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There are roughly 5,400 recognized mammal species in the world, comprising over 150 families and approximately 1,200 different genera. The largest known mammal is the Blue Whale which can reach a length of 33 metres, while the smallest mammal, the miniscule Bumblebee Bat, grows no larger than 30-40 millimetres.
Mammals belong to the class Mammalia and the name is derived from their distinctive feature: mammary glands. Other distinguishing features are hair, sweat glands, the configuration of the ear, and the presence of a neocortex region in the brain.
All mammals give birth to live young, except for five species of monotremes which are egg-laying. Two examples of well known monotremes species are the Platypus and the Echidnas.
The largest mammal group is the placentals, which all develop a placenta to feed their young during gestation.
Almost all species of mammals are endothermic, which means that they maintain a relatively stable body temperature (they are so called “warm-blooded” animals). Mammals are not the only endothermic animals in the world because birds are also endothermic, and we do actually know of one ectothermic mammal, the Naked mole rat.
Keeping the temperature up requires a lot of energy and mammals and birds must therefore eat much more than animals who can allow their body temperature to drop, such as lizards and snakes. The advantage of being endothermic is that you can live in much harsher climates; climates where so called “cold blooded” animals like large insects and snakes die each fall or are forced to go into a state of hibernation to survive the cold season.
The first mammals are believed to have been carnivores, but today mammals have adapted to all kinds of food niches. Many are herbivores that live on foods such as grass, leaves, fruits, seeds or nuts. Other mammals are still carnivorous, with prey size varying from tiny insects and plankton to large undulates devoured by big cats and big ocean dwelling species caught by even larger marine mammals like the Orca. There is also a large group of omnivore mammals who eat both prey and plants. Many omnivores are opportunistic and can make use of a long row of different food sources. Dogs have for instance adapted to eating not only meat but a lot of other food types found around human dwellings and brown bears will happily eat anything from roots, fruit and honey to ants, carcasses and garbage.
The digestive tract of carnivorous mammals is quite basic since it only has to digest proteins, fats and a few other things like minerals, while the digestive tract of omnivores and herbivores must deal with much tougher compounds like complex carbohydrates, including sturdy cellulose. To help breaking down complex carbohydrates and make it possible to extract nutrients from them, herbivores and omnivores have specialized bacteria living in their digestive tract.
Generally speaking, small mammals have a higher metabolic rate than large mammals since it is harder for small bodies to retain heat due to their proportionally larger surface area. Really small mammals, i.e. those weighing 500 grams or less, are usually not herbivore since they would have a hard time keep their body heat up while slowly digesting complex carbohydrates. Small mammals like this are therefore typically predators hunting for insects. There are naturally exceptions to this rule; many rats and mice are for instance omnivore despite weighing less than 500 grams. Larger mammals on the other hand have a proportionally smaller surface area and can therefore go with out food longer than their smaller counterparts. Some large mammals have opted for a slow digestive process and feed on plants, while others eat less frequently (slow collection process), i.e. carnivore mammals that kill or steal prey once in a while.