There are more than 6500 species of mammals on Earth, subdivided into 1200 genera, 155 families and 30 orders.
But who are they?
Mammals are a group of vertebrate amniote animals which are characterized by the presence of mammary glands, which in females produces milk for nursing their young, a neocortex area in the brain, fur or hair and three middle ear bones.
The appearance and expansion of mammals took place at the end of the Triassic. Their essence lies in their diversity of form and function and, above all, their individual flexibility of behavior. Their variety in shapes and sizes is extraordinary: ranging from 30 millimeters to 30 meters. Some have relatively short lives (2 years for the shrew) and others have very long ones (211 years for some whales).
Mammals are considered intelligent animals. They have emotions, some of them possess large brains, self-awareness and can even use tools.
Most mammals are quadruped and use their four extremities for terrestrial locomotion, but some species have special adaptations in their extremities, allowing them to fly, to live underground, to walk in a biped position or to swim and live under water.
Mammals can communicate and vocalize in so many ways: whales and dolphins for example communicate via echolocation, bats use a variety of ultrasonic ranging techniques to detect their prey, many species use scent-marking to mark their territories, meerkats for example use alarm signals to let the group know when there is a predator around, lions roar to mark their territories, and gibbons sing to attract possible mates.
Most mammals live in groups, that can be organized into fission-fusion societies (where animals are part of a same group but don't stay together all the time), harems (usually with one or two males, several females and their offspring) and hierarchies (linear social structure where there is a dominance), but some animals are solitary and territorial (most of the felid family). Most mammals are polygamous, but some can be monogamous. Although most are polygynous (where one male has an exclusive relationship with two or more females), there are some animals that are polyandrous (where one female has an exclusive relationship with two or more males) like house mice or marmosets. Promiscuity (where a member of one sex within the social group mates with any member of the opposite sex) is also common in many animal species that live in big mixed groups, specially in primates.
Each individual mammal maximizes its "fitness", its ability, relative to others of its kind, to pass on genes, usually by leaving viable offsprings.
Modern mammals are subdivided in 3:
SUBCLASS THERIA: the placentals are able to feed the fetus during gestation through the placenta, allowing it to be carried in the uterus of its mother to a relatively late stage of development.
INFRACLASS METATHERIA: the marsupials give birth to relatively undeveloped young that often reside in a pouch located on their mothers' abdomen for a certain amount of time.
SUBCLASS PROTOTHERIA: the monotremes lay eggs rather than bearing live young, but like all mammals, the female nurse their young with milk.
How did we ended up with cats and dogs as pets?
Well, domestication of many types of mammals played a major role on what we call the Neolithic revolution: one of the most important transitions in human history.
The domestication of animals and plants was triggered by several climatic and environmental changes that occurred after the Last Glacial Age, around 21000 years ago. These changes had a huge impact on animals, making very hard to obtain food!!
The long-term association between wolves and hunter-gatherers, more than 15000 years ago, was the beginning of agricultural societies. Thanks to that, farming replaced the old hunting and gathering system as the primary source of food for humans.
Dog evolution is closely linked with human evolution. Dogs suffer from the same common diseases (such as cancer, diabetes, heart conditions or neurological disorders) as do humans. Being the first domesticated species has created a very strong bond between dogs and humans and linked their histories.
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