Saturday, February 18, 2012

Professors of the art of disguise in nature: Larvae amazing disguised in the form of snakes to protect itself!

Posted by Unknown at 6:21 PM
Imagine that you are a one-half-ounce, two-inch-tall insect-eating bird foraging for dinner on the dimly lighted floor of a Costa Ricanrain forest. You come face to face with a pair of beady eyes. Study them for a moment.




If those eyes belonged to a snake, that moment of study would mean that you would be dinner by now.
The face, however, is not a snake’s, but the chrysalis of a skipper butterfly. An uncanny resemblance — but, as it turns out, not a unique disguise.
In one area of Costa Rica alone, a team of researchers led by Daniel H. Janzen and Winnie Hallwachs of theUniversity of Pennsylvania and John M. Burns of the Smithsonian National Museum of Natural History have discovered hundreds of species of moths and butterflies whose caterpillars or chrysalises display false eye and face patterns that mimic those of snakes, lizards or other animals. In a study published this week in The Proceedings of the National Academy of Sciences, they propose that this plethora of counterfeit patterns has evolved to exploit birds’ innate instinct to avoid potential predators.
The idea is a fresh twist on the well-established phenomenon of mimicry among animals. First described by the British explorer Henry Walter Bates in the 1860s (the subject of mycolumn on Feb. 16), the original insight was that harmless, edible species could gain protection from predators by resembling distasteful, noxious species.
Bates assumed that for this mechanism to work, the potential predators had to learn which prey in their range were to be avoided. And the potential prey — say, a large, colorful adult butterfly — must closely resemble the inedible species it mimics.
But when it comes to a deadly encounter with another species, there may be no second chances, no opportunity for learning. Hence, natural selection would favor instant recognition, and hard-wired rapid responses, in a close encounter with potential danger. Harmless creatures that evolved some general resemblance to the variety of creature features to be avoided (eyes, scale patterns) would then gain some protection.
Dr. Janzen and colleagues have cataloged a delightful assortment of striking false eye patterns on the front and rear ends of caterpillars and the front ends of chrysalises.
Their bounty and insights are the product of a dedicated, and somewhat accidental, long-term study of the denizens of the Área de Conservación Guanacaste, or A.C.G., in northwestern Costa Rica.
It began in 1978, when Dr. Janzen broke some ribs falling into a ravine while conducting field studies in the region. The road to the hospital was too rough to navigate, so he wrapped his sore rib cage and confined himself to a chair for a month.
Unable to explore the rain forest, he soon went a bit stir-crazy. The field station had only two hours of electricity each night, and just enough power to run a 25-watt light bulb. Fortunately for Dr. Janzen, that was a bumper year for moths, which were attracted to the light. So he passed the time building a moth collection.
When he recovered enough to wander back into the rain forest, he discovered that it was also a bumper year for caterpillars. The challenge was to identify which of the many different kinds of caterpillars belonged to which species of moths or butterflies. Now 71, he told me from his field station 32 years later that “my private insanity was to find all of the species before I die.”
To accomplish his goal, he had to set up a system of collecting the caterpillars, photographing each of them, raising them into adults, then identifying each of the species, at least half of which had not been described previously. He started by himself, then was joined by his wife, Dr. Hallwachs, an expert on rodents and now caterpillars. The operation continues to this day, 365 days a year, with the help of 33 trained Costa Rican assistants.
In an area of about 77 square miles, more than 450,000 caterpillars have been studied. As of a few years ago, the team had identified more than 12,000 species. That number ballooned to 15,000 species when the team discovered, through the use of DNA typing, or “bar coding,” that many of the species were actually made up of multiple distinct species,as many as 11 in one case. The total number of species in just this one region equals that of all of the moths and butterflies species of North America.
With caterpillars and chrysalises coming into the station at the rate of more than one hundred a day, Dr. Janzen began to discern a trend. In species belonging to many different groups, he saw caterpillars or chrysalises that bore all sorts of paired eyelike markings of various color schemes, with round or slit pupils. The variety of patterns suggested that the bugs do not have to match exactly the appearance of any particular predator for the ruse to work.
Moreover, the distinct behavior of many caterpillars when handled underscored that the whole game was to startle the many species of insect-eating birds that foraged in the dry, cloud and rain forests of the conservation area. Some eye patterns became visible only when the caterpillars were molested and expanded part of their body, and some large specimens wriggled and rattled like snakes.
Dr. Janzen and his colleagues estimate that a typical foraging bird might encounter tens to hundreds of false-eyed bugs each day. It is unlikely that a bird encountering such a spectrum of patterns could learn to discern which specific ones were safe and which were not, especially when one mistake would mean its demise. It would be better to leave suspicious items alone and to quickly move on.
For two centuries, naturalists have sought to catalog and make sense of the dazzling diversity of life, particularly as found in the tropics. Often, new insights have come from asking very simple questions, like “Why does this small caterpillar look like a snake on one end?”
But the answer to such questions requires finding many more creatures and understanding where and how they live. And that requires a special breed of human willing to live far from the comforts of home and eager to look at 450,000 bugs for 32 years.


