This is a diary about beauty, about developing complex beauty from humble beginnings and about keeping safe. It is a diary about science and nature and weird music and hopefully within you will find at least diversion and perhaps amazement.
Last week we learned a bit about how color is produced in plants and animals and a few basics of color genetics. This week we're going to follow up on that with more on color patterns. How does the complex pattern seen on this male wood duck get produced? How do patterns keep organisms safe from predators? Warning: Music Videos vary from mildly eccentric to really weird.
Part 1: It's A Beautiful World, How Did it Get that Way?
In this section we're going to briefly discuss how color patterns get made. Below and above are some examples of the beautiful color patterns of male ducks.
The bottom picture shows an American Wigeon swimming among a group of American Coots. These birds illustrate the crux of the problem. All of the coots' feathers are black. Each feather has the same pigment, resulting from the same genes. In contrast the Wigeon and the other ducks above, have very different colored feather in different parts of their bodies. Given that the entire duck's body is derived from a single cell and all resultant cells have exactly the same genes, how do different parts of the body 'know' to make feathers of particular colors. And it's not just that the feathers are different colors - there is a definite pattern to the color with precise borders that are repeated across different individuals.
Let's start out by looking a somewhat atypical example, the calico/tortoiseshell pattern in cats. In these cats, portions of the fur have orange coloration and other portions are tabby or black or some other color. How is this controlled?
The controlling gene is called orange and it is found on the X chromosome. The orange allele produces orange fur with slightly darker orange tabby stripes. The 'not orange' allele results in whatever color is produced by other loci such as tabby and agouti as discussed last week. The orange allele basically over-rides the effects of those other genes.
Male cats have only one X chromosome and thus will either have an orange allele or a 'not-orange' allele and be either orange or something else. Female cats have two X chromsomes. If a female has one orange allele and one 'not orange' she ends up as a calico/tortoiseshell. How?
In female mammals one X chromosome is inactivated in every cell at an early stage of development. If it is the X with the orange allele that is inactivated then every cell that is derived from that parent cell for the rest of the animal's life will only have the X with the 'not orange' allele working. So each patch of orange fur represents a population of cells with with 'not orange' X deactivated.
This is both similar to and very different from how most color patterns are determined. It is different in that an entire chromosome is deactivated or activated rather than individual genes. It is also different in that the pattern of activation is largely random.
It is analogous to the genetic control of other color patterns in that, like all development, it works by activating specific genes in specific areas of the body. Developmental genes work like a hierarchical series of switches. For example a master control gene may get switched on at the front end of an embryo and switched off at the back end of an embryo. Each position results in the activation of a different set of genes in each region. Interactions between neighboring cells then trip different genes at a third level. And so on moving down to more local areas of the body.
Developmental biologists studies butterfly wings have identified a number of genes also found in fruit flies that are involved in generating the color patterns of the wings, including eye spots. Many of these genes are expressed in different parts of the wings (and elsewhere) also control wing vein patterns. Basically each gene is expressed in a certain part of the wing and the interaction between these genes results in specific colors appearing in different places.
The eyespots are interesting in that they are overlaid on top of the wing pattern. Each one is the result of a focal point where a chemical called a morphogen is released. This morphogen radiates outward in decreasing concentrations and determines the features of the spot (such as the edge).
Evolutionary developmental biology or Evo-Devo is a new and rapidly expanding field. If you are interested in learning more about it than this very brief intro I could write a more detailed diary later or you could go straight to the source and read 'Endless Forms Most Beautiful' by Sean Carroll, a great introduction to the field that could be read by just about anyone.
Part 2: Staying out of sight while working for the Cryptic Corporation
The rest of this diary is about not getting eaten. Probably not something you worry about on a regular basis but it is a concern of most organisms. 'Nature Red in Tooth and Claw' may be a poetic statement but it is metaphorically accurate. Most individuals end up as somebody's dinner.
Color can increase the odds of survival in several ways. The most obvious is, perhaps somewhat paradoxically, by making the individual undetectable, in a sense invisible. Of course there is no such thing as invisibility but there is crypsis. Crypsis is having a color pattern, shape, and/or other characteristics that make an organism hard to detect.
Visual crypsis mostly works through two mechanisms: background matching and obscuring the body outline. Background matching is just like it sounds - the color of an organism matches the background of its environment. Pretty self evident. The other thing that makes an organism hard to detect is the lack of a distinct visual 'edge' to the body. We'll talk about several ways of accomplishing both of these through color.
Counter Shading: One color pattern that is so common in animals that you probably don't even notice it is having a dark dorsal (back) surface and a light ventral (front/underneath) surface. Penguins are an obvious and dramatic example of this if you look around you will notice many birds, fishes, and other animals with this pattern.
It is most common and most obvious in animals that swim in open water and in birds like swallows that fly about in the open a lot. These animals are commonly viewed from both above and below. Looking up from underneath they are pale and blend into the light. From above they are dark.
Animals that violate this pattern often look a bit odd to us such as a black-bellied plover or a bobolink.
Mottling: Lots of cryptic animals have a very uneven coloration with blotches, spots, and so on. In many cases this causes to animal to resemble the wood, bark, or rocks on which it lives. More generally though, the uneven coloration helps animals blend in under dappled light conditions. It also makes the shape of the animal less obvious than if it is all the same color unless the background provides a strong contrast.
