My husband and I just got a new Bullmastiff puppy last weekend, he's the fifth Bullmastiff we've owned, and we love the breed so much. One thing I've noticed is that all our Bullmastiffs, whenever they are very cozy and happy and content, quiver their lower jaws slightly, and at the same time the muscles on their forehead kind of contract and quiver. I love it when they do this. It means they are very happy, but I've never heard anyone else mention it and was just wondering what it is? Has anyone else noticed their Bullmastiffs doing this?