Hot bees in empty broodnest cells: heating from within
2003
SUMMARY Honeybee colonies maintain brood nest temperatures of 33–36°C. We
investigated brood nest thermoregulation at the level of individual worker
behaviour and the transfer of heat from workers to the brood. Worker bees
contribute to the regulation of brood nest temperature by producing heat while
sitting motionless on the caps of brood cells. We report here an additional,
newly observed heating strategy where heating bees enter empty cells between
sealed brood cells and remain there motionless for periods of up to 45
min. Individually marked worker bees on the surface of sealed brood cells
maintained thorax temperatures ( T th ) between
32.2±1.0°C and 38.1±2.5°C (mean ±
s.d.; N =20 bees) with alternating warming and cooling
periods. Most of the observed bees made one or several long-duration visits
(>2 min) to empty cells within the sealed brood area.
T th at the time bees entered a cell
[ T th(entry) ] was 34.1–42.5°C ( N =40). In
83% of these cell visits, T th(entry) was higher (up to
5.9°C; mean 2.5±1.5°C; N =33) than the mean
T th of the same bee. High values of
T th(entry) resulted from preceding heating activity on the
comb surface and from warm-ups just prior to cell visits during which
T th increased by up to +9.6°C. Bees inside empty cells had mean T th values of
32.7±0.1°C (resting bees) to 40.6±0.7°C (heat-producing
bees) during long-duration cell visits without performing any visible work.
Heating behaviour inside cells resembles heating behaviour on the brood cap
surface in that the bees appear to be inactive, but repeated warmings and
coolings occur and T th does not fall below the optimum
brood temperature. Bees staying still inside empty cells for several minutes have previously
been considered to be `resting bees9. We find, however, that the heating bees
can be distinguished from the resting bees not only by their higher body
temperatures but also by the continuous, rapid respiratory movements of their
abdomens. By contrast, abdominal pumping movements in resting bees are
discontinuous and interrupted by long pauses. Heat transfer to the brood from individual bees on the comb surface and
from bees inside empty cells was simulated under controlled conditions.
Heating on the comb surface causes a strong superficial warming of the brood
cap by up to 3°C within 30 min. Heat transfer is 1.9–2.6 times more
efficient when the thorax is in touch with the brood cap than when it is not.
Heating inside empty cells raises the brood temperature of adjacent cells by
up to 2.5°C within 30 min. Heat flow through the comb was detectable up to
three brood cells away from the heated thorax.
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