As of 2015, there are about 5,000 known kinds or species of mammals. Mammals as a group are divided into three main kinds: placental mammals, marsupials, and monotremes,Continue Reading
Species counts are always somewhat imprecise, as scientists often disagree on how to classify animals and not all animals species have been discovered. As of 2015, the most up-to-date and authoritative list of mammal species is found in the third edition of "Mammal Species of the World: A Taxonomic and Geographic Reference" by Don E. Wilson & DeeAnn M. Reeder. This book, which was published in 2005, counts 5,416 different species, but the actual current number is in flux due to new discoveries and species being reclassified.
There are three main major groups of mammals. Placental mammals are the largest group. They carry their young inside a uterus throughout their fetal development. Marsupials also carry their young in a uterus, but they are born at an early stage of development and are subsequently carried inside a special external pouch on the mother's body. Monotremes, the smallest group, lay eggs.Learn more about Zoology