Feb 8, 2016

Animal Coloration

One cannot help but be impressed by the diversity of colors seen in animals. In his Colour of Animals, written in 1890, Edward Poulton, an evolutionary biologist and Oxford professor of zoology, provided the first comprehensive text on animal coloration. As a subtext, the book was intended to actively support Charles Darwin’s theory of natural selection, which was then besieged by many of his contemporaries.

Poulton was not the first to comment on coloration in animals. Robert Hooke, a pioneer microscopist, first described the structure and brilliant colors of a peacock’s feathers in his classic 1665 work Micrographie. In Descent of Man, and Selection in Relation to Sex (1871), Darwin proposed that conspicuous coloration evolved to provide individual animals, in particular, male birds, with a reproductive advantage in attracting females. Moreover, duller colors provided birds and insects with camouflage to conceal themselves from the covetous eyes of predators, a concept elaborated upon by Poulton.

In the Colour of Animals and findings by others, coloration was observed to provide animals with a diverse array of survival benefits. Poulton was first to emphasize that camouflage coloration enabled prey to avoid potential predators but also enabled predators to conceal themselves or to lure unsuspecting prey. He acknowledged the work of Henry Bates (1862) on the use of coloration by butterflies to resemble another species and thereby deceive predators; and by Fritz Müller who, in 1878, introduced the concept that coloration served as a warning signal (aposematism) to an approaching predator that the would-be prey was prepared and capable of defending itself.

Feather from the male Indian peafowl (Pavo cristatus), the national bird of India. Male birds are generally more colorful or ornamented than females, perhaps because it confers a reproductive advantage, while females can more easily conceal themselves from predators while raising their young.

Coloration provides animals other survival benefits: Some use flashes of light, bold patterns, or motion to divert attacks by predators. Coloration can protect others against sunburn, while certain frogs lighten or darken their skin to control their body temperature. Male monkeys use coloration to assess the social status of their peers. Poulton concluded that pigments in animal tissues produced coloration and that the brilliant colors seen in some birds were the result of consuming carotenoidcontaining plants.

Germ Theory of Disease

Prior to the latter half of the nineteenth century, and dating back to ancient times in China, India, and Europe, it was widely accepted that such infectious diseases as cholera and the Black Death were caused by “bad air” or miasma. Infections were thought to be spread by contact with poisonous vapors filled with decomposed or rotting matter.

The “germ theory of disease” may be the most important contribution of microbiology to modern medical science and practice, and it has served as the basis for the use of antibiotics for the treatment of infectious diseases. The concept that microbes were the cause of some diseases evolved over several hundred years, with multiple scientists providing evidence that culminated in the theory and its acceptance by the medical and scientific community.

Using a simple microscope, microbes were first seen and described in the 1670s by the Dutch lens maker Antonie van Leeuwenhoek. Almost two centuries later, in 1862, Louis Pasteur conducted decisive experiments that refuted another long-held theory—namely, spontaneous generation—that living organisms could arise from nonliving matter. Pasteur demonstrated that microbes were present in the air but were not created by air.

When developing his germ theory of disease, Koch used Bacillus anthracis (shown in this digital illustration), employing purified cultures of the microbe that had been isolated from anthrax-diseased animals.

Robert Koch transformed from a simple practicing German physician to one of the pioneer founders of microbiology (as was Pasteur) after he received a late-twenties birthday gift of a microscope from his wife. From 1876 to 1883, he discovered the bacterial causes of anthrax, tuberculosis, and cholera, and devised methods for isolating pure cultures of disease-causing microbes. In 1890, he devised rules that are still used (with some modification) to determine whether a given microbe causes a disease. These postulates state that: (a) the microbe must be present in every case of the disease; (b) the microbe must be isolated and grown in pure culture; (c) the disease must be produced when the microbe is administered to a healthy individual; and (d) the microbe must then be reisolated from the individual. Koch was awarded the 1905 Nobel Prize for his work on tuberculosis, a disease that was responsible for one of every seven deaths in the mid-nineteenth century.

Negative Feedback

Homeostasis, a fundamental principle in biology, is a concept that was developed by Claude Bernard during the 1850s and expanded upon and popularized by W . B. Cannon during the 1920s to1930s. It is the process by which a living being maintains a constant internal environment when its external environment is changing. Negative feedback control systems, whether in biological or nonliving systems, consist of three integral components: a receptor that detects changes in the system; a control center that compares the change with a set or reference point, which in biological systems are the normal values; and an effector, which initiates appropriate action to return the system to its reference point. By analogy, consider the home furnace and thermostat. In 1885, Albert Butz invented the earliest functional thermostat. The furnace continues to heat the facility until a set temperature is detected by the thermostat, which shuts the furnace down and then turns it on when the facility’s temperature falls below the set temperature.

