What does the zombie monolith do on zombie farm?

There are several monoliths you can get on zombie farm,

Mutant Monolith - 8 brains - all mutilations successful

Zombie Monolith - 10 brains - NO MORE LIFELESS Zombies

Plowing Monolith - 10 brains - Plowing costs no gold

Speed Monolith - 10 brains - Farmers actions are super fast

NEW UPDATE - CLAY MONOLITH 12 BRAINS MAKES COMBINING ZOMBIES TWICE AS FAST!