Insulin is a hormone specifically produced by animals, particularly mammals, to regulate blood sugar levels.

Here's why:

* Evolutionary Development: Insulin evolved as a mechanism to control blood sugar in animals, particularly as they developed complex systems for using glucose as energy.

* Different Energy Systems: Plants and other organisms use different mechanisms for energy regulation. They may use sugars like sucrose, but they don't rely on insulin to control their sugar levels.

* No Pancreas in Plants: Insulin is produced by the pancreas, an organ found in animals. Plants do not have a pancreas.

While insulin is essential for maintaining blood sugar balance in mammals, it's not a universal feature of all living things.