Integument and water exchange

As in other vertebrates, the integument of reptiles serves as mechanical protection from the environment, shields the body from unwanted substances or toxins, and largely determines the nature and magnitude of mass and energy transfer between animals and their environment. The skin consists of a fibrous dermis overlain by an epidermis that has multiple layers of living or keratinized cells derived from an active germinative layer beneath. Thin layers of bone called osteoderms are deposited in the dermal layers of many lizards, turtles, and crocodilians. The outermost stratum corneum consists of highly organized layers of keratin derived from dead cells and is periodically shed, either as an entire unit (snakes, some lizards) or in flakes or pieces (turtles, crocodilians, lizards). Periodic replacement of older, keratinized, epidermal generations as a synchronous, whole-body ecdysis (sloughing or shedding) is especially characteristic of snakes.

There are relatively few glands in reptilian skin compared with that of fishes and amphibians. Known glands tend to be concentrated in particular regions such as the femoral and preanal scales of lizards. Glandular secretions mark territories or establish scent trails that are recognized by conspecific individuals. Several species of snakes secrete substances from the skin that contain lipids involved in sexual signaling or

Integument Animals
The radiated tortoise (Geochelone radiata) is considered to be one of the most beautiful tortoises. (Photo by Animals Animals ©Joyce & Frank Burek. Reproduced by permission.)

contain toxins thought to have defensive functions. These and other glands of reptiles are not well studied.

The external stratum corneum of scales often is highly sculptured with complex patterns of spiny projections or ridges. Patterns of microscopic sculpturing at micrometer and nanometer levels vary among species and may give rise to structural colors exemplified by the iridescence seen in many snakes immediately after they shed their skin and expose a new generation of epidermis. These colors are most likely incidental to other functions of the sculpturing, which remain largely unknown. The color patterns of reptiles are determined principally by pigments present in the skin. Structures called chromatophores bear pigments that reflect light differentially. Nervous or hormonal control of the dispersion of these pigments determines the color seen and gives rise to color changes related to camouflage, excitement, thermoregulation, and defensive and social behaviors. The color patterns of most reptiles probably are functional largely in relation to camouflage and protection from predators.

Water evaporates from the skin and lung and is lost in the urine and feces. Evaporative loss of water from the skin can account for more than half of the total water loss and is extremely important in species with relatively permeable integuments. Reptiles that live in drier environments have been shown to have skin that is more resistant to transepidermal water loss than is skin of reptiles that live in moist environments. The resistance to passage of water is determined largely by a discrete layer of lipids within the keratinized layers of epidermis. These lipids are organized into intercellular layers that surround the keratin filaments to form a complex association that has been likened to bricks and mortar. Thus the barrier to water is a highly ordered, laminated lipid-keratin complex. Variation in the quantity and kinds of lipids presumably accounts for differences in skin permeability among species. Contrary to popular opinion, differences in the thickness of skin, amounts of keratin fibers, or morphology of scales have relatively less influence on skin per meability than does the nature of the lipid barrier. Therefore the external appearance of the skin is not a reliable guide to how resistant an animal is to dehydration. Crocodilians have skin that is thick and tough in appearance, yet the lipid barrier is poorly developed, and the skin is quite permeable to water exchange.

Marine reptiles experience loss of water across the integument if the solute level in the external medium exceeds that of the body fluids. For example, 92% of total body water loss of the yellow-bellied sea snake (Pelamis platurus) is across the skin. However, the skin of marine reptiles generally has lower permeability than does that of freshwater species. The direction of net water movement is reversed for animals in freshwater, where the solute concentration of the medium is generally less than that of the body fluids, thereby promoting osmotic movement of water from outside to inside the animal. Excess water resulting from osmotic intake is normally eliminated by the kidneys.

A complete picture of water balance must include routes of water gains from food, metabolic water, drinking, and skin as well as losses due to excretion, respiration, and outward diffusion across the skin. The reptilian kidney cannot excrete urine that is more concentrated than the body fluids, although digestive and urinary fluids can sometimes be further concentrated by absorption of water within the cloaca. The cloaca is the terminal region of the hindgut where digestive and urinary contents are joined and temporarily stored before elimination through the anal opening. Fully marine reptiles and some terrestrial ones have excretory organs called salt glands in addition to the kidneys. These structures are located near the tongue (snakes, crocodilians), nasal region (lizards), or eye (turtles), and eliminate concentrated secretions of salts, principally sodium, potassium, and chloride, that might be ingested in excess. The salt gland, however, does not necessarily enable marine reptiles to maintain water balance by drinking seawater. Some marine species have been shown to become dehydrated if kept in saltwater indefinitely. They rely on water contained in their food and drinking of freshwater from rainfall to maintain water balance.

Snake Salt Gland
Generalized epidermis of a squamate reptile. (Illustration by Patricia Ferrer)

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    What is water exchange in animals?
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