Bob writes:

> To a lesser degree can be explained by sick bees flying out to die (a hypothesis I do not buy into personally)

Actually, this idea has been gaining traction over the past couple of years. Recent studies have not only demonstrated how it works but the reasons why a colony would be likely to evolve such a behavior. Of course, we have no way of knowing why bees do what they do, but "Some circumstantial evidence is very strong, as when you find a trout in the milk." (see note at the end)

Social insect colonies represent distinct units of selection. Most individuals
evolve by kin selection and forgo individual reproduction. Instead, they
display altruistic food sharing, nest maintenance and self-sacrificial colony
defence. Recently, altruistic self-removal of diseased worker ants from their
colony was described as another important kin-selected behaviour. Here, we
report corroborating experimental evidence from honey bee foragers and
theoretical analyses. We challenged honey bee foragers with prolonged CO2
narcosis or by feeding with the cytostatic drug hydroxyurea. Both treatments
resulted in increased mortality but also caused the surviving foragers to
abandon their social function and remove themselves from their colony,
resulting in altruistic suicide. 

Our experimental studies revealed a suite of behavioural
changes in honey bee foragers that are consistent
with the hypothesis that workers that feel ill or otherwise
compromised actively abandon their social role as foragers
and remove themselves from their colony.

Preventing the spread of diseases that are transmitted
by direct contact among nestmates,
the benefits of altruistic self-removal outweigh
the potential cost of its erroneous execution in
individual cases. However, error accumulation at the
colony level may result in a large portion of adult
workers leaving their colony, resembling the phenomenology
of the recently reported colony collapse disorder
in honey bees

Altruistic self-removal of health-compromised honey bee workers from their hive
O. RUEPPELL, et al. 2010. Journal of Evolutionary Biology


Note: "Some circumstantial evidence is very strong, as when you find a trout in the milk." Henry David Thoreau. The mere finding of a fish in a tank of milk is not proof that the milk is watered down, but it would be a real good bet that it is. Sort of like seeing a tanker truck filled with corn syrup pull up at the loading dock of a honey packer...


Peter Loring Borst
Ithaca NY USA
peterloringborst.com

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