Dog's reacts to wolf howling sounds

Ever notice when police sirens pass by your house your dog might have the tendency to howl? I used to think that they howled because the sirens were too loud and were hurting their ears. Until recently when my friend go all her dogs together and showed me a howling ritual. I was told I was not allowed to laugh. It was really hard not to laugh but I made it through because I was amazed and amused that she was able to get all three of her dogs to howl together. How did she do it? She and her mom howled and after a few howls the dogs joined in - including the chihuahua. So it doesn't surprise me that this cute German Shepard howled in response to hearing the sound of other howls. It's funny and it's cute.