Stripes: Tigers, zebras and many more mundane animals are striped or barred. This can provide background matching in a grassland but again, one of the big advantages is that it ends to obscure the outline of the animal, especially when it is in motion.
Part 3: 'Everybody Looks Like Ernest Borgnine'
An alternative use of color to avoid big hungry animals is to advertise to the big hungry animals that they don't want to eat you. Even a cursory survey of the animal world will reveal many many species that are both brightly colored and down right nasty in some way.
They can sting or bite you
They are just plain bad to eat.
All of these animals have what is called aposematic coloration. The coloration is a warning to potential predators that what they are going to get is not a tasty morsel but something much less pleasant. Aposematic coloration is always conspicuous and tends to involve bright colors and lots of contrast.
The allusion to the John Cooper Clarke song is as follows. If you are tough but don't look tough then you are going to get challenged a lot. If you look like Ernest Borgnine then people will leave you alone.
The evolution of aposematism is an interesting topic. The idea is that either predators coevolve with the toxic 'prey' and evolve an instinctual avoidance OR they learn as young predators not to eat things that can hurt them. Either of these scenarios requires a large population of toxic, brightly colored individuals. Why? Because, in order for the toxic/bright type to be successful each individual has to have a low probability of being eaten by a naive predator.
Another way to look at it is to imagine what happens to the very first individual that evolves the bright color. It may be toxic but none of the predators 'know' that or have evolved to avoid it. Our aposematic pioneer is a sitting duck. The predator that eats it won't be eating another individual with the same color but neither will anyone else because there aren't any more.
Several solutions have been proposed for this problem. They are really worthy of an entire diary (or two) unto themselves so I will just briefly mention a few. First if aposematism arises in families then the benefits of helping your similarly colored relatives might outweigh the high risk to yourself. Secondly the coloration could evolve as a plastic response and only be expressed under high density conditions. Thirdly the toxin or other hazard may evolve first and predators already avoid the species - the coloration evolves gradually to make recognition easier.
Part 4: Checking into the Bates Motel under an assumed name
Henry Walter Bates was a British naturalist in the mid-19th century. As a young man he and his friend Alfred Russel Wallace (later the co-discoverer of Natural Selection) set off for South America in an attempt to start careers as collectors of natural history specimens for European Museums. After a short period where they worked as a team they decided it would be more efficient to split up. After a few years Wallace returned to England and eventually returned to the tropics in southeast Asia. Bates remained in the Amazon region for eleven years. He eventually returned to England in 1859, the year of the publication of Darwin's 'Origin'. Ill health drove him to leave Amazonia, but he reported feeling a deep regret at leaving a land of eternal summer.
Bates had primarily collected insects and he noticed a striking pattern. Within a particular geographic location he would often find one or more species of unrelated butterfly with strikingly similar color patterns. As he travelled through Amazonia he discovered that there was often geographic variation in color such that species A would look different in two different locations and species B (or some other species) would resemble the local form of A.
Bates had discovered mimicry, more particularly a type of mimicry that bears his name: Batesian Mimicry. Mimicry is a widespread mechanism to avoid predation: caterpillars and spiders resemble bird droppings, caterpillar butts look like snake heads, and so on.
What Bates discovered was mimics of species with aposematic coloration. These systems have a distasteful/toxic/dangerous model and a tasty/nontoxic/harmless mimic. It is to the advantage of the mimic to resemble the model as closely as possible.
A syrphid fly, a harmless mimic of bees and wasps
A second 19th century naturalist in South America, Fritz Muller (the u should have an umlaut but I don't know how to do that), expanded on Bates idea. Muller travelled from Germany to the coastal forests of southern Brazil and remained there for the rest of his life. Like Bates he was a strong advocate of Darwinian ideas. Mullerian mimicry is similar to Batesian except that both species involved are genuinely distasteful/toxic/dangerous. In a sense they are both mimics and both models. Mullerian mimicry explains the specific similarity in pattern of Monarch, Viceroy, and Queen Butterflies but also the common black alternating with yellow/orange/red seen in so many aposematic species. Why reinvent the wheel?
Batesian mimicry is explicitly based on the idea that predators learn from eating the model species to avoid a particular color pattern. This poses a problem for some examples of aposematism. Coral snakes for example are highly venomous. A predator that managed to eat one without being bitten would get a good meal. A predator that gets bitten will be dead. No learning in either case.
Snakes with the 'coral snake pattern' of red, yellow, and black bands are widespread in the New World. Coral snakes are highly venomous. Other species are either harmless (milk and king snakes among others) or mildly venomous. For some time it was proposed that the mildly venomous species were the models and both the highly venomous and the harmless species were the mimics. However there are quite a few places where the putative mimics exist without the models.
Clever biologists have studied predation on these snakes by putting out plasticine models with different color patterns. Bird attacks can be seen as marks in the clay. Lab studies indicate that many birds have evolved a non-learned avoidance of the banded color pattern. Field studies indicate that the response to the banded pattern varies depending on the presence of toxic species in the local area.
Part 5: Neon meat dream of a octafish
I know I promised to discuss octopi this week but I ran out of time so it will have to wait until later.