A lower limit on the size of mammals is set by their warm-bloodedness and the surface area to volume ratio, which increases with decreasing size. The rate at which body heat is lost depends on surface area, but the rate at which body heat is produced depends on volume. At some size, a species will lose body heat faster than it can produce it.
The smallest mammal that ever lived may have been Batodonoides vanhouteni, a shrew-like animal that lived about 53 million years ago.¹ Its weight is estimated at only 1.3 grams. Only a jaw has been found (in Wyoming); the weight of the animal was estimated from the size of its lower first molar.
The smallest known living mammal is the bumblebee bat, at 2.0 grams. It may be that higher temperatures in the Eocene permitted smaller mammals.
1. Jonathan I. Bloch, K.D. Rose, and P.D. Gingerich.
New species of batondonoides (lipotyphia, geolabididae) from the early Eocene of Wyoming: Smallest known mammal?
Journal of Mammology, volume 79, page 804 (1998).
Bloch is at the University of Michigan, Ann Arbor.
fotalia/ © Ericos
The largest mammal that has ever lived is the blue whale, Balaenoptera musculus, which reaches lengths of over 31 meters (100 feet) and weights of more than 200 tons. It is also probably the largest animal that has ever lived, even including the dinosaurs.
The largest living land mammal is the African elephant. An extinct mammal, the rhinoceros Indricotherium (also called Baluchitherium), which lived about 35 million years ago, was about twice as big as an elephant. It probably weighed about 11 tons and stood 5.5 meters (18 feet) high at the shoulder.
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