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Fur versus feathers: The different roles of red kangaroo fur and emu feathers in thermoregulation in the Australian arid zone

Authors:

Abstract

Fur or feathers provide protection against heat loads from solar radiation for birds and mammals. The red kangaroo (Macropus rufus) and the emu (Dromaius novaehollandiae) are conspicuous in arid environments of Australia where there is potential for high solar loads. The diurnal D. novaehollandiae feeds in the open yet it has a dark coat with a high absorptivity (83%), which contrasts with that of M. rufus (61%), but M. rufus generally shelters in shade during the day. We examined the effect of coat characteristics on the heat load from solar radiation at skin level. Coat depth and density (thermal conductance or insulation) and the level of penetration of solar radiation into the coat were important determinants of solar heat load. For M. rufus less than 25% of incident radiation reached the body at low wind speeds and this diminished to below 15% at moderate wind speeds. In the modest shade M. rufus seeks on summer days, their heat load from solar radiation appears minimal. Colour differences among M. rufus did not affect thermal load. D. novaehollandiae on the other hand is exposed to the full incident solar load in the open but its plumage provides almost complete protection from solar radiation. Solar radiation is absorbed at the feather surface and the insulation provided by the deep coat prevents heat transmission to the skin.
FUR VERSUS FEATHERS: THE DIFFERENT ROLES OF RED
KANGAROO FUR AND EMU FEATHERS IN THERMOREGULATION IN
THE AUSTRALIAN ARID ZONE
T
ERENCE
J. D
AWSON
AND
S
HANE
K. M
ALONEY
Dawson TJ and Maloney SK, 2004. Fur versus feathers: the different roles of red kangaroo fur
and emu feathers in thermoregulation in the Australian arid zone. Australian Mammalogy 26:
145-151.
Fur or feathers provide protection against heat loads from solar radiation for birds and
mammals. The red kangaroo (Macropus rufus) and the emu (Dromaius novaehollandiae) are
conspicuous in arid environments of Australia where there is potential for high solar loads.
The diurnal D. novaehollandiae feeds in the open yet it has a dark coat with a high absorptivity
(83%), which contrasts with that of M. rufus (61%), but M. rufus generally shelters in shade
during the day. We examined the effect of coat characteristics on the heat load from solar
radiation at skin level. Coat depth and density (thermal conductance or insulation) and the
level of penetration of solar radiation into the coat were important determinants of solar heat
load. For M. rufus less than 25% of incident radiation reached the body at low wind speeds and
this diminished to below 15% at moderate wind speeds. In the modest shade M. rufus seeks on
summer days, their heat load from solar radiation appears minimal. Colour differences among
M. rufus did not affect thermal load. D. novaehollandiae on the other hand is exposed to the
full incident solar load in the open but its plumage provides almost complete protection from
solar radiation. Solar radiation is absorbed at the feather surface and the insulation provided by
the deep coat prevents heat transmission to the skin.
Key words: Macropus rufus, Dromaius novaehollandiae, fur, plumage, insulation, thermal
conductance, reflectance, penetrance, solar radiation, fur colour, thermoregulation.
TJ Dawson, School of Biological, Earth and Environmental Sciences. University of New South
Wales, NSW 2052, Australia. Email: t.dawson@unsw.edu.au. SK Maloney, Physiology: School
of Biomedical and Chemical Science, University of Western Australia, 35 Stirling Highway,
Crawley, WA 6009, Australia. Email: shanem@cyllene.uwa.edu.au.
TWO very different large native animals occur in the
arid rangelands of Australia; kangaroos, notably the
red kangaroo (Macropus rufus) and the emu
(Dromaius novaehollandiae), a giant bird. Their
coats are obviously different but how this impacts on
their adaptation to these thermally harsh conditions is
unknown. The functions served by the fur and
feathers of homeotherms are varied and complex.
Other than insulation against heat loss in the cold, the
pelts are involved in physical protection, sensory
inputs, water proofing, and of course, cryptic
colouration, display or camouflage. A role less often
considered is the modulation of radiation interception
and heat flow at the surface, particularly solar
radiation. In desert regions solar radiation levels can
exceed 1000 W
m
-2
for many hours of the day. If
fully absorbed such radiation would impose heat
loads that are many times the resting metabolism.
Our interest in this aspect of thermal biology
originated in studies on the micro-meteorology of
habitat selection by arid zone kangaroos (Dawson
and Denny 1969). Species of kangaroo are largely
nocturnal and in summer seek to avoid high solar
heat loads while resting during the day. Of all
kangaroo species M. rufus inhabits regions with the
highest incident solar radiation intensity. It also exists
as two colour morphs, ‘red’ (a rusty brown) and
‘blue’ (a smoky grey) with intermediate forms also
occurring. The blue form (mostly seen in females) is
more common in the southern parts of the species’
range and rare in northern Australia, particularly in
the more tropical regions (Dawson 1995). Red fur is
more reflective to solar radiation (Dawson and
Brown 1970) and was presumed to result in lower
solar heat loads to the animal, raising questions about
the incidence of the blue morph. In the south of its
range M. rufus lives in saltbush / bluebush shrublands
and the blue colouration provides good cryptic
colouration. Also, the blue form is common to
females and may have a role in sexual dimorphism.
Consequently, it was thought that trade-offs between
AUSTRALIAN MAMMALOGY
146
competing adaptive forces in coat selection may have
out weighed the disadvantages associated with a
darker coat in the less extreme areas of their range.
We accepted these possibilities until we became
interested in D. novaehollandiae. While the M. rufus
shelter in sparse shade on summer days (Dawson and
Denny 1969; Watson and Dawson 1993), D.
novaehollandiae are diurnal and spend the daylight
hours feeding in the open (Dawson et al. 1984). Yet,
the surface layer of the feathers is dark grey to black
and apparently not very reflective. Initially, it was
considered that dark coloured mammals and birds
absorbed more radiation and experienced higher heat
loads (see references in Maloney and Dawson 1995).
If so, dark coloured desert inhabitants would be
thermally maladapted. Several authors dismissed the
significance of thermal selection (e.g., Cloudsley-
Thompson 1979) while others urged that thermal
influences not be considered in isolation from other
selective pressures (Wunder 1979).
We found from D. novaehollandiae that the
situation was complex (Maloney and Dawson 1995).
That colour and absorptivity were not the whole story
had been suggested by earlier work on solar loads in
cattle. Hutchinson and Brown (1969) had examined
the implications of forward reflection, sometimes
called penetration, of solar radiation into the fur of
white cattle with sparse hairs as compared with what
happens with black and brown furs. The heat load at
the skin depended not only on coat reflectivity, but
also on fur thickness and density and additionally,
wind speed. Subsequent work by Cena and Monteith
(1975) and Walsberg and coworkers (Walsberg
1988a,b; Walsberg and Wolf 1995; Wolf and
Walsberg 2000) has supported these ideas.
Penetrance of solar radiation deeper into the fur or
feather coat, partially by re-reflection, is important
and markedly influences the solar heat load. Light
coloured coats reflect more visible radiation, but not
all of it back to the environment, some of it is
reflected further into the coat. The penetration of
solar radiation deeper into light coloured coats means
that the resultant absorbed heat is somewhat
protected from loss to the environment by insulation
of the coat, while radiation absorbed close to the
surface in dark coats is predominantly transferred to
the environment. The latter is what we found for D.
novaehollandiae (Maloney and Dawson 1995).
In the light of these data from D.
novaehollandiae we became interested in the relative
roles of the differing body coverings of M. rufus on
the thermoregulatory balance, particularly in the sun
in summer. We re-examined the thermal and optical
properties of the fur of M. rufus and compared their
characteristics with those of D. novaehollandiae
plumage. We also re-examined the thermoregulatory
impact of coat colour of the different morphs within
M. rufus.
MATERIALS AND METHODS
Pelts from 10 M. rufus (5 of each of the two colour
morphs) were collected in north-western New South
Wales (NSW) during summer. They were processed
and prepared according to the procedures used for D.
novaehollandiae pelts by Maloney and Dawson
(1995). Measurements were made on central back fur
and largely followed the techniques of Maloney and
Dawson (1995).
Spectral reflectances of the pelt samples were
measured using an Optronics model 746-IRD
spectroradiometer (Optronics Inc, Orlando, FL, USA)
with an Optronics Model 740-20 lamp housing
attached to the monochromator entrance port and an
Optronics Model 740-70 reflectance attachment at
the exit port. This reflectance attachment included an
integrating sphere coated with barium sulphate paint.
Measurements were made with reference to a
compressed barium sulphate disc using standard
published values for its reflectance. Measurements
were made with a 10 nm half band width between
350 and 750 nm, and with a 50 nm half band width
between 800 and 2100 nm.
Solar reflectances of the coats were calculated
using the relative spectral power of sunlight given by
the American Society for Testing & Materials ASTM
E424-71. The relative spectral distribution of the
ARRI daylight lamp used in the wind tunnel
experiments was measured by the spectrophotometric
facilities of the Australian Broadcasting Corporation
at Gore Hill, NSW. The spectral transmission of the
glass used to make the wind tunnel was measured by
the spectrophotometric facilities at the School of
Optometry, University of NSW. These spectra were
used to calculate the relative spectral distribution of
light impinging on the plumage sample in the wind
tunnel and thus the total reflectivity of the samples in
the wind tunnel.
Measurements of thermal conductance (and
therefore insulation) of pelts were made by mounting
samples on the upper surface of a heat flux
transducer / temperature controlled plate apparatus
(Fig. 1). The water filled plate was maintained at
38°C. Voltage output from three 2 x 3 cm heat flux
transducers (Thermonetics Corporation, USA),
embedded in the plate’s upper surface, were logged
on a personal computer via a Datataker analog/digital
converter (Data Electronics Australia P/L, model
100F). Skin surface temperature (T
s
) was measured
with two thermocouples fed through oblique holes
from beneath the skin. These and similar
thermocouples measuring plate, pelt surface and air
temperatures (T
p
, T
e
and T
a
) were referenced to a
DAWSON & MALONEY: FUR VS FEATHERS IN THERMOREGULATION
147
Fig. 1. Experimental set-up for measuring the effects of
wind speed on the thermal conductance and solar heat load
of animal pelts. Site of anemometer, a. Arrows indicate the
direction of wind. The sheet of painted glass has a hole
above the coat. Placement of thermocouples is given in the
text.
Datataker Isothermal Block and logged on a personal
computer via the A/D converter.
Conductance of furs was measured as a function
of wind speed inside a glass wind tunnel (Fig. 1).
Wind speed was adjusted via a fan controlling air
flow through the tunnel and was measured 2 cm
above the sample with a Datametrics 810L
thermoanemometer. Each sample was measured at
six wind speeds (1, 2, 4, 6, 8, and 10 m
s
-1
). T
a
was
controlled at 20 ± 1°C by placing the wind tunnel
inside a temperature controlled room. Thermal
conductance (C) was calculated as: C (W
m
-2
°C)
= Q/(T
s
- T
a
), where Q = heat flow through the
sample. The contribution of the air boundary layer to
total insulation was obtained from calculations of fur
conductance, C
p
= Q/(T
s
- T
e
), and air boundary layer
conductance, C
a
= Q/(T
e
– T
a
).
To examine the impact of solar radiation, the
experiments were then repeated with 590 W
m
-2
of
short wave radiation incident on the sample.
Radiation was supplied by an ARRI spotlight (ARRI
Daylight 575W). The relative spectral distribution of
radiation from this lamp is similar to the solar
spectrum (Maloney and Dawson 1995). To minimize
heating of the wind tunnel surrounding the sample
and transmission of infra-red radiation to the sample
from the hot lamp body, a piece of glass, painted
black except for a hole allowing light to penetrate in
the area of the sample, was placed between the lamp
and the wind tunnel. The lamp and second sheet of
glass were cooled by forced convection. Radiation at
the level of the top of the coat was measured with a
CSIRO SRI4 radiometer and maintained at
590 W
m
-2
. The proportional heat load from radiation
(PHLR, expressed as a percentage of radiation
incident on the coat surface) was calculated as:
PHLR = ((HF
without radiation
) - (HF
with radiation
))/Incident
Radiation, where HF was heat flow through the
sample.
Results were usually analysed using a Two-Way
Repeated Measures ANOVA for species or colour
morph and wind speed. A Student-Newman-Keuls
(SNK) multiple range test was applied when
significant differences were indicated by the
ANOVA (using Statistica/Mac software). Values are
given as mean ± SE. Means considered significantly
different have a p < 0.05.
RESULTS AND DISCUSSION
To a mammal in open desert the important thermal
aspects of fur are its thermal conductance (insulation
is the reciprocal of conductance) and its influence on
the percentage of heat from solar radiation (%HLR)
that reaches the skin, and thereby impacts on the
body. For the summer furs of the two colour morphs
of M. rufus these characteristics are shown in Fig. 2.
There was no significant difference between the
colour morphs in the conductance of heat through
their furs. Conductance generally increased
significantly with each increase in wind speed. The
%HLR at skin level also was not significantly
different between the colour morphs. The %HLR
decreased significantly with each increase wind
speed.
The result for conductance was not unexpected
because conductance is largely dependent on fur
depth and fur density. For the red and blue colour
morphs of M. rufus depths were not different; the pelt
depths of red and blue furs were 8.8 ± 1.2 mm and
7.9 ± 1.2 mm, respectively (p = 0.7; t-test). Densities
were also similar (Dawson and Brown 1970). The
two fur colours have significantly different
reflectance across the solar spectrum, of which about
half is outside the visible spectrum (Fig. 3). This
results in red furs having a higher total solar
reflectance (Dawson and Brown 1970). We measured
the reflectance of solar radiation of the red fur as
39.2 ± 0.9% and that of the blue fur as 35.1 ± 1.9%
(p < 0.05) (Fig. 3). However, the %HLR at skin level
was the same (p = 0.94). The level of penetrance into
the different furs largely contributes to this result as
outlined below.
Radiation falling on the coat surface can be
reflected or absorbed and converted to heat (Fig. 4).
The fate of this absorbed heat depends on the relative
insulation of the coat from the point of absorption to
the environment or to the skin, and thus depends on
the wind speed. Forced convection will reduce the
insulation provided by the air boundary layer and
may also reduce insulation of the coat. The other
component, the reflected radiation can be reflected
Insulating foam
ARRI
Daylight
a
painted glass sheet
water-filled hot plate
heat flow transducer
pelt
artificial "sun"
AUSTRALIAN MAMMALOGY
148
Fig. 2. The influence of wind speed on the thermal conductance of the furs of the ‘red’ and ‘blue’ colour morphs of M. rufus.
Also shown for the two fur types is the effect of wind speed on the amount of solar radiation that reaches the skin as a heat
load.
Fig. 3. The variation in the incident solar radiation at different wavelengths together with the reflectance characteristics of
‘red’ and ‘blue’ M. rufus fur and D. novaehollandiae plumage across the solar spectrum. Values given for the coat types are
the % of total solar radiation reflected.
either back to the environment (net reflection) or
deeper into the coat where it is absorbed and
converted into heat. This forward reflection manifests
as penetrance of radiation. The magnitude of
penetrance influences where the radiation is
ultimately absorbed and depends on hair absorptivity
and coat density. The overall situation of the heat
gain from radiation can be dealt with in a simple
model (Fig. 4). In this model it is assumed that there
is an average depth to which solar radiation
penetrates before being absorbed, indicated as a
single layer, z, such that the heat load resulting from
this average penetration equals the heat load
experienced with non-localised absorption. The heat
resulting from radiation absorption at z flows either
to the environment or to the skin, in inverse
proportion to the insulation in each direction. The
heat load at skin level which impacts on the body is
then given by
HLR = RA(I
z
+ I
e
)/(I
c
+ I
e
)
Where R = the intensity of radiation incident on the
coat, A = the coat’s absorptivity, I
c
= the insulation of
0
2
4
6
8
10
0
5
10
15
20
25
024681012
Cond red
Cond blue
HLR red
HLR Blue
Conductance (W. m
-2
. °C)
Heat Load From Solar Radiation
(% Incident)
Wind Speed (m. s
-1
)
-2
0
2
4
6
8
10
-20
0
20
40
60
80
100
0 500 1000 1500 2000 2500
Solar spectrum
Solar reflectance (%)
Wavelength (nm)
Incident solar
emu 17%
'blue' 35%
'red' 39%
DAWSON & MALONEY: FUR VS FEATHERS IN THERMOREGULATION
149
the coat, I
z
= the insulation of the coat between the
point of absorption and the coat’s surface, and I
e
=
the insulation of the air boundary layer.
If HLR is expressed as a % of incident radiation
then:
%HLR = (HLR. 100/R) = A(I
z
+ I
e
)/(I
c
+ I
e
).
