Snakes with two heads and cats with two faces may seem like morbid entertainment, but they have a lot to teach us about development and evolution.
Two headed snake
This British grass snake exhibits dicephalus, a condition related to conjoined twinning. It is caused during development by high incubation temperatures.
Dicephalus is common in reptiles and particularly in British grass snakes, who lay their eggs inside compost heaps where decomposing matter produces heat. Eggs heated above 40 ° Celcius usually fail to hatch, but a small percentage of those that do will yield snakes with two heads.
Because the condition occurs in response to environmental influences rather than genetic mutations, this snake is not a "mutant", but merely a "freak".
Laloo and his parasitic twinThis photograph of Laloo, a circus performer whose child-sized parasitic twin seems to emerge from his abdomen, was published in Anomalies and Curiosities of Medicine, a 19th century book on teratology - literally "the study of monsters" - by George Gould and Walter Pyle.
"In studying monstrosities", they wrote, "we seem to catch forbidden sight of the secret work-room of Nature, and drag out into the light the evidences of her clumsiness".
Zoologist Harris Hawthorne Wilder developed this conceptual framework, which he called "Cosmobia", to understand certain developmental abnormalities.His scheme runs the spectrum from cyclopia (having only a single eye, in the middle of the forehead) to diprosopia (having duplicated facial features), suggesting that what we consider "normal" is nothing more than the middle ground in the vast range of nature's creations.
Wilder insisted that conditions like cyclopia and diprosopia should not be considered deformations, but rather "symmetrical anomalies on either side of a normal being".
The figures beyond diprosopia, labeled XII through XIV, illustrate a condition called dicephalus, which turned out not to be an extension of diprosopia, but rather a variant of conjoined twinning
Young girl with ameliaThis image, from a collection at Philadelphia's Mutter Museum, shows a young girl with amelia, a condition marked by undeveloped limbs.
"For a child born with all her limbs, development follows a path that is typical of our species", Blumberg writes. "But when a child is born with limbs missing, she traverses a distinct developmental path that entails novel brain organization and associated behavioral adjustments".
As the fetal brain and the functional organization of the nervous system interacts with its sensory and motor systems during development, it responds with remarkable adaptability to any obstacles it encounters - such as missing limbs
Franklin Dove's "fabulous unicorn"In the early 1930s, University of Maine biologist Franklin Dove surgically transplanted the two horn buds of a one-day-old male calf from their usual position to a single, central location on the calf's skull. He suspected that the two horns would fuse together into one.
That is precisely what happened - Dove created a real live unicorn.
Blumberg writes, "The success of Dove's experiment depended upon the ability of an isolated horn bud, like the regenerating limb of a newt, to grow independently of other tissues in the calf's body." It highlights the body's remarkable ability to successfully deal with strange developmental challenges early in life.
Two-headed kitten
The kitten in this photograph has diprosopia, a rare congenital malformation that duplicates facial features.
While genetic mutations cause some abnormalities in humans, many, including diprosopia, are caused by obstacles faced during development.
By focusing only on genes, Blumberg argues in Freaks of Nature, we miss out on everything developmental anomalies can teach us about biological form and behavior.
These anomalies reveal that development is not predestined, but is a step-by-step process of formation that can take alternate paths in response to environmental and genetic influences. "Freaks" are no less natural or ingenious than "normal" biological forms
