Animals of a particular type will not attack each other. Tamed boars do not attack wild boars, tamed wolves do not attack wild wolves and vice-versa. It sounds like your boars are dying while you're away. The most likely culprit is a skeleton or greydwarf. Make sure they are fully enclosed by a good fence. A fully-enclosed barn will work for Boars and Wolves. You need a really high roof to enclose Lox. A too-low roof will prevent animals from eating.