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Perspiring patterns

Men not only seem to sweat more than women but they seem to sweat in different places. Is the pattern of sweat glands different, and what distinguishes a sweaty person from a non-sweaty one?

• The average adult loses about 0.7 litres of water through sweating per day, but can lose as much as 2.5 litres per hour. Men sweat at twice the rate of women. The average person has 2.6 million sweat glands in their skin, distributed all over the body except for the lips, nipples and external genital organs.

There are two types of sweat gland. Eccrine glands are the most numerous and are found over the whole body, particularly on the palms of the hands, the soles of the feet and the forehead.

The second type are the apocrine sweat glands, mostly confined to the armpits and the anal-genital area. These typically end in hair follicles rather than pores. Apocrine secretions are those where the tip of the secreting cell is lost along with the secretion. This type of sweat contains protein and carbohydrates which, when acted on by bacteria on the skin, gives the sweat an odour. Sweating in humans (the smallest animals that sweat) is controlled through the sympathetic nervous system.

In a 1997 study on Korean adults published in Acta Anatomica (vol 158, p 112), Kun Hwang and Sang-Ho Baik discovered seven regions of the human body where the density of sweat glands is significantly different between men and women. Men have more sweat glands than women on the soles of their feet and in the umbilical region, while women have more sweat glands than men on the palms, the fingertips, the front of the thighs, the front of the legs and the back.

Androgens have been implicated in setting the capacity for perspiration, which may explain why males sweat at a much greater rate than females in similar situations.

This difference is minimal in children before puberty and becomes obvious during puberty. Receptors for androgen have been identified in both eccrine and apocrine sweat glands of humans.

However, the rate of sweat release is not directly influenced by androgens. J. Rees and S. Shuster concluded in a 1981 study in Clinical Science (vol 60, p 689) that the rate of sweating in men is set by androgen-induced events at puberty, and not by direct modulation by androgens in adult life.

Polly Miele

Washington DC

Old blue eyes

I recently took a photo of my husband Ally with my terrier Jock. The flash of the camera meant Ally had red eyes when the photo was printed, yet Jock’s eyes were blue. Why?

• Had there been a cat in the picture its eyes would have been yellow or green. Cats, dogs and other animals, but not humans, have a reflective layer at the back of their eyeball behind the rod or cone cells that detect light and send nerve impulses to the brain. The reflective layer enables the animal to see in very dim light because it sends photons that have missed the light-detecting cells back to the rods and cones a second time. Humans don’t have this adaptation for dark vision.

Flashing a bright light into the eyes of such a dark-adapted animal reflects light back through the pupil before the aperture has had a chance to contract. Shining a torch into a cat’s eyes in the dark has the same effect – you see the shiny reflective layer, which is a different colour depending on the animal.

Human eyes show red because the light from a camera flash reflects from the red blood vessels behind the rods and cones.

Hillary Shaw

University of Leeds

• In animals with good night vision this reflection comes in many different colours. These are due to the tapetum lucidum, a highly reflective pigmented layer that acts almost like a mirror at the back of the animal’s eye. The tapetum enhances low-light vision by reflecting light outwards, giving the visual pigments a second chance to absorb it.

In dogs, at least, the tapetum fluoresces, shifting the wavelengths of incoming light into a better alignment with the peak spectral sensitivities of the photoreceptors.

Johan Uys

Bellville, South Africa

This week’s questions

Large cone please…

Why do tornadoes have the shape of an inverted cone?

Brian LaBelle

Brea, California

Living fence

If I put a wooden fence post into soil it will eventually rot and collapse, sometimes within months. What prevents trees, which can live for hundreds of years, doing the same?

Richard Meldrum

Cardiff