Biology Reference
In-Depth Information
60
50
e
r
40
30
20
10
10
20
30
40
50
60
70
Inter-flower time (s)
Fig. 3.3
(a) The relationship between load size (expressed as number of flowers visited)
carried home by worker bees and flight time between flowers in a patch. Each dot is
the mean of an individual bee and the two lines are predictions based on maximizing
efficiency (
e
) and maximizing rate (
r
). From Schmid-Hempel
et al
. (1985). (b) By placing
tiny weights on the bee's back while it is foraging Schmid-Hempel was able to study
the bee's rule of thumb for departure from a patch to go home to the hive with a load
of nectar. The weights, in the form of brass nuts, are placed on a fine rod that is
permanently glued to the bee's back. They can be added or removed to simulate
loading and unloading. From Schmid-Hempel (1986). With permission from Elsevier.
Paul Schmid-Hempel (Schmid-Hempel
et al
., 1985) tested whether these diminishing
net returns influence the bee's decision about when to go home and empty its crop. He
trained bees to fly from the hive to a cluster of artificial flowers, each containing 0.6 mg
of nectar. By varying the amount of flight the bee had to do between each flower in the
cluster, he could alter the total cost of carrying the crop load and, therefore, the extent
to which the bee experienced diminishing net returns. If, for example, the bee could
collect a load of 10 flower's-worth of nectar while flying for a total of five seconds, it
would experience little decrease in returns as it loaded up, while a bee collecting the
same load by flying for a total of 50 s would suffer sharply diminishing returns. As
predicted, the bees went home with smaller loads when they were forced to fly a greater
distance between flowers (Fig. 3.3a). Fig. 3.3a also shows two predicted lines based on
maximizing two different currencies. One is based on the currency used for the starlings,
net rate of energy delivery, while the other is based on a currency that did not work for
the starlings, energetic efficiency. In contrast to the starlings, the second currency but
not the first accounts for the bees' behaviour.
Why should there be this difference between bees and starlings? A simple example
shows why the 'starling currency' is normally a sensible one to consider. Compare a
starling that forages for one hour, spending 1 kJ and gaining 9 kJ with one that spends
10 kJ and gains 90 kJ. Both have the same efficiency (nine) but the former has 8 kJ to
spend on its chicks and the latter has 80. In other words net rate ( (gain - cost)/time)
In bees,
diminishing
returns arise
from the cost of
carrying nectar
How much nectar
should a bee carry
home?
Bees maximise
efficiency, not
rate of energy
gain
