Evolution
Within the genus Mustela, the least weasel is a relatively unspecialised form, as evidenced by its pedomorphic skull, which occurs even in large subspecies. Its direct ancestor was Mustela praenivalis, which lived in Europe during the Middle Pleistocene and Villafranchian. M. praenivalis itself was probably preceded by M. pliocaenica of the Pliocene. The modern species probably arose during the Late Pleistocene. The least weasel is the product of a process begun 5-7 million years ago, when northern forests were replaced by open grassland, thus prompting an explosive evolution of small, burrowing rodents. The weasel's ancestors were larger than the current form, and underwent a reduction in size to exploit the new food source. The least weasel thrived during the Ice Age, as its small size and long body allowed it to easily operate beneath snow, as well as hunt in burrows. It probably crossed to North America through the Bering land bridge 200,000 years ago.
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