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Comes in handy

Q: Why do we have fingerprints? What beneficial purpose could they have
evolved to serve?

* * *

A: Fingerprints help us in gripping and handling objects in a variety
of conditions. They work on the same principle as the tyres of a car. While
smooth surfaces are fine for gripping in a dry environment, they are useless
in a wet one. So we have evolved a system of troughs and ridges, to help
channel the water away from the fingertips, leaving a dry surface which
allows a better grip. The unique pattern is merely a useful phenomenon that
is used by the police to identify individuals.

James Curtis Bradford, West Yorkshire

* * *

A: Fingerprints are the visible parts of rete ridges, where the epidermis
of the skin dips down into the dermis, forming an interlocking structure
(similar to interlaced fingers). These protect against shearing (sideways)
stress, which would otherwise separate the two layers of skin and allow
fluid to accumulate in the space (a blister). They appear on skin surfaces
which are subject to constant shearing stress, such as fingers, palms, toes
and heels. The unique patterns are simply due to the semi-random way in
which the ridges and the structures in the dermis grow.

Keith Lawrence Staines, Middlesex

Thick-skinned

Q: I have heard that the entire skin of a human is completely replaced
over a period of between three and six months. If this is the case, why
is it that tattoos can last for so long without fading or vanishing completely?

* * *

A: It is true that the epidermis is replaced very frequently and marks
made on it would be lost swiftly. This is why tattoos are injected deeper,
into the dermis where the change is much more gradual.

Peter Brooks Bristol

* * *

A: Skin is translucent and tattooing works best in areas where the skin
is thin. Tattooing causes bleeding because the dyes are driven below the
outer skin. In this position the tattoo is stable. Only dye lying within
the skin will get carried away as the skin grows, leaving the fully injected
pigment behind. This results in slight fading of the tattoo when it is new.

Adrian James Bridge of Weir, Strathclyde

Fishing well

Q: A number of different species of fish are often found in wells in
Somalia. These wells are dug by hand, usually in limestone or gypsum rocks.
How did the fish get into these wells and how long have they been there?

* * *

A: I think that the fish are in the wells because they were put there.
My experience is from West Africa where, in villages with no piped water
supply, it was customary to store water in very large pots for use in the
long dry season. It was also customary to put a small fish in each pot,
presumably to deal with mosquito larvae and so on. I imagine this practice
is widespread throughout Africa.

G. K. Berrie Fareham, Hampshire

* * *

A: Unfortunately, I have never been to Somalia and so cannot comment
on the wells there. However, here in Britain I have been told not to be
surprised when I see fish in our new pond in the back garden and I understand
that birds are to blame. At the moment we have no fish but we already have
found an extraordinary variety of wild life turning up in the pond, the
latest being a 7 centimetre leech.

Maria Fremlin Colchester, Essex

* * *

A: Any object that comes into contact with the muddy shallows of a lake
or river has the potential to pick up fish eggs. For example, water birds
can transport eggs to virgin waters on their feet.

In the case of a well in Somalia, it is possible that a vessel used
to collect water from the shallows of a nearby river or pool before the
start of a dry season is then used to collect water from the well. Fish
eggs, probably adapted to being encased in mud until the next rainy period,
could easily end up in the well.

Patrick Fox Chipping Norton, Oxfordshire

Watery grave?

Q: I have recently encountered the practice of drinking only distilled
water for health reasons. Are there any long-term consequences of this practice,
especially in comparison to drinking mineral water or filtered water? Are
there minerals and trace elements to be found in non-distilled water that
are useful, or even necessary, for health?

* * *

A: Drinking only distilled water could not be justified on health grounds.
Trace levels of chemicals in water supplies can provide an important source
of some essential elements. For example, a daily intake of between 1 and
5 milligrams of copper is an essential component of the adult diet. This
may be derived from foods rich in copper such as liver, tea, chocolate and
coffee, or from water supplied via copper piping. There may be reasons for
avoiding some water supplies such as inadequate treatment or because of
known contaminants such as dissolved lead or pathogens. However, these conditions
are rare, and would not justify the long-term ingestion of distilled water
which, in any case, tastes awful.

David Kay Department of Environmental Science University of Leeds

* * *

A: This fad is based on the belief that there are harmful trace compounds
such as heavy metals to be found in mains water. The effect of drinking
only distilled water, as with all health issues, depends on your lifestyle.
If you have a balanced diet, then there is little likelihood of experiencing
any problems in drinking distilled or deionised water. Drinking water provides
only a small proportion of beneficial minerals necessary for good health.

However, distilled water can often be unpleasant to drink and it is
very flat, with little or no taste. An alternative to distilled water that
would still allow one to avoid drinking tap water would be to use a reverse
osmosis system. This removes 99 per cent of high valency ions and metals,
but produces what is known as a ‘light’ water, as the gases dissolved in
solution are not removed.

Michael Ledger Lightwater, Surrey

This week’s questions

Tea-souper: When I take a vacuum flask full of a watery beverage such
as tea or coffee on a picnic, it is still hot hours later. However, if I
take a more viscous liquid such as soup, it cools far more quickly. Why?

Fiona Holland Coventry

* * *

In the green: The benefits of camouflage would suggest that there should
be green mammals. Are there any – and if not, why not?

A. C. Henderson Braco, Tayside

* * *

Steamy movies: Why do American drains steam? This is not just a feature
of Hollywood movies because I have observed steam emerging from drains in
several American cities, but never in Europe. The temperatures involved
seem quite high as some drain covers are hot to the touch.

Tim Ashton Inchinnan, Renfrewshire