We measured all of these components except I
z
, and
so we can calculate I
z
and estimate the depth of layer
z, the average depth of penetration of incident solar
radiation. In kangaroos there was no difference
between the colour morphs in the penetrance into the
furs. It was approximately 1.4 mm or 16 – 18% of fur
depth at the lowest wind speed and decreased slightly
(though significantly) to 0.8 1.0 mm or 9 13% of
depth at the highest wind speed.
Fig. 4. Simple model of fate of solar radiation incident on
fur or plumage.
The reason for this low penetration appears
related to the relatively high absorptivity of the coats,
more than 60% and, importantly, the fur density. M.
rufus have a very high fur density. Dawson and
Brown (1970) report it to be 62 fibres
mm
-2
as
compared with 20 fibres
mm
-2
for M. r. erubescens.
In a larger study of the thermal characteristics of the
fur of most species of kangaroo the penetrance of
solar radiation into M. rufus fur was significantly
lower that other species (unpubl. data). The slightly
higher absorptivity (lower reflectance) of the blue fur
is probably balanced by a slightly lower penetrance
relative to the red furs, but the data are inconclusive.
Since coat colour has little influence on the
thermal characteristics of M. rufus fur, is it
significant in the case of D. novaehollandiae
plumage? We have compared data from the ‘red’ M.
rufus with that for D. novaehollandiae from Maloney
and Dawson (1995). The reflectances of total solar
radiation of their back coats were very different (Fig.
3). M. rufus had more than double the reflectance,
39.2 ± 0.9%, of the D. novaehollandiae coat,
17.0 ± 2.0%. In other words D. novaehollandiae
plumage, which appears dark grey absorbs 83% of
solar radiation as compared with 61% for M. rufus.
The resultant heat load at the skin of the two species
was statistically different at all wind speeds but the
pattern was different from what might be anticipated
(Fig. 5). For D. novaehollandiae at low wind speed
(1 m
s
-1
) only 9% of incident radiation reached the
skin as a thermal load, as compared with 23% for M.
rufus. How is this explained given that the
reflectance of the M. rufus coat is much greater,
meaning that the M. rufus coat absorbed much less
heat in total? Penetrance is important but is not the
only significant characteristic. The average
penetrance of solar radiation for D. novaehollandiae
back plumage was 5 mm, but that is only 10% of the
coat depth of 45 ± 3.7 mm. The dark layer at the
surface of D. novaehollandiae plus other plumage
characteristics limits penetrance and results in the
solar radiation that is absorbed being converted to
heat near the coat surface. Penetration into the more
reflective, though dense, M. rufus fur is only
1.4 ± 0.33 mm but that is 16 ± 2.5% of the much
thinner (9 mm) coat. The proportion of heat from
absorbed sunlight that reaches the skin as compared
with that which flows back to the surface and is then
lost by radiation and convection depends largely on
the ratio (I
z
+ I
e
)/(I
c
+ I
e
), that is, the ratio of
penetrance depth (including air insulation) to total
coat depth (including air insulation). The effect of
this can be seen in Fig. 5, which shows that %HLR is
much lower in D. novaehollandiae than M. rufus. D.
novaehollandiae plumage offers more effective
insulation against the absorbed solar radiation.
At higher wind speeds the %HLR for D.
novaehollandiae became negligible while for M.
rufus it dropped from 23% at 1 m
s
-1
to 11% at
10 m s
-1
(Fig. 5). The reason for this influence of
wind on %HLR is that the insulation of the air
boundary is much reduced and heat absorbed near the
coat surface is rapidly lost to the environment.
Mathematically this reduction in I
e
brings the
proportion of heat flowing to the skin closer to I
z
/
I
c
Z, Average penetrance
Incident radiation
Reflected
Skin surface
Fur surface
Absorbed
Air boundary
HLR = Heat load from radiation at skin surface
R
= Intensity of incident radiation
A
= Absorptivity of coat
I = Insulation of coat
I = Insulation of air boundary layer
I = Insulation of coat between Z and coat surface
%HLR=HLR. 100/
R
=
A
(I + I )/(I + I )
ze ce
c
e
z
AUSTRALIAN MAMMALOGY
150
Fig. 5. The influence of wind speed on the amount of solar radiation that reaches the skin as a heat load for the fur of M.
rufus, and the plumage of D. novaehollandiae. Also shown for the two coat types is the effect of wind speed on thermal
conductance.