What Meets The Eye May Not Actually Be An Eye


What looks like a snake head is actually the back end of this xxx caterpillar. Photo courtesy of Dan Jenzen.

We’ve got news for all of Costa Rica’s insect-eating birds: Those fierce eyes that popped out of the foliage may not actually belong to a snake. They likely belong to an insect just inches tall.
Hundreds of species of butterflies and moths in Costa Rica have evolved to develop markings or “false eyes” that trigger an instant reaction in their bird predators to make them fly away, according to a study published last week in the Proceedings of the National Academy of Sciences by University of Pennsylvania scientists, and husband-and-wife, team Dan Janzen and Winnie Hallwachs, along with John Burns, the curator of lepidoptera at the National Museum of Natural History.
The form of mimicry was studied over several decades in the Área de Conservación Guanacaste in northwestern Costa Rica, where Janzen and Hallwachs have trained native Costa Ricans to gather the insects from the forest and raise them.  The study argues that the caterpillars have actually manipulated birds’  instinct, over time, to avoid potential predators.
We spoke with John Burns in his office at Natural History, where he explained how exactly such tiny creatures can make much larger bird predators head for the hills.
Tell us a little of the history of insect mimicry?
One [theory] is what’s called Batesian mimicry, proposed around 1852 by Henry Bates, an English naturalist who spent a lot of time in the Amazon. He noticed that many butterflies looked like other butterflies even though they weren’t really closely related. He figured out that many butterflies that had showy color patterns were using these color patterns as a warning to the fact that they were distasteful, or poisonous. Birds would learn to leave these kinds of color-patterned butterflies alone, which they learned by trial and error: Eating the butterflies would make the birds sick. So Bates realized that through evolution, there were perfectly edible, non-toxic butterflies who looked almost exactly like, if not exactly like, these poisonous butterflies, and the non-toxic butterflies were copying, or mimicking, them and thereby gaining a degree of protection from their potential predators. 
What did your recent study find?
In this case with Dan Jenzen, we’re studying caterpillars. Daniel Janzen and his wife, Winnie Hallwachs, are rearing many lepidoptera caterpillars in the Área de Conservación Guanacaste in northwestern Costa Rica. They’ve been doing this for a few decades now. They actually have a huge team of trained Costa Ricans who go out into the forest and hunt for the caterpillars and bring them back and rear them individually to the adult stage. Many of the caterpillars, and the pupa they turn into as they metamorphose to butterflies, develop paired structures that look like the eyes of a snake, or a vertebrate animal. Now most of these caterpillars, or pupa, are perfectly good food for the small insectivorous birds that feed on them. But if you can imagine a small bird suddenly coming across a pair of eyes on something it’s thinking about attacking, it’ll have second thoughts because those eyes might belong to a snake or a larger bird that would attack it and it would become the prey. We figured out that the birds would have to be already genetically programmed to fly away when they are faced with these false eyes. Much earlier in evolution birds have run up against this kind of threat and if they’re caught, they’re killed, so the birds have developed this innate response—an instant startle and a fear flee reaction. Because if they hesitate in this kind of real situation and decide, “Well you know is that something I can eat, or is that going to hurt me?” In that instant they might get killed. It’s in the bird’s favor to reject that little piece of food and go look for another one rather than linger. This is a form of mimicry—the development of eyes that aren’t real eyes—but it’s not a case where birds have to learn to leave these alone. They are already genetically programmed to do that.
So what do these eyes look like?
Photo courtesy of Dan Janzen
Actual caterpillar eyes are tiny little structures, they don’t look anything like eyes as we know them, or like vertebrae eyes. They are just very small structures several on each side of the head. But the false eyes we’ve seen, there’s been everything from a pair of little black dots that are sort of the beginning of a suggestion of eyes, to ones that are just extraordinarily complex. There can also be features of the body surrounding the false eyes that will even resemble that of the birds’ predator. There are a few pupa who have markings that look just like the scales of a snake and it’s just amazingly good mimicry. It usually is not that well advanced.
How does this study help the evolution of birds and insects? What does it do for future research?
I would say it’s an interesting result of evolution so far, that this kind of thing has arisen. I can’t say exactly where it’s going except that it will certainly persist as long as birds look for caterpillars. I can imagine over time in many species that the false eyes that caterpillars have may look more and more like an eye—not like those of another caterpillar or any particular kind of snake, but it may become a better copy.
To learn more about the caterpillars and conservation efforts to save their environment, 
To you this collection of images of some species that have been discovered:



Snake camouflage caterpillar



This is the head end of a caterpillar at the back of my mother-in-law's house in

 Kajang, Malaysia. It has camouflage mimicing a snake.



0 comments:

Post a Comment

Related Posts Plugin for WordPress, Blogger...
 

Copyright © 2011 Just amazing | Design by Kenga Ads-template