Complex sentence: When rats are in high numbers the snakes grow very sleek and fat, however once the rats disappear the snakes must depend on less prevalent prey and/or draw upon their fat reserves until the rats return.
Split into simpler sentences: When rats are in high numbers, the snakes grow very sleek and fat. However, once the rats disappear, the snakes must depend on less prevalent prey and/or draw upon their fat reserves until the rats return.