About Animalia
Characteristics: Animals share multiple characteristics with other living organisms: like plants and fungi, they are eukaryotic, multicellular, and aerobic. Unlike photosynthetic plants and algae, animals cannot produce their own food, a trait they share with fungi. Instead, animals ingest organic material and digest it internally.
Reproduction: Nearly all animals reproduce sexually. They produce haploid gametes via meiosis: smaller, motile gametes are spermatozoa, while larger, non-motile gametes are ova. These gametes fuse to form zygotes, which develop through mitosis into a hollow embryonic structure called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and grow into a new adult sponge. In most other animal groups, the blastula undergoes more complex rearrangement. It first invaginates to form a gastrula with a digestive chamber and two distinct germ layers: an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between these two layers. These germ layers then differentiate to form specialized tissues and organs. Repeated close-relative mating during sexual reproduction typically causes inbreeding depression in populations, because it increases the prevalence of harmful recessive traits. Animals have evolved many different mechanisms to avoid close inbreeding. Some animals can reproduce asexually, which usually produces a genetic clone of the parent. Asexual reproduction can occur through fragmentation, budding (seen in Hydra and other cnidarians), or parthenogenesis, where fertile eggs develop without mating (seen in aphids).
Ecology: Animals are grouped into ecological categories based on their trophic level and how they consume organic material. These categories include carnivores (further divided into subcategories like piscivores, insectivores, and ovivores), herbivores (subcategorized into folivores, graminivores, frugivores, granivores, nectarivores, algivores, and more), omnivores, fungivores, scavengers/detritivores, and parasites. Interactions between animals in a given biome form complex food webs within that ecosystem. In carnivorous or omnivorous species, predation is a consumer-resource interaction where a predator feeds on another organism, its prey. Prey species often evolve anti-predator adaptations to avoid being eaten, and reciprocal selective pressures lead to an evolutionary arms race between predator and prey, resulting in various forms of antagonistic or competitive coevolution. Almost all multicellular predators are animals. Some animals use multiple feeding methods: for example, parasitoid wasp larvae feed on their host's living tissues and kill the host in the process, while adult parasitoid wasps primarily feed on flower nectar. Other animals have very specific feeding behaviors, such as hawksbill sea turtles, which mainly eat sponges. Most animals depend on biomass and bioenergy produced by plants and phytoplankton (collectively called producers) via photosynthesis. Herbivores, as primary consumers, directly eat plant material to digest and absorb nutrients, while carnivores and other animals at higher trophic levels get nutrients indirectly by eating herbivores or other animals that have eaten herbivores. Animals oxidize carbohydrates, lipids, proteins, and other biomolecules during cellular respiration, which allows them to grow, maintain basal metabolism, and power biological processes like locomotion. Some benthic animals that live near hydrothermal vents and cold seeps on the dark seafloor consume organic matter produced via chemosynthesis (the oxidation of inorganic compounds like hydrogen sulfide) by archaea and bacteria. Animals originated in the ocean; all extant animal phyla have at least some marine species, except Micrognathozoa and Onychophora. Several arthropod lineages began colonizing land around the same time as land plants, likely between 510 and 471 million years ago, during the Late Cambrian or Early Ordovician. Vertebrates such as the lobe-finned fish Tiktaalik started moving onto land in the late Devonian, about 375 million years ago. Other notable animal groups that colonized land environments are Mollusca, Platyhelmintha, Annelida, Tardigrada, Onychophora, Rotifera, and Nematoda. Animals occupy almost all of Earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interior of other organisms. However, animals are not particularly tolerant of high heat; very few can survive at constant temperatures above 50 °C (122 °F) or in the most extreme cold deserts of continental Antarctica. The collective global geomorphic influence of animals on Earth's surface-shaping processes remains largely understudied, with most research limited to individual species and well-known examples.
Practical uses: Human populations exploit a large number of animal species for food, including domesticated livestock raised through animal husbandry, and wild species mainly hunted at sea. Many species of marine fish are caught commercially for food, and a smaller number of species are commercially farmed. Humans and their livestock make up over 90% of the biomass of all terrestrial vertebrates, and almost as much biomass as all insects combined. Invertebrates including cephalopods, crustaceans, insects (mainly bees and silkworms), and bivalve or gastropod molluscs are hunted or farmed for food and fibres. Chickens, cattle, sheep, pigs, and other animals are raised as livestock for meat worldwide. Animal fibres such as wool and silk are used to make textiles; animal sinews have historically been used as lashings and bindings, and leather is widely used to make shoes and other goods. Animals have been hunted and farmed for their fur to make items like coats and hats. Dyestuffs including carmine (cochineal), shellac, and kermes are produced from the bodies of insects. Working animals including cattle and horses have been used for work and transport since the earliest days of agriculture. Model organisms such as the fruit fly Drosophila melanogaster play a major role in scientific research as experimental models. Animals have been used to develop vaccines since vaccines were discovered in the 18th century. Some medicines, such as the cancer drug trabectedin, are based on toxins or other molecules originally produced by animals. Humans have used hunting dogs to help chase down and retrieve game, birds of prey to catch birds and mammals, and tethered cormorants to catch fish. Poison dart frogs have been used to poison the tips of blowpipe darts. A wide variety of animals are kept as pets, including invertebrates such as tarantulas, octopuses, and praying mantises, reptiles such as snakes and chameleons, and birds including canaries, parakeets, and parrots. The most commonly kept pet species are mammals, specifically dogs, cats, and rabbits. There is an ongoing tension between the role of animals as human companions and their status as individuals with their own rights. Many types of terrestrial and aquatic animals are hunted for sport.
Symbolic uses: The signs of both the Western zodiac and the Chinese zodiac are based on animals. In China and Japan, the butterfly has been viewed as the personification of a person's soul, and in classical Western representation the butterfly is also a symbol of the soul. Animals have been subjects of art from the earliest periods, including historical works such as those from ancient Egypt, and prehistoric works such as the cave paintings at Lascaux. Famous major animal artworks include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's c. 1762 horse portrait Whistlejacket. Insects, birds, and mammals appear as characters or subjects in literature and film, for example in giant bug movies. Animals including insects and mammals feature in mythology and religion. The scarab beetle was sacred in ancient Egypt, and the cow is sacred in Hinduism. Among other mammals, deer, horses, lions, bats, bears, and wolves are subjects of myths and religious worship.
