A change in their food, from trees to grass in the Great Plains.
Answer: Local chemical conditions
Explanation: A Precapillary Sphincter is a smooth muscle that is located in circles around the capillaries.
The precapillary sphincter helps to coordinate blood flow to the into the capillaries by causing the capillaries to decrease the flow of blood to tissues that are dormant or not active.
In some reactions, a single-reactant substrate is broken down into multiple products.