"Not a single male mammal walks around waving its penis, apart from when a male feels a great desire to mate. In most mammals, the dartos muscle neatly tucks the flaccid penis inside the body. When Homo sapiens walked around naked, their penis was also hidden from view. **When you’re climbing over sharp rocks or running through thorny bushes, you want to keep your genitalia as close as possible to the body. Only with sexual arousal did the dartos muscle relax and the penis come out**. The muscle also runs as far as the skin of the scrotum, where it helps with the temperature regulation of the testicles. Each testicle is connected to a vas deferens which is also surrounded by a muscle. When the testicles get too warm, the vasa deferentia let the testicles hang down; if it suddenly gets cold, they tuck the testicles in. At the same time, the dartos muscle contracts the skin of the scrotum. That’s why your penis looks small if you swim in cold water." ([Location 376](https://readwise.io/to_kindle?action=open&asin=B089QJDCV4&location=376)) --- **Tags** -- [[quotes]], [[human-biology]], [[sexuality]], **Source** -- [[20250426112058 - B - Members Club]]