Mammals: Characteristics, Types, and Evolution
Mammals: Characteristics and Evolution
Mammals are the most evolved of all vertebrates. The majority live on land; their bodies are covered with hair or fur, and the females have glands on their chest, called mammary glands, which secrete milk when the animal gives birth to its young. Among non-land mammals are whales and dolphins, which live in the aquatic environment, and bats, which are flying mammals.
Mammals are homeothermic, or warm-blooded animals, so their body temperature stays fairly constant.
The Body of Mammals
The body of mammals is more developed than that of other vertebrates. Some of its distinguishing characteristics are the following:
- Mouth: It has different types of teeth for grinding food, and lips. Their teeth are replaced once during their lifetime.
- Fur or hair: It covers the body of the majority of mammals and allows them to maintain the right body temperature. Some mammals, such as whales and dolphins, lost their fur when they adapted to the aquatic environment. The loss of fur helps them to swim.
- Brain: It is the largest of all vertebrates, so it can perform more complex actions.
- Limbs: Land mammals have four limbs, usually with five toes each, although they may have fewer.
- Mammary glands: These are on the female’s chest. The milk that they secrete allows them to feed their young during the first few months of life.
- Lungs: They are the main organs of their respiratory system.
- Anus: Unlike the cloaca of the rest of vertebrates, the anus is only the exit orifice of the digestive tract.
Mammalian Habits
Mammals’ feeding habits are very diverse and vary depending on the species. Some mammals are carnivores (e.g., the lion), while others are herbivores (e.g., the giraffe), or omnivores (e.g., monkeys).
The respiration of mammals is pulmonary, which means that their lungs are the central organ of their respiration system.
The organs that participate in their interaction, such as the brain and cerebellum, or the sensory organs, are highly developed, so mammals are able to perform more complex actions than the rest of animals.
The reproduction in this group of vertebrates is viviparous; that is, the young develop inside the mother’s body in an organ called the uterus before they are born. The fetus and the mother are connected by the placenta and the umbilical cord, and nutrients and oxygen pass through these. The only non-viviparous mammals are those that belong to the monotremes group which, like the platypus, are oviparous.
Human beings belong to the mammal vertebrate group.
Mammalian Groups
The various species are classified into different groups:
- Rodents: These are mammals whose incisors wear down and grow continually, such as mice, squirrels, and hamsters.
- Ungulates: Their toes have been modified, and they have hooves. They may have one or three toes on each hoof (e.g., horses, zebras, and rhinoceroses) or two or four (e.g., cows, deer, and hippopotamuses).
- Cetaceans: These are mammals that are adapted to aquatic life, so they have fins. They have a very thick layer of fat in order to maintain their body temperature. They are very intelligent animals. In this group, we can find whales, sperm whales, dolphins, and porpoises.
- Chiroptera: These are mammals that are able to fly because their upper limbs have developed as wings thanks to a membrane. They feed on fruit, insects, or the blood of other animals. Bats belong to this group.
- Insectivores: They feed particularly on insects. This group is made up of the most primitive mammals, which are sea urchins, shrews, and moles.
- Carnivores: These are mammals that feed on meat. They are fast, they have a highly developed sense of smell, and they are more intelligent than other groups. This group includes dogs, wolves, foxes, bears, otters, leopards, lions, cats, lynxes, and seals.
- Marsupials: These are very peculiar animals that are characterized by being born before they are fully developed. They complete the development process in a pouch on their mother’s abdomen. They are kangaroos and koalas.
- Monotremes: This is an oviparous group of mammals, which means that they lay eggs. In addition, their digestive and reproductive tracts end in a single opening called a cloaca. Platypuses and echidnas are monotremes.
- Primates: These mammals have a highly developed brain. This group includes monkeys, apes, and human beings.