They are pink with wrinkled skin, and they have white fur. At fourteen days, the white hair begins to thicken, and size differentiation makes it easy to tell males from females. At 36 days the weasels are weaned and can eat food brought back to the nest by the mother. They learn how to kill prey from the mother, and by 56 days they are able to kill prey on their own. One male's home range may overlap a few female home ranges, but the home ranges of adults of the same sex never overlap.
Weasels exhibit very aggressive behavior to intruders of their home ranges.
Long-tailed weasels are very quick, agile, and alert animals. They are very good climbers and swimmers. Long-tailed weasels hunt their prey by picking up on a scent or sound. They then follow the animal and make a quick attack. They kill their prey by a quick bite at the base of the skull.