I have an 18 month old black lab and she is very aggressive around her food and bedding. She also tenses considerably when approached and petted on the head. She has never bitten me, but she will become very tense, the fur on her hind end will go up, and she will bare her teeth and growl if I try to pet her when she is like this. Typically, it only happens around food or her bed, but she will also be like this at other times when she is approached. I would love to keep her, but can't in good conscience have a pet that I need to worry about with people around. Other than that, she is a very smart, and athletic dog.