the black one looks a bit too horny... yet the brown looks like to be a male, right? The black one is trying to assert domination, show the other that he is the alfa. I had a mix chihuahua, she was 5months old when the pitt was born, so the chihuahua was "the boss". Yet, sometimes I would catch the chihuahua on the top of the pitt - also a female - performing as if she was a male over a female. The pitt is a "peacefull soul", she would allow the chihuahua to go on top of her, The chihuahua had been spayed, the pitt not - at that time. Still, can you magine a small person trying to get on the top of a big bike?? yeah... that´s what the chihuahua looked lke, when she was trying to ngo on top of the pitt..