Meet Plethodon cinereus - the Northern Red-backed Salamander. These relatively plain little fellows belong to the "woodland salamander" family, a nod to their forested terrestrial home. Unlike some salamanders who, like many amphibians, lay their eggs in water, at no point in the Red-back sallie's life cycle are they aquatic. A cluster of 4-14 eggs is laid in the soft nook of a rotting log or tree stump, from which young develop fully inside their eggs until they hatch; the "infants" wobble out like dainty versions of their parents.
The painting above documents 5 stages of the Red-back reproductive cycle - zig-zag top to bottom: beginning with a clutch of eggs (fiercely defended by mamma for up to 2 months); followed by a view of the embryo development within those eggs (dark eye spots and long tail coiled around a creamy round yolk clearly visible); a pile of wee hatchlings (roughly 1" in length); a juvenile, starting to look a bit more proportional; and lastly, coming full circle, an adult Red-back, poised to find a mate of his own. The prominent Hemlock needles and cones in the painting are in reference to the Eastern Red-back's preferred forest type.
A unique feature of woodland salamander physiology not obvious in the illustration - they are lungless. That's right - no lungs! Instead of breathing through organs in the chest, a lungless salamander gets oxygen through its skin, via a process known as cutaneous respiration, or cutaneous gas exchange (remember, breathing in oxygen goes hand in hand with breathing out carbon dioxide). A seemingly bizarre strategy, but Red-backs aren't the only ones...
Lungs are what enabled the ancestors of all tetrapods (4-limbed animals) to climb out of the Devonian soup some 400 million years ago and begin sucking in great quantities of air for a living. Since they utilize the vast surface area of alveoli for gas exchange, rather than the relatively limited amount of surface area provided by a creature's skin, lungs are a much more efficient way for an organism to respirate - especially large creatures, or those requiring bursts of energy and high O2 consumption. For comparison, an average human adult's skin has a surface area of roughly 16-21 square feet - the surface area of the alveoli inside that same human is about 100 square yards!
Amphibians are neither large, nor particularly active, and many spend almost their whole lives in or around water. For these reasons, amphibians of the order Anura (ie: frogs) evolved FOUR mechanisms for breathing: 1. through gills (present in the larval stages of many aquatic species, but they are generally lost prior to adulthood); 2. lungs (slightly more primitive than those of mammals); 3. through their very permeable skin, and; 4. through sensitive skin inside the mouth (aka buccal cavity). Lungless salamanders (about 431 species of the family Plethodontidae, and making up an entire 2/3 of all salamander diversity!) took things one step further and got rid of the primitive lungs all together, to rely solely on cutaneous gas exchange. Similarly, two known Caecilians (bizarre, smooth-skinned amphibians resembling a combination of a worm and a snake) made themselves a sans-lung niche, one-upping the sallies by also losing their legs! ("squeam" factor is a bit high on these guys...). Until fairly recently, these examples were thought to be the only tetrapods lacking lungs, but in 2008, the Bornean Flat-headed Frog became the first frog (they're actually toads...) to be discovered without lungs. So yeah - Nature: stranger than fiction, EVERY TIME.
Still iffy on how this whole "lungless breathing" concept works? I was, and since the interweb failed to deliver the all-encompassing visual diagram I was hoping for, I went ahead and fashioned my own (facts carefully double-checked with the Harvard researcher who studies the Plethodons in the painting above)...
Hungry for more deets on breathing in and breathing out? WATCH: this quick 3D medical animation of respiration in human lungs; this insanely authentic Encyclopedia Britannica film on the respiratory system requires about 25min, but the 80s edu-film effects are priceless (and it's actually jam-packed with all the basics) - highly recommended if you want something thorough and entertaining; and for a gander at how a few additional amphibians have solved the oxygenation problem using... creatively alternative methods... check out the bizarrely bristly Hairy Frog, the excessively wrinkled Lake Titicaca Frog, and the amusingly sweet-faced Axolotl.
My great thanks to Zachary Lewis, currently a Phd student in the Hanken Laboratory at Harvard University (where he studies the evolution of development - via salamanders!), and Carolyn Eng, doctoral candidate in the Harvard Human and Organismic Evolutionary Biology departments (Carolyn studies anatomy and physiology and frequently puts animals like goats on treadmills to observe muscle function through locomotion - I covet her job).