Mammals are a group of vertebrates that have evolved over more than 200 million years, giving rise to a wide variety of shapes and sizes as an adaptive response to the different lifestyles and environments where they have lived. developed. Placentals originated in the Cretaceous, about 130 million years ago. In this class there are extreme sizes, as in no other group of animals, from small bats that barely exceed 4 grams to the largest animal that has ever existed: the great blue whale (Balaenoptera musculus), which can reach 30 meters in length. and more than 150 tons. There are species that fly, others are aquatic and others have fossorial habits and spend almost all their lives underground. They inhabit all regions of the planet, such as the oceans, the polar areas, the high mountains or the driest deserts.
If you want to know everything about the placental mammals, their classification, characteristics and examples, continue reading this article that we present in our place.
What are placental mammals?
Mammals are those animals that feed their young through breast milk, which emanates in most cases from the mother's breasts. They are divided into three large groups: the Metatheria (marsupials), where we find kangaroos among the different types of marsupials, the Protothheria (monotremes), a group to which the platypus and other mammals that lay eggs belong, and the Placentalia (placentals). Together, these three groups currently number more than 5.100 species.
Placental mammals are viviparous mammals and, unlike marsupials, they do not have a bag or marsupium where the embryo develops, instead, it remains inside the uterus where it develops and is nourished by a chorioallantoic placenta
The time of gestation varies in each species, being generally longer in larger mammals, although there may be exceptions. Gestation can range from several days, as in the case of mice, whose gestation lasts about 21 days, to almost two years, as happens in elephants, for example. The young can be born completely covered with hair and with their eyes open, like antelopes, which are also able to run from the first moment, or they can be born without hair, with their eyes closed and totally defenseless, like many small rodents.
Characteristics of placental mammals
Although placental mammals make up very diverse groups, they share certain characteristics in addition to the placenta in which the fetus develops. Thus, the characteristics of placental mammals are:
- The skull is synapsid, that is, it has a pair of openings in the roof, where the jaw muscles are inserted. It has a milk dentition in the offspring and during the first part of life, to later be replaced by the definitive dentition of the adult.
- They have hairs at some stage of their development and can be of two types: like a fluff, which are insulating, soft and dense, or bristles, which are thick, longer guard hairs. Hair in mammals is of epidermal origin and is made up of a protein called keratin. They may be adapted as whiskers or whiskers, which are sensory hairs that provide them with a tactile sense, or in the porcupine they are adapted for protection.
- They have a skin with different modifications, since they are adapted to each type of life they lead. Like hair, which is made of chitin, nails, claws, and hooves are also made of chitin. Or like the antlers or horns of ruminants, which are hollow sheaths of epidermis, covered by keratin. These do not change or molt, are unbranched, and are present in both sexes. On the other hand, the antlers present in the deer family are entirely bony when fully formed. Each year they grow under a covering of very soft, vascularized skin called velvet. In mating season they molt, scratching against trees, and stray after each breeding season.
- The mammary glands produce milk to feed the young and give this group its name. Milk is made up of fats and proteins that allow the progeny to develop and grow in the initial stage of their lives. They are present in all females and in a rudimentary way in males.
- There are also sweat glands in various parts of the body and only found in mammals. They can be eccrine, which secrete a watery sweat that absorbs heat from the skin and cools it and are generally found in areas without hair, or apocrine, present in areas with hair and their secretion is whitish.
- Their feed is very varied depending on the group they belong to, so they can be carnivorous, with teeth adapted to tear meat and with claws to catch their prey, herbivores, which feed on vegetation, insectivores, which eat small invertebrates such as snails, earthworms or ants, or omnivorous animals and feed on both animals and plants.
- They have a estrous cycle (or heat) in the case of females, that is, a periodic cycle in which they are suitable for fertilization, since many males are capable of fertile copulation at any time of the year. Estrus is divided into different stages where changes occur in the ovaries, uterus and vagina, and a preparation phase, when she is fertile and copulation occurs.
Classification of placental mammals
The placentals or eutherians are an underclass of mammals and is the most diverse group of the three groups of mammals that exist. Eutheria (Eutherios) are a clade (grouping) that includes placentals, plus all marsupial mammals (Metatheria). This group is divided into 18 orders of placental mammals, all of them very diverse in terms of physical characteristics and habits. Next, we will see how placental mammals are classified and some examples of each of them:
- Xenarthra (29 species): they are exclusively American. Here we find anteaters, armadillos and sloths. They have very varied morphologies, such as elongated bodies in the case of the anteater (Tamandua mexicana), which also has an elongated snout and a long tongue that allows it to hunt ants and termites, as well as strong claws with which to break termite mounds or anthills. On the other hand, sloths (Choloepus didactylus) also have claws to climb and have a very slow metabolism. They are present throughout the American continent.
- Pholidota (7 species): These animals are characterized by having their bodies covered with large scales. They have powerful claws, a prehensile tail and a large sticky tongue. Its representative is the pangolin (Manis crassicaudata), which inhabits Africa and Asia and feeds on termites and ants. Although there is only one genus of pangolin, there are seven different species. They all have nocturnal habits and are solitary animals.
- Lagomorpha (80 species): Hares and rabbits are found here. They resemble rodents only because of their long, continuously growing incisors, which force them to constantly gnaw. The difference between one and the other is that lagomorphs have two rows of incisors. They inhabit Europe, Africa and North America, but were introduced to other continents, and are now almost cosmopolitan.
