Zoology Terms Starting With R

Radial Symmetry is a body plan in which multiple planes passing through a central axis divide the body into similar halves or repeated sectors.

Radial symmetry occurs in sessile, drifting, or slow-moving animals that interact with the environment from many directions rather than mainly from a head-first axis. Cnidarians such as jellyfish, corals, hydras, and sea anemones show radial or biradial organization, with tentacles and sensory structures arranged around the mouth. Adult echinoderms display pentaradial symmetry, usually organized into five sectors, even though their larvae are bilaterally symmetrical and their ancestors were bilaterians.

This body plan suits feeding, sensing, and defense around a central point, but it is less suited to fast directional locomotion than bilateral symmetry.

Rattle Quill is a short, hollow tail quill of porcupines. It occurs in crested porcupines of the genus Hystrix. Each one is thin-walled and open at its free end. It sits on a narrow stalk. A threatened porcupine shakes its tail. The rattle quills knock together and make a hissing rattle. This sound warns predators away.

Rattle quills grow only on the porcupine’s short tail. They look nothing like the long, spear-like body quills. Each one is a hollow cup on a thin stalk.

A threatened porcupine raises its quills and shakes its tail. The hollow quills clatter together like a baby’s rattle. It may also stamp its feet at the same time.

This noisy display warns predators before any contact. If the threat stays, the porcupine may charge backward. Its long defensive quills then stab the attacker.

Remote Touch lets probing birds sense buried prey before contact. The bill tip feels tiny vibrations in wet mud. Ibises, kiwi, and many sandpipers use this sense.

A probing bill pushes water through wet sand. A buried clam or worm disturbs that flow. Burrowing prey also sends out tiny vibrations.

Herbst corpuscles in pits at the bill tip detect them. Researchers first showed this ability in Red Knots in 1998. Ibises, kiwi, and many sandpipers have similar bill-tip organs.

The sense works best in wet mud or sand. Dry sand carries the signal poorly. Birds probing drier soil may need more sensitive bills.

Reptile is a scaly amniote vertebrate traditionally grouped with lizards, snakes, turtles, crocodilians, and the tuatara; in modern cladistics, birds are nested within the broader reptile lineage, making the traditional grouping paraphyletic when birds are excluded.

Modern phylogenetics shows that crocodilians are more closely related to birds than to lizards, a relationship supported by molecular clock estimates placing the crocodilian-bird divergence at roughly 240 million years ago. Reptiles and their relatives dominated terrestrial ecosystems during the Mesozoic, with dinosaurs, pterosaurs, and marine reptiles occupying nearly every major ecological niche. Reptile scales are keratinous epidermal structures, distinct from fish scales, which incorporate dermal bone; most living non-bird reptiles are ectothermic and regulate body temperature through behavioral means such as basking.

Rodent is any mammal of the order Rodentia. Rodents have one pair of ever-growing incisors per jaw. They have no canine teeth. A gap called a diastema separates incisors from cheek teeth. More than 2,700 rodent species are recognized. They make up about 40 percent of living mammal species.

Squirrels, mice, beavers, and porcupines are all rodents. So are capybaras, guinea pigs, and chinchillas. Their shared trademark is ever-growing gnawing incisors.

Strong jaw muscles drive these teeth with great force. The masseter muscle does most of this work. Rodents also slide the lower jaw forward and back.

This lets them switch between gnawing and chewing. Most rodents are small, but sizes vary enormously. Many eat seeds, while others eat bark or insects.

Rostrum is an elongated anterior projection of the head, snout, or mouthparts in multiple animals, used for piercing, probing, feeding, or defense depending on the lineage.

In plant-feeding true bugs (order Hemiptera), the rostrum is a tubular piercing structure containing stylets that penetrate plant or animal tissues to extract sap or blood. The rostrum of crustaceans such as crayfish and shrimp is a rigid anterior projection of the carapace positioned between the eyes, used in defense and species recognition. Among vertebrates, the elongated upper jaw of the sawfish (family Pristidae) bears tooth-like denticles along its margins and can reach more than 1.8 meters in length, functioning both to detect prey through electroreceptors and to slash through schools of fish.

Ruminant is a hoofed mammal that digests plant material through a specialized multi-chambered stomach, including a rumen where microbial fermentation breaks down cellulose before further digestion.

The ruminant stomach has four compartments: rumen, reticulum, omasum, and abomasum. Inside the rumen, dense microbial communities of bacteria, archaea, protozoa, and fungi ferment cellulose into volatile fatty acids, which the animal absorbs directly through the rumen wall as its primary energy source. Rumination involves regurgitating rumen contents as a bolus for further mechanical breakdown by chewing cud, increasing microbial access to plant cell wall polysaccharides and improving fermentation efficiency.