Mimicry and patience are not the fringed jumping spider’s only tricks; its camouflage as forest detritus allows it to employ “cryptic stalking”, where it hides its palps, moves slowly and freezes if its victim looks towards it. It will also use environmental conditions as a kind of camouflage; if gusts of wind are shaking the prey’s web, it will change its slow-and-steady walking technique and advance in speedier bursts, using the vibrations caused by the breeze as a cover.
The most famous form of mimicry, however, is for defence, not attack. Batesian mimicry refers to animals that gain protection from predators by imitating the appearance of a noxious or dangerous organism, often in the form of conspicuous colours. It was named for 19th-century naturalist Henry Bates, a travelling companion of Alfred Wallace (see Darwin, Wallace & the princess in the south, page 94), who first suggested it after noticing that several Amazonian butterfly species looked the same.
Mimicry an advantage
The green tree ant mimicking spider is employing Batesian mimicry, gaining protection by imitating the appearance of the ferocious green tree ant. In Australian waters the harmless harlequin snake eel sports the same striking black and white bandings as the highly toxic yellow-lipped sea krait, ensuring no predator will take it on. But the success of Batesian mimicry depends on the ratio of mimics to originals. “If a predator encounters too many that are edible, it will just assume none of the animals with those markings are dangerous,” Martyn explains.
An imitation of a more dangerous creature doesn’t need to be exact, just enough to make a potential predator hesitate long enough to effect escape, as is the case for the many butterflies with highly conspicuous markings on their wings. The pale brown hawk moth caterpillar, found around Brisbane, has markings resembling a snake’s eyes on its abdomen. When confronted, the caterpillar pulls its head in and the “eyes” flash open. Whether the potential predator thinks it has seen a snake or is simply startled is unclear, but the outcome is that the caterpillar lives to see another day.
The development of such markings is a cumulative process, Mark says. “Imagine you have a population of individuals of a particular colour and then one has particularly intimidating markings, which makes it slightly less likely to be eaten by predators. That individual is advantaged, so it reproduces and its offspring also have that characteristic. Over time, that characteristic spreads throughout the population. Those markings become more striking in some individuals, putting them at further advantage, and so the cycle continues.”
In the tropical tablelands of north Queensland, the chameleon gecko has another way of making predators hesitate: a built-in decoy. Its body is leaf-litter brown but its tail is banded in contrasting black and white.
“If attacked, it will drop its tail, which will wriggle around, as do many lizards’ [tails], but in this case the tail bones actually rub against one another, so it squeaks,” Martyn says. “The predator is, of course, thoroughly absorbed by this black-and white-striped, wriggling, squeaking thing on the ground, and the gecko can sneak away.” It’s a one-time-only trick: the regrown tail is hued speckled brown, the same as the gecko’s body.
Diversion is also the aim of the broken-wing display used by ground-nesting birds, including Australia’s red-capped plover, whose eggs and young are particularly vulnerable to predation. The adult bird flaps around on the ground, feigning injury to convince an approaching predator it is easy pickings, and leads the predator away from the nest.
“The birds are reasonably well camouflaged but they have no impressive claws or beaks, so they have evolved this defence,” says Gisela Kaplan. “Sometimes the female will drag the wing on the ground and limp, or twist it or extend it to one side, draw the predator away quite a distance and then, as the predator goes to attack, fly away in the opposite direction of the nest. The behaviour is probably both genetic and learned, but definitely deliberate, because the display can be varied from one encounter with a specific predator to the next.”
Such precise camouflage, sophisticated mimicry and elaborate trickery illustrate the genius and limitless possibilities of evolution and nature. But, as Martyn points out, only the most successful illusionists are here to tell the tale.
“A lot of it is sheer accident,” he says. “There’s an extraordinary number of species that have been on planet earth, the majority of which are now extinct. Maybe they came up with the wrong adaptation or maybe luck was against them but either way they didn’t survive. Considering that, the complexity of these adaptations seems a little less unbelievable.”
Source: Australian Geographic Oct – Dec 2009