Think of it this way. In the Atacama they experience seasonal fluctuations
in temperature, water/humidity availability, day length, and food
availability. They use one, some or all of these to entrain their annual
cycles, to synchronize their lives with the rest of Mother Nature. Their
species evolved in this absolutely predictable waltz of variations. Each
individual tarantula has grown up in these conditions.
Then somebody snatches them out their lair and ships them to the other
side of the planet. Worse yet, we keep them in a house with
thermostatically controlled heat. There goes any temperature clues to let
them readjust to the new time table.
We get up and turn the lights on every morning at 6:30 or 7:00 AM and the
house is well lit until we turn the lights off at 10:30 or 11:00 PM. And
this never changes regardless of what season of the year it is. We've just
removed day length as a clue.
Worse yet, in nature they're preprogrammed to eat as much food as
available in preparation for the coming famine season. (There's *ALWAYS* a
coming famine season!) During the famine season they may go hungry for
several months before food becomes plentiful again, another seasonal clue.
In captivity we give them all the food they'll eat and, out of instinct,
they eat everything that we throw at them. We overfeed them thinking that
they're starved and they don't stop eating until they're obese. Even then
the food *STILL* keeps coming! There is no string of light meals followed
by a few months of fasting. This destroys any food availability clues
completely.
Lastly, in the Atacama, as dry as it is, there are dry seasons and damp
seasons. It may not rain often, but from time to time fog banks roll in
from the Pacific and generally moisten everything for a few hours to
several days. And, this tends to happen seasonally. In your home its
always bone dry, but you always keep a dish of water in the cage. Ooops!
There goes another clue.
The result is that this species more than almost any other gets really
confused about what season of the year it is. Because we've removed all
their clues they don't know when to start eating again once they get too
fat and stop. Neither do they know when it should be time to molt. They
may go 2 years or more without eating or molting, before they finally pick
up the few very subtle clues available to synchronize with the local
seasons.