Beaks are increasingly recognised as important contributors to avian thermoregulation.
Several studies supporting Allen’s rule demonstrate how beak size is under strong selection
related to latitude and/or air temperature (Ta). Moreover, active regulation of heat transfer
from the beak has recently been demonstrated in a toucan (Ramphastos toco, Ramphastidae),
with the large beak acting as an important contributor to heat dissipation. We hypothesised
that hornbills (Bucerotidae) likewise use their large beaks for non-evaporative heat
dissipation, and used thermal imaging to quantify heat exchange over a range of air temperatures
in eighteen desert-living Southern Yellow-billed Hornbills (Tockus leucomelas). We
found that hornbills dissipate heat via the beak at air temperatures between 30.7°C and
41.4°C. The difference between beak surface and environmental temperatures abruptly
increased when air temperature was within ~10°C below body temperature, indicating
active regulation of heat loss. Maximum observed heat loss via the beak was 19.9% of total
non-evaporative heat loss across the body surface. Heat loss per unit surface area via the
beak more than doubled at Ta > 30.7°C compared to Ta < 30.7°C and at its peak dissipated
25.1Wm-2. Maximum heat flux rate across the beak of toucans under comparable convective
conditions was calculated to be as high as 61.4Wm-2. The threshold air temperature at
which toucans vasodilated their beak was lower than that of the hornbills, and thus had a
larger potential for heat loss at lower air temperatures. Respiratory cooling (panting) thresholds
were also lower in toucans compared to hornbills. Both beak vasodilation and panting threshold temperatures are potentially explained by differences in acclimation to environmental
conditions and in the efficiency of evaporative cooling under differing environmental
conditions.We speculate that non-evaporative heat dissipation may be a particularly important
mechanism for animals inhabiting humid regions, such as toucans, and less critical for
animals residing in more arid conditions, such as Southern Yellow-billed Hornbills. Alternatively,
differences in beak morphology and hardness enforced by different diets may affect
the capacity of birds to use the beak for non-evaporative heat loss.
S1 Data. Numerical data. Numerical data used in preparation of Figs 1 and 4; Tables 1, 2, 3
and 4; S1, S2 and S3 Figs.
S2 Fig. Heat loss (% of total) in Southern Yellow-billed Hornbills. Heat loss as a proportion
of total body heat loss (%) plotted against air temperature (Ta) of torso, gular skin, the beak as
a whole and lower mandible of the beak in Southern Yellow-billed Hornbills (Tockus leucomelas).
Data above the panting initiation temperature (Ta = 37.4 ± 2.1°C) has not been included
in this graph since evaporative heat loss has not been assessed and this makes total heat loss
after initiation of panting incomplete.
S3 Fig. Relative humidity (%) and water vapour pressure (kPa) in the temperature cabinet.
Relative humidity (%) and water vapour pressure (kPa) in the temperature cabinet in response
to air temperature (°C). Data are combined from all the individual experiments. Error bars represent
S1 Text. Additional methods for heat transfer calculation
S1 Video. Thermal imaging sequence of the Southern Yellow-billed Hornbill. Thermal
imaging sequence of the Southern Yellow-billed Hornbill (Tockus leucomelas) during the experiment.