The Cloud Effect
What purpose do you think is served by the behavior known as the "cloud effect"? That is, why do you think the butterflies suddenly fly into the air the moment the sun goes behind a cloud? What are they trying to do?
Here's
Dr. Calvert's explaination:
"These butterflies have no way of knowing how long the cloud will
last. If they were to remain in the exposed, basking positions without
the sun, they soon may cool down and not be able to fly. This would compel
them to stay there as long as the cloud persists--which could be overnight.
By flying, they keep their body temperatures high enough to maintain mobility--either
to return to basking position when the sun returns, or to go back under
the canopy if clouds remain.
"What they are undoubtedly perceiving is a sudden lack of solar radiation
and rapidly falling air temperatures. They are poikilotherms and cannot
raise and maintain elevated internal temperatures without incoming radiation.
They need high internal temperatures if they are to perform daily functions
such as flying, crawling, mating, etc. At many locations and times during
the winter season, nighttime radiational cooling takes surfaces exposed
to the night sky down below freezing. If trapped there, the butterflies
might freeze to death. Within their clusters, radiation and re-radiation
trapped within vegetative mass of the forest and understory foliage keeps
temperatures higher - as much as 5°C higher - than the exposed surfaces.
The butterflies are clearly safer within the forest. This is where they
try to go when radiation is blocked by a cloud for a long period."
