Scrotum: tight or hanging low

The sack is a thermostat with two little muscles. Why it shrinks in the cold, drops in the heat and almost never hangs level

The scrotum is the least glamorous part of the package and the hardest working. It is a climate-controlled bag that keeps two very temperature-sensitive organs a few degrees cooler than the rest of you, and it changes shape all day long to do it. Tight and wrinkled one minute, loose and swinging the next: it is the same sack doing its job.

That is why "tight" and "hanging" are not really two types of men. They are two settings of the same thermostat, with some men set a little lower than others.

A thermostat in a bag

Sperm production works best a little below body temperature, which is why the testicles live outside the body in the first place. In a normally dressed man walking around, the skin of the scrotum sits at about 34 °C, and the testicles inside are only a fraction of a degree warmer. Your core runs at about 37 °C.

Small things move that number. Clothing alone adds about half a degree to a degree compared with being naked. Sitting with the thighs apart pushes the scrotum to about 35 °C, and sitting with the thighs pressed together takes it above 36 °C, close to the temperature inside the abdomen. Your mother's advice to sit up straight never covered this.

Two little muscles

The shape changes are done by two muscles. The first, the dartos, is a thin layer of smooth muscle inside the scrotal skin. When it contracts, the skin wrinkles, thickens and shrinks around the balls, cutting heat loss. When it relaxes, the skin smooths out and the bag lengthens.

The second, the cremaster, wraps around the cord each testicle hangs from and can pull the testicle up towards the body. It reacts to cold, to fear and to touch: stroke the inside of the upper thigh and the ball on that side rises. Doctors use this cremasteric reflex as a quick test of the nerves in that part of the spine.

Exercise plays tricks too. In one study, eleven long-distance runners ran naked on a treadmill for an hour. While the skin everywhere else cooled down at the start of the run, the scrotum warmed up, probably because both muscles pulled it up against the body. After the run the pattern reversed.

Low hangers

Some men simply hang lower than others, even on a cold day. Size of the balls, length of the cords, how much skin there is and how relaxed the muscles are all play a part. It is widely said that the scrotum drops with age, as the skin loses elasticity and the muscles lose tone. That makes sense, but nobody seems to have measured it.

The two sides are rarely level. In most men the right ball sits higher and the left hangs lower, so a hanging scrotum is usually lopsided. The Balls chapter tells the story of how Greek sculptors noticed this and then got the explanation wrong.

As for what gay men find attractive, there is no study at all. Some men like a tight high sack, others a heavy low one, and some admit they had never thought about it until they read this paragraph.

Grooming the sack

The scrotum is a popular target for the razor. In a national survey of US men, two thirds of those who groomed their pubic hair also did the scrotum. It is also the trickiest place to do it: thin, loose, wrinkled skin is exactly the kind a blade loves to nick. Stretching the skin flat and using clippers rather than a razor is the common advice from people who learned the hard way. The Pubic hair chapter has the full numbers.

Shaved or not, the skin of the scrotum has its own normal features: it is darker than the rest of the body, it carries a central seam called the raphe, and with age it may show small dark red dots called angiokeratomas, which are harmless and become more common after the third and fourth decades of life. More on spots and bumps in Skin and colour.

When to check

A scrotum that changes shape with the temperature is normal. One that changes shape for no reason is worth a look. See a doctor for a swelling that does not go down, a lump on a testicle, or a soft mass above it that feels like a bag of worms when you stand: that last one is usually a varicocele, a swelling of the veins, and nine in ten are on the left side. Sudden, severe pain in the scrotum is an emergency.