Answer this interesting question!! ;)?

Ello I'm a local Brit from London I just recently moved to new York city. This may seem like a rather odd question of sorts, and all you Americans probly already know the answer to this question so I feel like a large idiot. But anyways I am going to stop this brambling and ask the question! Why do we have those liltle dingoes under our arms, I mean the lines. You know just bellow the inner elbow. So please answer this because I am rather curious!