"What is the life expectancy of a typical water bear?"
At first sight this seems to be a rather simple question. Nevertheless there
is no clear answer. In fact it turns out to be rather tricky because of the
water bear desiccation periods. Those dry periods might last up to at least 7 years
(each one!). It is quite obvious that in theory 10 of those long desiccation
periods would sum up to 70 years and we would arrive at the life expectancy
of man, elephant or even tortoise.
Water bears individuals have been kept alive in culture dishes for time periods
of up to one year, so it is quite clear that their minimum life expectancy
is one year at least.
But nobody knows whether those water bears kept in artifical environment
encountered some kind of natural death or possibly died too early because
of the restrictions within the culture environment.
It is impossible to study water bear individuals in their natural environment.
Terrestrial tardigrades can feed on the moss plants they live in. And, as a rule,
they will be able to fill up their stomach within five minutes or less.
They are completely surrounded by food in the moss cushion - land of milk
and honey. Furthermore the moss tends to protect them against sudden humidity
changes, it provides optimum oxygen supply and serves as a light barrier.
Possibly the water bears simply feel protected and at home there, too,
and therefore might live longer.
Because of all those complications we will not come to a conclusion here
as far as life expectancy is concerned. But it is a tempting task to study
water bear aging in general and to study the varying properties of the
ageing water bears in particular.
Human experience has taught us to interpret signs of age: age spots,
"misplaced" thick hairs emerging out of nose and ears, even forming
tiny eye-brow jungles. Furthermore old humans tend to move in a strictly
controlled and economic manner whereas youngster simply jump around. And,
last but not least, each year of human life has a tendency to deposit
additional fat on the bone structure.
And, with increasing age there is a continously increasing need for a nap
from time to time.
Funny enough, we do find many of those signs of age with water bears, too.
Old individuals appear to be more hairy, their body is more massive
and forceful. And the old ones tend to have pigment spots.
In contrary the young ones have a slim body and a broad head and
some of them move like elegant dancers.
Let's look a the massive backside of a senior water bear: