Animals

Ape: biology, classification, behavior and conservation Ape Overview of apes (Hominoidea): anatomy, classification, behavior, evolution, and conservation, with distinctions from monkeys and notes on human relationships.

Apex predator Apex predators are animals at the top of a food chain with no natural predators. They shape ecosystems through direct hunting and indirect effects, and are central to conservation…

Aplacophora: worm‑like, shell‑less molluscs of the deep sea Aplacophora Aplacophora are small, vermiform, shell‑less marine molluscs with a mantle covered in calcareous spicules. Found worldwide, they illuminate molluscan evolution and benthic ecology.

Apodiformes: swifts, hummingbirds and their specialized flight adaptations Apodiformes Apodiformes are an order of highly aerial birds—swifts and hummingbirds—known for long wings, reduced feet, diverse flight styles, and about 450 species worldwide. This article cov…

Apostatic selection: predator-driven maintenance of prey polymorphism Apostatic selection Apostatic selection is negative frequency-dependent predation where predators focus on common prey morphs, giving rare variants a survival advantage and helping maintain polymorphi…

Apterygota: primitive wingless insects (bristletails and silverfish) Apterygota Apterygota are early-diverging, wingless insects—ametabolous forms such as bristletails and silverfish—known for lifelong molting, primitive anatomy, and a fossil record reaching t…

Aquarium An aquarium is a controlled aquatic environment where fish and other water-dwelling animals are kept for display, study, conservation, or as a hobby, ranging from small home tanks…

Aquilops — an early North American ceratopsian Aquilops Aquilops is a small, early horned-dinosaur relative known from a tiny skull found in Montana, dating to the Lower Cretaceous (about 109–104 million years ago) and important for cer…

Arabian horse: history, characteristics, and cultural importance Arabian horse An overview of the Arabian horse breed: origins, physical and behavioral traits, historical development, uses in sport and breeding, and notable distinctions.

Arachnid: overview, anatomy, diversity and role in ecosystems Arachnid Arachnids are a class of chelicerate arthropods including spiders, scorpions, mites and ticks, notable for eight legs, chelicerae and diverse ecological roles from predators to par…

Arachnocampa — glowworm fungus gnats Arachnocampa Arachnocampa are a genus of bioluminescent fungus gnats whose larvae, known as glowworms, produce light and hang silk threads to capture prey in caves and forest litter.

Araneomorphae — the modern 'true' spiders Araneomorphae Araneomorphae are the largest and most diverse infraorder of spiders, including orb‑weavers, jumping spiders and cobweb weavers. Overview of their anatomy, diversity, evolution, ec…

Arapaima (pirarucu, paiche): Large Amazonian bonytongue fish Arapaima Arapaima are very large freshwater bonytongue fish native to South American river basins, noted for air-breathing, thick scales, economic importance, and conservation concerns.

Archelon — the giant sea turtle of the Late Cretaceous Archelon Archelon was a car-sized marine turtle from the Upper Cretaceous Western Interior Seaway. Known from fossils in the central United States, it is the largest sea turtle yet discover…

Archosaurs: evolution, key features, and Mesozoic dominance Archosaur Archosaurs are the reptile group that gave rise to crocodiles, pterosaurs, dinosaurs and modern birds; defined by shared skeletal traits, an early Triassic radiation, and a major r…

Archostemata: the primitive, basal suborder of beetles Archostemata Archostemata is a small, ancient suborder of beetles with fewer than fifty living species. They retain primitive traits, have a rich fossil history, and include unusual life cycles…

Arctic char (Salvelinus alpinus): biology, distribution, and conservation Arctic char Arctic char is a cold‑water salmonid with a circumpolar range. This article describes its appearance, life histories (landlocked and anadromous), habitat, uses in fisheries and aqu…

Arctic fox (Vulpes lagopus): appearance, adaptations and ecology Arctic fox Compact overview of the Arctic fox (Vulpes lagopus): physical traits, seasonal adaptations, diet, reproduction, ecological role and notable facts.

Arctic hare (Lepus arcticus) Arctic hare A cold‑adapted, tundra‑dwelling lagomorph of the high Arctic. Covers appearance, behavior, diet, predators, adaptations, distribution, reproduction and conservation.

Arctic tern (Sterna paradisaea): migration, appearance, and range Arctic tern Overview of the Arctic tern, including identification, remarkable migratory behaviour, breeding range, ecology, and cultural notes such as its appearance on postage stamps.

Arctiidae — tiger moths, lichen moths and woolly bears Arctiidae Arctiidae (commonly tiger moths) is a diverse group of moths known for bright patterns, hairy 'woolly bear' caterpillars, chemical defenses and mimicry; it is now often treated as…

Argali (Ovis ammon): the wild mountain sheep of Central Asia Argali Argali (Ovis ammon) is the largest wild sheep, native to Central Asian highlands. This article covers appearance, habitat, behavior, subspecies (including Marco Polo sheep), and co…

Argentavis magnificens — the giant teratorn of the Miocene Argentavis magnificens Argentavis magnificens was an enormous soaring bird from the late Miocene of South America. Known from Argentine fossils, it is among the largest flying birds ever and illustrates…

Argentinosaurus — giant titanosaur of the Late Cretaceous Argentinosaurus Argentinosaurus was a colossal titanosaurian sauropod from Argentina. Known from fragmentary remains, it is considered among the largest dinosaurs, though size estimates remain unc…

Argonauta (paper nautiluses): pelagic octopods and their eggcases Argonauta Argonauta are pelagic octopods whose females secrete a delicate, paper-thin eggcase. They display extreme sexual dimorphism, live in tropical and subtropical seas worldwide, and re…