- Rodentia (2024 species): constitute the largest order of placental mammals, comprising more than half of the species of mammals. Their size is generally small and they inhabit the entire earth, especially house mice, which are cosmopolitan. They are species that adapt very easily to available food and environments.
- Macroscelidea (15 species): these are the elephant shrews such as Elephantulus brachyrhynchus. They are small animals with a long snout and elongated hind legs. They only inhabit the African continent.
- Primates (236 species): they are classified into two large groups, on the one hand there are the Strepsirrhini with lemurs from Madagascar, the galagos from Africa and the lories from India and Southeast Asia, and on the other hand there are the Haplorrhini, with tarsids, monkeys and apes, including humans. They are widely distributed throughout the world, such that we have the monkeys of Central and South America (Plathyrrhini), such as the marmoset Saimiri oerstedii or the howler monkey Aloutta caraya, and the monkeys and apes of Africa, Europe and Asia, such as the macaque Macaca mulatta, the chimpanzee Pan troglodytes or the human Homo sapiens.
- Scandentia (19 species): these are tree shrews, found in the jungles of Southeast Asia. These placental mammals are adapted for life in trees, as they have a long tail and small claws for climbing, like Anathana ellioti.
- Dermoptera (2 species): They have membranes similar to those of bats, but their anatomy is different from that of bats. They are relatively large arboreal gliders that feed on shoots, fruits, leaves and flowers, such as the kaguang or colugo (Cynocephalus variegatus).
- Chiroptera (928 species): Bats are the only mammals that have active flight, as they have true wings. They are present on all continents except Antarctica. They possess echolocation, which allows them to move in the dark. Some are pollinators of the plants they visit, other species are insectivorous, frugivorous and some can consume blood, they are the so-called vampire bats, such as Desmodus rotundus, which lick the blood of animals such as cows or pigs.
- Carnivora (271 species): they are animals present all over the planet. Here you will find seals, elephant seals, walruses and sea lions. These species are found in almost all the seas, but they are especially grouped in the cold waters near the poles, due to the high concentration of fish and crustaceans that make up their diet. In general, they have a clumsy and heavy body on dry land, but great agility in the water. On the other hand, here are felids, such as cats, panthers, lions and cheetahs, and canids, like foxes, dogs and wolves, which are characterized by having an agile body, flexible spine and specialized limbs for running, because they must capture their prey to get food. Also here you can find the mustelids , such as otters, minks, skunks and the like, lthe ursids, where the bears are, the procyonids, like raccoons, coatis and pandas, the viverrids, which are genets, civets, mongooses, meerkats, and hyaenids , which are hyenas. Within this group, however, there is a mainly vegetarian species: the panda.
- Insectivora (429 species): they are the most primitive order of placental mammals, as they retain many characteristics of the ancient insectivores that They lived alongside the dinosaurs. They are represented by animals such as the shrew (Crocidura leucodon) present in Asia, the hedgehog (Erinaceus europaeus) from Europe, Asia, Africa and that have been introduced in New Zealand, and the Talpa europaea mole present in North America, Europe and Asia.
- Artiodactyla (220 species): has an even number of toes (2 or 4) that are covered by a horny layer called the hoof. Ruminant artiodactyls can be found, such as oxen, moose, buffalo, gazelles and giraffes, which are characterized by having a stomach with several chambers, ruminating and the presence of antlers that they use as means of defense. Non-ruminant artiodactyls include hippopotamuses and pigs. On the other hand, camelids (camels, dromedaries, vicuñas, alpacas, guanacos, and llamas), for example, have adapted to extreme environments, such as high altitudes or arid climates. They are present in America and Africa.
- Cetacea (78 species): Cetaceans are the only mammals that live exclusively in water. Here we find dolphins, sperm whales and whales. The body of cetaceans is extremely voluminous and they achieve their propulsion thanks to the muscles of the caudal fin, which is large and fleshy. They are devoid of hair, they only have a few touches near the mouth, so, as a method of thermal insulation, they have a layer of fat several centimeters thick.
- Tubulidentata (1 species): The aardvark (Orycteropus afer) is found here. It feeds almost exclusively on insects such as termites. It has a sticky saliva and a long tongue with which they catch them. It lives in the prairies or in the forests. It is native to Africa.
- Perissodactyla (18 species): This order includes large animals whose feet have an odd number of toes (1), which it is covered by a horny hoof. The best known representative is the horse. Other species of this order are donkeys, zebras, tapirs and rhinoceroses. They inhabit America, Africa, Asia and Europe.
- Hyracoidea (6 species): they have similarities with elephants and other groups of placental mammals, however, their shape and habits are similar to those of rodents. Here are the hyraxes (Procavia capensis), which live in Africa and are adapted to any type of environment and have a herbivorous diet.
- Proboscidea (2 species): here we have the elephant, with a proboscis or trunk that originates from the fusion of the nose with the upper lip and is used for breathing, sniffing and as a prehensile organ. They are currently represented by two species: the Asian elephant and the African elephant. The female of the Asian elephant does not have tusks and the male has them less developed than the African. His ears are small and triangular. The African elephant, on the other hand, has large ears. All elephants are exclusively herbivorous.
- Sirenia (5 species): these are placental mammals that, together with cetaceans and pinnipeds, have populated the aquatic environment. They live along the coasts or in rivers with abundant aquatic plants, since their diet is exclusively herbivorous. Due to the disappearance of their hind limbs, they swim using their enormous tail and their forelimbs, which have been transformed into fins. Representatives of this order are the manatee Trichechus manatus, which lives in America and Africa, and the dugong Dugong dugon, which lives in Africa, Asia and Australia.
