It shall not be necessary for the appellees to file an answer to the petition unless some affirmative defense is made thereto, but they may do so. [S13, §1989-a14; C24, 27, 31, 35, 39, § 7518; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, 79, 81, §455.97 ] 89 Acts, ch 126, §2 CS89, §468.87 Referred to in §357.33 , 468.126 , 468.135 , 602.8102(65)