Mouth to mouth: saliva transfer can help animals communicate, medicate, or even kill. Evolution has given rise to a variety of salivary mixtures that are being mined for ways to help save human lives

Natural History, Nov, 2004 by Lawrence A. Tabak, Robert Kuska

That familiar refrain in country-and-western songs, "You don't miss it 'til it's gone," is a good way to describe saliva. It's taken for granted, sometimes mocked, and often shunned in polite conversation. But in scientific circles there is good reason to speak with excitement about the subject of saliva. Ordinary spit, according to several lines of recent research, turns out to be far more than just a way to wet your lips.

In fact, saliva is a remarkably complex biological fluid with an extraordinary natural history It addresses a laundry list of obvious but necessary functions: lubricating the mouth, moistening dry food, assisting in digestion, protecting the teeth from decay, wetting the taste buds, and buffering the inside of the mouth against continuous assault from microorganisms. But beyond those basic functions, saliva exhibits tremendous variation in nature, often with special adaptations that can correspond to the dietary habits of the various species.

Giraffes, for instance, have evolved thick, mucus-like saliva that enables them to chew thorns without damaging their mouths. Creatures that drink blood--vampire bats, mosquitoes, and many ticks--have evolved amazingly efficient anticoagulant agents in their saliva to help them feed on their hosts. The Komodo dragon, the world's largest lizard, has more than fifteen infectious agents suspended in its saliva. When the powerful bite of the Komodo doesn't topple its victim, the salivary, pathogens Call finish the job [see "The Lizard kings," by Samuel S. Sweet and Eric R. Pianka, November 2003]. Pigs and many other animals rely on pheromones secreted in their saliva to woo their mates.

Such a broad range of functions collected and mixed in one fluid is particularly intriguing to evolutionary biologists. Salivary glands have evolved quite rapidly compared with other organs. They could well be one of the primary ways many species adapt to their environments, outcompete their rivals, and fill new ecological niches. Furthermore, salivary glands are pervasive in creatures large and small. It the glands were not advantageous, they would probably have been lost somewhere along the way.

The study of saliva, however important it is to evolutionary biology, also holds great promise for medical science. For one thing, the rich variety of proteins that occur in nonhuman saliva promise treatments for diabetes, strokes, and other diseases. But studies of human saliva are pointing to what could be even more exciting possibilities: Saliva can serve as a useful qualitative diagnostic tool, enabling tests for disease antibodies to be done without the risk and discomfort of drawing blood. And saliva itself, produced by genetically modified salivary glands, might someday serve as a twenty-four-hour internal pharmacy dispensing individually tailored medications to people who need them.

Saliva is notable not only for the variety of its ingredients, but also for the diverse structures of the glands that produce it. Other important secretory glands of the digestive system, such as the liver and the pancreas, are similar ill structure across various life forms, from fishes to mammals. The genetic blueprint for those glands could probably not be modified very much without unduly affecting their vital contributions to life. In contrast, as studies of more than 300 mammalian species have shown, the structural diversity of the salivary organs is striking. Even within single families of organisms there is often a great deal of variability among the salivary glands of different species.

In spite of that variety, mammals generally have the same three sets of salivary glands, which are each composed of clusters that resemble grapes still attached to their stems. Secretory cells inside the grapelike nodules release the initial salivary fluid, made up of water and some proteins. The fluid passes along the "stem," or narrow duct, of the cluster, where other cells modify its salt balance before it passes into the mouth.

Human beings have four kinds of salivary glands, including the three kinds common to other mammals. Parotid glands, which are opposite the front lower molars, nearly parallel to the ear lobes, secrete a thin, watery substance that is rich in antibacterial proteins and compounds that help remineralize the teeth. Submandibular glands, egg-shaped structures embedded below the floor of the mouth just above the throat, produce a more viscous fluid that helps lubricate the throat and mouth. Sublingual glands, a pair of almond-shaped structures also located below the floor of the mouth, produce secretions much like those of the submandibular glands. Finally, a fourth kind of salivary gland occurs in humans, comprised of hundreds of smaller, minor salivary glands cover the tongue and lining of the mouth. Some of them form small bumps on the inner lip. Their secretions play a major role in helping lubricate the mouth and protecting against infections.

The sum of these secretions, the pooled product called whole saliva, is about 99 percent water, but it also comprises a diversity of biochemicals that one gland alone could not produce efficiently. Variations on this theme between and even within species tell a compelling story of evolution's inventiveness.