The tongue is a recognizable and versatile organ found in nearly all terrestrial vertebrates, from frogs to mammals. This muscular structure is typically involved in manipulating food, tasting, and grooming. However, the presence of a mobile, fleshy tongue is not universal across all animal life. A significant number of species, spanning both vertebrates and invertebrates, have evolved to thrive without this common organ.
Essential Functions of the Tongue in Animal Biology
The tongue’s primary role in most vertebrates is tied to the process of eating. It serves as a muscular accessory organ that helps gather food, mix it with saliva, and maneuver the resulting bolus toward the pharynx for swallowing. Without the ability to manipulate food within the mouth, chewing and swallowing would be difficult for many species.
The tongue is also the main bearer of taste buds, acting as the chemical sensor for the digestive system. It allows animals to evaluate the quality and safety of potential food sources before ingestion. In some species, such as cats, the tongue’s surface is covered with specialized, keratinized papillae that function like a rasp for grooming or removing flesh from bone.
The tongue is also adapted for specialized tasks beyond eating. Dogs use panting, which involves rapid tongue movements, as a means of thermoregulation to cool their bodies. Amphibians like frogs possess tongues that can rapidly launch outward to capture insect prey.
Identifying Key Species Without a True Tongue
A “true tongue” is defined as a mobile, muscular organ on the floor of the mouth, characteristic of most tetrapods. The most notable vertebrate exception is the entire order Crocodilia, which includes alligators, crocodiles, and caimans. Unlike other reptiles, the crocodilian tongue is fixed to the floor of the mouth along its entire length, rendering it immobile for typical food manipulation.
This fixed, fleshy structure in crocodilians cannot extend or retract. Certain amphibians, like the African Clawed Frog, also lack a tongue. This fully aquatic species uses specialized claws and head movements to push food into its mouth instead of relying on a tongue for capture or manipulation.
The vast majority of animals without a true tongue are invertebrates, constituting over 90% of all animal species. Creatures like jellyfish, sea stars, and sponges completely lack the complex organ systems found in vertebrates. As simple or entirely aquatic life forms, their feeding strategies evolved without the need for a muscular appendage in the mouth.
Specialized Structures Replacing Tongue Function
The absence of a mobile tongue in crocodilians is compensated for by the palatal valve, or gular valve. This structure, which includes the fixed tongue and a fold of tissue from the palate, seals off the throat from the mouth. This valve allows the animal to open its powerful jaws underwater to capture prey without inhaling or swallowing water, a necessary survival mechanism for their ambush hunting style.
Since crocodilians cannot use a tongue to move food, they rely on violent head movements and water currents to maneuver prey within the mouth. They will often thrash their heads or quickly submerge and surface to position the food item correctly before swallowing it whole. Their fixed tongue also contains specialized lingual salt glands in some species, which help excrete excess salt ions, linking this organ to osmoregulation.
In aquatic invertebrates, the functions of the tongue are replaced by different feeding mechanisms. Filter feeders, such as sponges and baleen whales, use specialized structures to strain tiny organisms from the water. Sponges use numerous pores and internal canals to draw water through their bodies, while baleen whales use large, comb-like plates in their mouths to sift plankton.