Counter Current Heat Exchanger Controls Abdominal Temperature
We have already discussed the heat regulation mechanism which shunts heat between the thorax and abdomen, but this mechanism has been adapted by the bumblebee to aid in reproductive development. The aorta of the bumblebee makes a large loop in the thorax of the bee, where most of the heat is produced in order to transfer as much heat as possible into the blood by a better surface area to volume ratio (Figure 1 below). The heated blood is then shunted into the abdomen; the countercurrent heat exchanger, however, is bypassed by means of a special alternation of pumping which prevents blood from going in both directions at once, thus avoiding any heat exchange (Heinrich 1976). The two vessels are visible in Figure 2 below; blood passes through the two vessels as shown but never at the same time. The heart and diaphragms beat in such as rhythm as to make certain that no counter current effect is felt (Heinrich 1976). In this manner, the heated blood transfers its energy to the abdomen, increased the abdominal temperature greatly.
Figure 1 Adapted from Heinrich 1976
Figure 2 Adapted from Heinrich 1976
Increased Abdominal Temperature Benefits Brood Rearing
This increased abdominal temperature benefits the queen
in several ways. First of all, it increases metabolism within the ovary,
which is located in the abdomen, causing it to develop much more quickly
than normal. The ovary, increased in size and production, is now in essence,
jump-started, and able to produce more offspring more quickly (Vogt 1994).
The heated abdomen also allows the queen to regulate the temperature of
her brood patch. By pressing the abdomen and its heated blood against
the patch, she can warm it very effectively, increasing the rate of development
for the offspring (Vogt 1998). It has also been found that a large supply
of honey and pollen for consumption great accelerates ovary development,
perhaps by allowing the queens more metabolic energy for heat production
(Vogt 1998). Additionally, larger queens also produce broods more quickly
than smaller ones perhaps because they are able to regulate their abdominal
temperatures at higher and more stable levels than smaller queens. These
adaptations go hand in hand with others displayed in arctic bumblebees,
allowing them to get a head start and function in a cold environment by
accelerating their usual patterns, another example of how endothermy allows
hymenopterans to expand to new niches and territories.