What is the world’s smallest mammal?

With mammals as large as the Blue whale (Balaenoptera musculus) and the African elephant (Loxodonta africana), it may be hard to imagine that the world’s smallest mammal is only about the size of your fingernail. This miniature mammal, Batodonoides vanhouteni, was an ancient ancestor of the modern day shrew and weighed about 1.3 grams— the equivalent mass of a single dollar bill! This species of tiny rodents is said to have inhabited the underbrush of prehistoric forests.

In 1998, paleontologist James Bloch discovered remnants of Batodonoides in ancient old limestone deposits in Wyoming, dating back to over 53 million years ago. From the size of the lower first molar, a typical mammalian indicator of body size, paleontologists were able to estimate the Batodonoides’ weight. Initially, experts were doubtful that mammals could exist at such a small size because this body structure would cause the loss of too much body heat. However, paleontologists such as Bloch rationalized that the weather was much warmer during the Eocene Epoch, which would have allowed such minuscule warm-blooded organisms to thrive. In addition, Batodonoides compensated for their larger heart size to body size ratio by rapid heart activity and frequent food intake. Since then, further evidence has supported the lightweight’s existence and Batodonoides has been featured in the American Museum of Natural History’s “Extreme Mammals” exhibits across the United States.