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

For decades, the biologists Carleton J. Phillips of Texas Tech University, in Lubbock, and Bernard Tandler of Case Western Reserve University, in Cleveland, Ohio, have studied salivary glands in various species of bats. Among all the mammals, the 800-odd species of bats--nearly a quarter of all existing mammalian species--have been superlative at adapting to a wide range of food sources, from fruit, nectar, and pollen, to insects and blood.

Phillips and Tandler have documented many variations in the protein content and the physical structure of salivary gland cells throughout the bat order. Most bats possess not one but two sets of submandibular glands, and some also have salivary glands at the corner creases of their mouths. Those additional glands have assumed a broad range of functions. In the white-winged vampire bat (Diaemus youngi), for instance, the extra pair of glands ejects a foul-smelling liquid, not unlike a skunk's spray, that wards off unwanted advances. In frog-eating bats (Trachops cirrhosus) another salivary gland produces toxin-inhibiting proteins that seem to counteract the deadly poisons present in frog skin.

The simplest model that can account for the emergence of such adaptations is the appearance, at various points on an organism's genes, of single mutations that alter the function of cells and tissues. Some such changes are beneficial, some have no effect, and some are detrimental to the organism in which they take place. What separates the genetic wheat from the chaff, of course, is natural selection. Mutations that enabled bats to exploit new and hitherto dangerous food sources would have spread throughout the populations as the animals possessing them reproduced, presumably at greater rates than the bats without the mutations.

But Phillips and Tandler hypothesized that such simple mutations could not have happened fast enough to give rise to the diversity of bats that exist today. Instead, they proposed, the bats' DNA must have been undergoing changes on a larger scale, making their evolution a far more dynamic process. Such large-scale genetic alterations would have been a highly useful response to, say, some environmental change that forced a population to switch to a new food source. In that event, the bats' salivary glands may have needed to evolve rapidly if the animals were to thrive in their newfound dietary niche.

To explore this idea, the investigators turned to the heterogeneous family of Phyllostomidae, the New World leaf-nosed bats. Originally made up of insect-eaters, the family has adapted to diets of fruit or blood. The question that intrigued Phillips and Tandler was: how do the salivary glands that evolved among fruit-consuming phyllostomid bats differ in structure and function from the glands that evolved in their insect-eating cousins.

When fruit bats were first adapting to their current diets, they placed new and stressful demands on the salivary glands. For one thing, the glands had to combat the unfamiliar bacteria that invaded the existing community of organisms after the dietary shift. Furthermore, on their original insect-based diet, the bats had subsisted on foods rich in protein. Fruits, however, are extremely low in protein, and they also contain tannins that inhibit digestion. So when they switched to a fruit-based diet, fruit bats had to cat copious quantities of food, and process it quickly, just to ingest enough protein to survive. The salivary glands might have had to cope with rowel nutrients, which meant they suddenly had to manufacture many new proteins.