Intro to Anatomy 7: The Integumentary System

An Introduction to Human Anatomy and Physiology
Chapter Seven: The Integumentary System

Introduction: Where Have All the Frogs Gone?

Over the past 50 years or so, biologists have noted a sharp decline in the size of amphibian populations throughout the world. The numbers of frogs, toads, salamanders, and newts appear to be decreasing at an alarming rate, and several species have gone extinct.

The obvious culprit would seem to be loss of habitat due to human activities. Amphibians, after all, are dependent on wet (or at least moist) habitats, and every time a marsh or swamp or pond is destroyed in order to put up a new housing development or shopping mall, the local amphibian populations suffer. But amphibian populations are declining in protected areas too, such as national parks and nature reserves. Why is this?

There are doubtless several reasons, but one very important factor has to do with the nature of amphibians’ skins.

The first vertebrates to live on land were animals that we’d call amphibians. Modern amphibians, presumably like their ancient ancestors, have relatively poorly-developed lungs compared to reptiles, birds, or mammals. In fact, some land-dwelling salamanders lack lungs entirely as adults. So, amphibians absorb oxygen largely (in some cases, entirely) across their skin surfaces. They can do this because their skins are thin, moist, and permeable.

The Southern Gastric-Brooding Frog (Rheobatrachus silus).
Native to the rainforests of southeastern Queensland,
Australia, the species was last seen in the wild in 1981.
The last-known captive specimen died in 1983, so the
species is presumed to be extinct.
No one knows exactly why.

The downside of having such thin, permeable skin is that it offers very little protection. Water readily crosses amphibians’ thin skins, so they will quickly dehydrate and die in arid environments. Similarly, most amphibians are quickly killed by exposure to salt water. (Just as an aside, this inability to tolerate exposure to saltwater implies that amphibians are almost certainly descended from ancient freshwater fish, not marine fish. The popular notion that the first land-dwelling vertebrates crawled out of the ocean is almost surely false. Both the fossil record and the physiology of modern amphibians strongly suggest that the first land-dwelling vertebrates crawled out of ancient swamps and ponds.)

Their thin, permeable skins also make amphibians very sensitive to air- and water-borne pollutants. Perhaps the frogs and salamanders are like the canaries miners once used to alert them of toxic gases. When the canaries (which are more sensitive to these toxic gases than are humans) fell off their perches, the miners knew that they had to get out of the mine fast. Maybe we should be concerned that the frogs and salamanders are trying to tell us something.

Something like 350 million years ago, the amphibians gave rise to another lineage, the amniotes. The amniotes have quite a few features that distinguish them from the amphibians, but the particular feature we’re concerned with here is that amniotes, unlike their amphibian ancestors, have relatively thick, water-proof skins.

The first amniotes were animals that we’d call reptiles today, and from them descended modern reptiles, birds, and mammals. With their relatively thick, waterproof skins, amniotes can survive and even thrive in environments that are much too harsh for even the hardiest of amphibians.

An amphibian's skin contains mucous glands that help keep the skin moist and lubricated, and poison glands that help to deter would-be predators. In some species, chromatophores in the skin allow them to change their skin color. But that’s really about it.

The skins of reptiles and birds, by contrast, are much more complex. Reptiles and birds have relatively thick skins covered by protective scales made mostly of the protein keratin. (Birds’ feathers are modified scales, and most of the feather-free body surfaces – the feet in most species, for instance – are covered by scales.) The scales of reptiles and birds (and some mammals) are not the same thing as the scales of fishes, by the way. The scales of most fishes are modified bone (in sharks, they’re modified teeth), and they form from completely different tissues than do the scales of amniotes.

Of all the amniotes, though, it is mammals such as ourselves that have the most complex skins. Mammals’ skins are underlain by an insulating layer of fat that helps to conserve body heat even in very cold environments, and the skin itself contains many different kinds of specialized glands that aren’t found in other amniotes.