Yea, sorry for being so naive right now.

code/modules/mob/living/carbon/monkey/life.dm

@@ -30,10 +30,9 @@
 
 /mob/living/carbon/monkey/handle_mutations_and_radiation()
 	if(radiation)
-		if(radiation > RAD_MOB_MUTATE)
-			if(prob((radiation - RAD_MOB_MUTATE) / 25))
-				gorillize() 
-				return
+		if(radiation > RAD_MOB_MUTATE && prob((radiation - RAD_MOB_MUTATE) / 25))
+			gorillize() 
+			return
 		if(radiation > RAD_MOB_KNOCKDOWN && prob(RAD_MOB_KNOCKDOWN_PROB))
 			if(!IsKnockdown())
 				emote("collapse")