What is a mammal?

Mammals are a type of vertebrate creatures defined by the existence of mammary glands in females that produce milk for nourishing (nursing) their young, a neocortex (a portion of the brain), fur or hair, and three middle ear bones. These traits set them apart from reptiles (including birds), from whom they separated around 300 million years ago during the Carboniferous period. There are around 6,400 living species of mammals classified into 29 orders. The rodents, bats, and Eulipotyphla are the orders with the most species (hedgehogs, moles, shrews, and others). The Primates (humans, apes, monkeys, and others), Artiodactyla (cetaceans and even-toed ungulates), and Carnivora are the next three groups (cats, dogs, seals, and others).

Most mammals are intelligent, with some having huge brains, self-awareness, and the ability to utilize tools. Mammals may communicate and vocalize in a variety of ways, including ultrasonic generation, scent-marking, warning signals, singing, and echolocation. Mammals may form fission-fusion communities, harems, and hierarchies, yet they can also be solitary and territorial. The majority of animals are polygynous, while others are monogamous or polyandrous.

Mammals are also hunted and raced for sport, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation.