To find out how many sheep are ewes, we first need to determine how many of the 125 sheep are rams.
A fifth of the sheep are rams, so we calculate:
\[ \text{Number of rams} = \frac{1}{5} \times 125 = 25 \]
Now, we can find the number of ewes by subtracting the number of rams from the total number of sheep:
\[ \text{Number of ewes} = 125 - 25 = 100 \]
Therefore, the number of ewes is \( \boxed{100} \).