Tooth

Teeth are hard, calcified structures found in the jaws of many vertebrates. They serve many vital roles in the survival of an animal. Their primary function is to break down food for digestion. However, many species use them for other purposes as well. Carnivores and omnivores use teeth to capture or wound prey. They also use teeth to tear food or for defense. Some animals use their teeth to intimidate rivals or to carry their young.

Teeth are not actually made of bone. Instead, they consist of multiple tissues with varying density and hardness. These tissues originate from the ectoderm, which is the outermost embryonic germ layer. The general structure of teeth is similar across vertebrates, but their form and position vary. Mammals typically have teeth with deep roots. In contrast, many teleost fish have teeth attached to the outer surface of the bone. Lizards attach their teeth to the inner surface of the jaw. Cartilaginous fish, like sharks, use tough ligaments to attach teeth to their jaw cartilage.

Animals can be categorized by how they develop their teeth. Monophyodonts are animals that develop only one single set of teeth. Diphyodonts grow an early set of deciduous teeth followed by a permanent adult set. Most extant mammals, including humans, are diphyodonts. Polyphyodonts grow many different sets of teeth throughout their lives. For example, sharks can grow a new set of teeth every two weeks to replace worn ones. Some mammals, such as elephants, kangaroos, and manatees, are exceptions to the mammalian rule and are polyphyodonts.

Scientists have different theories regarding the evolutionary origins of teeth. The "outside-in" theory suggests teeth evolved from ectoderm denticles, which are scales similar to those on shark skin. These scales may have folded and integrated into the mouth. The "inside-out" theory proposes that teeth evolved from endoderm pharyngeal teeth in the throat of jawless vertebrates. Another theory focuses on the neural crest gene regulatory network. Fossil evidence from the extinct fish Romundina stellina supports the scale theory. The teeth and scales of this fish were made of the same tissues found in mammal teeth.

In mammals, tooth shape is closely linked to diet. Herbivores often have many molars for grinding hard plant matter. Carnivores possess canine teeth for killing prey and tearing meat. Zoologists use a standardized dental formula to describe these specific patterns. In humans, the primary or deciduous teeth usually appear around six months of age. This process, known as teething, can be painful. Adults typically have up to 32 permanent teeth, including four wisdom teeth.

Some animals possess teeth that grow continuously to compensate for wear. Rodent incisors grow constantly through gnawing, which helps maintain their length. Certain rodents, like voles and guinea pigs, also have continuously growing molars. Lagomorphs, such as rabbits and hares, share this trait. This is necessary because their abrasive diet causes significant attrition.

In elephants, tusks are specialized incisors used for fighting and digging. Elephant molars work like a conveyor belt. As one tooth wears out, another moves forward to take its place. An elephant has 28 molar plates at birth, organized into four sets. When the last tooth wears out, the elephant can no longer chew and will die.

Whales also show incredible dental diversity. The narwhal possesses a giant tusk that is actually a single tooth. This tooth contains millions of sensory pathways and is the most neurologically complex tooth known. In contrast, beaked whales are almost toothless, with only strange teeth found in males. These may be used for feeding or for showing aggression.

Even the structure of whale teeth differs from humans. While humans have cementum on the outside of the root, whales have cementum covering the entire surface. This allows them to adapt to their unique environments in the ocean.