Mammals (Mammalia) are a diverse class of warm-blooded vertebrates characterized by the presence of mammary glands, which females use to nurse their young. They also typically have hair or fur, a neocortex (a region of the brain), and three middle ear bones. Mammals inhabit a vast range of environments, from the ocean depths to the highest mountains, adapting to diverse ecosystems across the globe. They evolved around 200 million years ago, with current species ranging from the tiny bumblebee bat to the colossal blue whale. Mammals play crucial roles in ecosystems, including as predators, herbivores, and pollinators, and many have complex social structures.
Mammals are a varied group sharing key features like warm-bloodedness, fur or hair, and mammary glands for nursing their young. They have a well-developed brain, facilitating complex behaviors and enhanced senses such as sight, hearing, and smell. Mammals are typically four-limbed, with movements adapted to their environment, from running and swimming to flying in the case of bats. Many can vocalize, aiding in communication and social interactions. Their skeleton provides a sturdy framework, with a spinal column divided into distinct regions for flexibility and support. Mammals breathe using lungs, and their circulatory system is highly efficient, featuring a four-chambered heart.
Humans' relationship with mammals is complex and multifaceted, spanning companionship, agriculture, scientific research, and conservation. Domesticated species like dogs and cats are beloved pets, featuring widely in pop culture, from Lassie to Garfield. Farm animals like cows and pigs are vital for food and materials. Wildlife fascinates and inspires, populating nature documentaries and films like "The Lion King."
Conservation efforts are critical, as many mammals face extinction due to habitat loss, climate change, and poaching. Organizations worldwide work to protect them, promoting biodiversity and ecological balance. Mammals are integral to human life, culturally, economically, and environmentally.
Mammals first appeared during the Triassic period which was about 252 million to 201 million years ago and were members of the reptilian order Therapsida. This order had a subclass, Synapsida which are sometimes referred to as mammal-like reptiles. They were part of the Carboniferous period and are considered one of the earliest reptilian groups.
The only 2 mammal species that lay eggs are the duck-billed platypus and the echidna, also known as the spiny anteater. The reasons these 2 mammals still lay eggs may be due to their distant ancestors as well as other primitive traits like their reptile-like shoulders.
The most common mammal species are humans as it is the most numerous species of mammal on Earth. As of 2022, the human population reached 7.9 billion. Human beings are the only species that are close to exceeding the number of members of the brown rat and the house mouse.