Many endocrine systems, such as blood glucose levels, are linked to control centers by homeostatic negative feedback mechanisms that operate in a cyclical and continuous manner. After eating a carbohydrate-rich meal, blood glucose levels rise, stimulating the release of insulin from the beta cells of the pancreas. Glucose enters body cells, and the liver takes up the excess sugar, which it stores as glycogen. Blood glucose levels are detected and compared with set levels (70–110 mg glucose/100 ml blood). If levels are too low, insulin secretion stops and glucagon is released from the alpha cells of the pancreas stimulating the breakdown of liver glycogen to glucose, which is released in the blood.

An illustration of a complex machine with a steam boiler, gears, levers, pipes, meters, furnace, flue, and presumably a thermostat to provide a negative feedback loop that will keep the temperature of the machine at a reasonable level.

Negative feedback inhibition also controls the amount of final product that is synthesized in many enzyme-catalyzed biochemical pathway reactions. After an optimal amount of end product is formed, the end product reacts with an enzyme in the pathway, interfering with synthesis of additional compounds.

Gram Stain

One year after graduating from medical school, in 1884, while working in a Berlin mortuary, the Danish scientist Hans Christian Gram developed a stain that permitted him to visualize some but not all bacteria in lung tissue. This simple but major discovery subsequently led to the finding that many bacteria can be differentiated into two broad categories based on the thickness of their cell walls, aiding in the diagnosis and treatment of bacterial infections.

Cell walls, found in bacteria, plants, and fungi, but not in animals or protozoa, provide protection and support of the cell, and perhaps most important, prevent bursting if excess water enters the cell. It is the cell wall that traps certain dyes, permitting their bacteria to be visualized.

After applying the Gram stain, Gram-positive Bacillus cereus are dyed violet and appear in a series of chains, while Gramnegative Escherichia coli are the small pink clusters in the background.

In the Gram stain procedure, Gentian (crystal) violet is poured over a slide containing bacteria and Lugol’s (iodide) solution is added to fix the dye. The slide is then washed with ethanol. Certain bacteria (such as the pneumonia-causing Streptococcus pneumoniae) retain the dye and appear purple; these are Gram-positive bacteria. Other microbes (the typhus- and syphilis-causing bacteria, for example) become decolorized by the alcohol and assume a red or pink color—Gram-negative bacteria. Gram-positive bacteria have thick cell walls and trap the purple stain in their cytoplasm, while Gram-negative bacteria have much thinner cell walls from which the dye is readily washed. The Gram stain is routinely used in medicine as a diagnostic tool to differentiate infections caused by Gram-positive or Gram-negative bacteria and provides a rational basis for the selection of antibiotics.


Most antibiotics are preferentially effective against either Gram-positive or Gram-negative bacteria. For example, penicillin combats many Gram-positive bacteria by interfering with their ability to synthesize cell walls that are essential for their survival. (Animal cells lack cell walls and, therefore, penicillin is not toxic to them.) The thick outer membrane of Gram-negative bacteria protects it against the body’s defenses and also impedes the passage of many antibiotics into the cell. The aminoglycosides are a class of antibiotics used for the treatment of these bacteria.

Eugenics

Francis Galton, a man of many intellectual talents, made significant contributions to such diverse areas as meteorology (weather maps), statistics (correlation and regression analysis), and criminology (fingerprinting). Upon reading his cousin Charles Darwin’s Origin of Species, he became inspired by the notion that if natural selection enables the fittest organisms to survive and pass on their traits, it must also apply to humans—human ability and intelligence must be hereditary.

In 1883, Galton initiated a social movement, which he called eugenics (“good birth”), intended to improve the genetic composition of the human population. Eugenics, called “social Darwinism” by some, enjoyed its greatest popularity during the early decades of the twentieth century. It was practiced throughout the world and actively promoted by governments and some of society’s most influential and respected individuals. While its advocates argued that the results would lead to more intelligent and healthier people by eliminating such hereditary diseases as hemophilia and Huntington’s disease, its opponents viewed eugenics as a justification for state-sponsored discrimination and human rights violations.

Practices arising from the eugenics movement varied among countries. Great Britain sought to decrease the birth rate among the urban poor. In the United States, many states enacted laws prohibiting the marriage of epileptics, the “feebleminded,” and mixed-race individuals. Thirty-two states had eugenics programs that resulted in the sterilization of 60,000 individuals from 1909 to the 1960s.

Ultrasound, commonly performed during the eigteenth to twentieth weeks of pregnancy, can be used to detect birth defects such as spina bifida and Down syndrome.

By far, the most egregious interpretation of eugenics was responsible for the racial policies of Nazi Germany seeking to promote a pure and superior “Nordic race” and eliminate the less fit and undesirable, which led to the annihilation of millions of Jews, Romani (Gypsies), and homosexuals. By the end of World War II, because of its association with Nazi Germany and concerns that what is improved or beneficial is highly subjective and often based on prejudice, the active pursuit of eugenic programs fell into disfavor. More recently, some have argued that medical genetics, with in utero testing for mutations leading to diseases or fetal gene manipulation, is the new eugenics. These are decisions made by the individual, however—not the state.