Armadillo: overview, anatomy, behavior, and conservation Armadillo Armadillos are New World placental mammals with a protective armored carapace. This article summarizes their anatomy, behavior, distribution, taxonomy, interactions with people and…

Army ant (legionary ant) Army ant Army ants are a group of over 200 ant species noted for large cooperative raids, nomadic colonies that form living nests (bivouacs), and a major role as tropical predators exhibiti…

Aromobatidae: neotropical frogs related to poison-dart frogs Aromobatidae Aromobatidae is a family of small neotropical frogs native to Central and South America. Many species are terrestrial, exhibit parental care, and differ from classic poison-dart fr…

Arthrodires (Arthrodira) — jointed-neck placoderm fishes of the Devonian Arthrodire Extinct order of armored, jawed placoderms with a distinctive head–thorax joint; diverse in form and ecology during the Devonian and lost in the Late Devonian extinctions.

Arthroleptidae (squeaker frogs of sub-Saharan Africa) Arthroleptidae Arthroleptidae is a family of small, primarily terrestrial African frogs known as squeakers. Many species develop on land without a free‑living tadpole stage and inhabit leaf litte…

Arthropod: characteristics, diversity, evolution and ecological importance Arthropod Arthropods are invertebrates with jointed legs and an external skeleton. This article reviews their anatomy, diversity, evolutionary history, ecological roles, and differences amon…

Artiodactyls — even-toed ungulates Artiodactyl Artiodactyls are even-toed hoofed mammals including cattle, deer, pigs, camels, giraffes and hippos; the group is central to terrestrial ecosystems, agriculture and evolution studi…

Ascaphidae — the tailed frogs Ascaphidae Ascaphidae is a small family of primitive frogs native to western North America, known for a male 'tail' used for internal fertilization and adaptations to fast, cold mountain stre…

Ascaris: biology, life cycle, and public health impact of giant intestinal roundworms Ascaris Ascaris is a genus of large parasitic roundworms that infect humans and animals. The article explains anatomy, life cycle, clinical effects, diagnosis, treatment, and prevention in…

Asian black bear (Ursus thibetanus) Asian black bear Medium-sized forest bear of Asia with a distinctive white chest mark. Overview of appearance, behavior, range, threats, and conservation, plus notable differences from other bear s…

Asian elephant (Elephas maximus) Asian elephant Large mammal native to South and Southeast Asia; social, forest-dwelling herbivore used historically by people and now threatened by habitat loss, conflict, and poaching.

Asian garden dormouse (Eliomys melanurus) Asian garden dormouse A small nocturnal rodent of the Gliridae family found across North Africa, the Middle East and Turkey; occupies forests, Mediterranean shrubland and rocky areas and is largely omni…

Asian tiger mosquito (Aedes albopictus) — overview, biology, public health Asian tiger mosquito Aedes albopictus, the Asian tiger mosquito, is a black-and-white, day‑biting species native to Southeast Asia that has become globally invasive and can transmit several arboviruses…

Asiatic linsang (Prionodon): small arboreal carnivores of Southeast Asia Asiatic linsang Asiatic linsangs (genus Prionodon) are two slender, tree‑loving carnivores of Southeast Asia. Molecular studies place them in their own family, Prionodontidae, closely related to c…

Asiatic lion (Panthera leo persica) Asiatic lion A subspecies of lion native to South Asia, the Asiatic lion is now confined to a small region of India and is the focus of conservation due to its endangered status and limited ran…

Asiatic salamanders (Hynobiidae): overview, biology and conservation Asiatic salamander Asiatic salamanders (family Hynobiidae) are a basal group of salamanders from temperate Asia, known for external fertilization, aquatic larvae and numerous Japanese endemics; many…

Asiatic wildcat (Felis silvestris ornata) Asiatic wildcat A small wildcat subspecies native to arid and steppe regions of Central and South Asia, recognised by a pale, spotted coat and facing habitat loss and hybridization threats.

Aspidogastrea: primitive flukes with a distinctive adhesive disc Aspidogastrea Aspidogastrea are a small subclass of trematode flatworms (about 80 species) parasitizing molluscs and vertebrates; notable for a large ventral adhesive organ and evolutionary impo…

Assassin bug (Reduviidae): predatory true bugs and their significance Assassin bug Assassin bugs (family Reduviidae) are a diverse group of predatory true bugs known for their raptorial proboscis, varied hunting strategies, and importance as both beneficial preda…

Astor Markhor (Flare‑horned Markhor): overview, range and conservation Astor markhor Subspecies of wild goat with distinctive flared horns, native to parts of northern Pakistan, Kashmir and adjacent Afghanistan; threatened by hunting and habitat loss.

Astrodon — Early Cretaceous sauropod known for star-shaped teeth Astrodon Astrodon is an Early Cretaceous sauropod known from teeth and fragmentary bones in Maryland. Its distinctive star-shaped teeth and debated taxonomy, including links to Pleurocoelus…

Atelopus (harlequin frogs): appearance, habitat, decline, and conservation Atelopus Atelopus, the harlequin frogs, are small, often brightly colored toads of Central and South America that live near mountain streams. Many species have declined from chytrid and hab…

Jambato Toad (Atelopus ignescens): Rediscovery and Conservation in the Ecuadorian Andes Atelopus ignescens Jambato Toad (Atelopus ignescens): rediscovered in the Ecuadorian Andes, this high‑elevation toad faces fragmented populations—learn about its habitat, threats, and conservation ac…