The boundary layer effect, however, is not the
whole story because there was a different response to
wind speed between the species (Fig. 5). At lower
wind speeds the conductance of the D.
novaehollandiae coat was significantly lower than
that of M. rufus, that is, it offered greater insulation.
This difference diminished at the two higher wind
speeds and the difference between the species
became not statistically different. At high wind
speeds the D. novaehollandiae feathers appeared to
become disturbed. We have not estimated the density
of D. novaehollandiae feathers relative to that of
highly dense M. rufus hairs, but this effect likely
stems from the lower element density of emu feathers
compared to kangaroo hairs. The result of feather
disturbance means that not only was I
e
reduced as
wind speed increased, but wind breaking into the
actual feather layer also reduced I
c
. The combined
effects reduce the insulation from layer z to the
environment considerably and result in the bulk of
the heat from absorbed radiation being convected
away to the environment. This impact of wind thus
extends beyond the influence on the air boundary
layer and also affects the thermal conductance
(insulation) of the coat itself of the two animals (Fig.
5). The effect on the coat then is influenced by coat
structure.
In summary, colour differences among M. rufus
have little impact on the thermal load from solar
radiation. Over the general body fur less than 25% of
incident radiation reaches the body as a heat load and
this diminishes to below 15% at even moderate wind
speeds. Given even the modest shade of small desert
trees that M. rufus seek on summer days (Dawson
1972; Dawson and Denny 1969), their heat load from
direct solar radiation should be minimal. D.
novaehollandiae on the other hand is often feeding in
the open during summer (Dawson et al. 1984) but its
plumage provides its body with almost complete
protection from solar radiation (Maloney and
Dawson 1995). Therefore, in their usual situations on
hot summer days both species are largely protected
from direct solar heat loads, but with D.
novaehollandiae active in the open.
While the pelage that we examined was the main
body covering, which would play the major role in
regard to solar radiation, pelage is not constant over
the body, either in depth or reflectance for M. rufus
(Dawson and Brown 1970) nor D. novaehollandiae
(Maloney and Dawson 1994). Obviously, the full
story of the role of fur or feathers in radiation
exchange is quite complex. What happens at the
large, lightly-furred tail of M. rufus; it is tucked
between the legs under the body of a standing
kangaroo if thermal loads become extreme (Russell
and Harrop 1976). Under similar circumstances the
nearly naked legs of D. novaehollandiae are
generally shaded by the body feathers but in the
hottest part of such days D. novaehollandiae may
resort to sitting in shade, which markedly changes
heat transfer characteristics of the body covering
(Maloney and Dawson 1994).
ACKNOWLEDGEMENTS
We thank Drs J Hallam and K Webster for assistance.
This research was funded in part by a grant from the
0
2
4
6
8
10
0
5
10
15
20
25
024681012
Cond Red kang
Cond Emu
HLR Red kang
HLR Emu
Conductance (W. m
-
2
. °C)
Heat Load From Solar Radiation
(% Incident)
Wind Speed (m. s
-1
)
DAWSON & MALONEY: FUR VS FEATHERS IN THERMOREGULATION
151
Australian Research Council to TJD. Animals were
taken under a licence (A18) from the NSW National
Parks and Wildlife Service. The study was carried
out under approvals given by the University of NSW
Animal Care and Ethics Committee.
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... Fur may provide a waterproof layer (Dawson and Fanning 1981), prevent physical abrasions, aid in sensory reception (Diamond and Arabzadeh 2013) and may affect camouflage via colouration (Caro 2009). Here, we focus on the role of fur in thermoregulation (Scholander et al. 1950a(Scholander et al. , 1950b(Scholander et al. , 1950cHammel 1955;Dawson and Maloney 2004). Fur properties relevant to mammalian thermoregulation include the length, diameter, depth, density, thermal conductivity and solar absorptivity. ...
... Fur properties relevant to mammalian thermoregulation include the length, diameter, depth, density, thermal conductivity and solar absorptivity. These traits interact in complex and non-linear ways to affect heat transfer, depending on the size of the organism, its physiological response and the environment it is experiencing (Conley and Porter 1986;Walsberg 1988a;Dawson and Maloney 2004;Dawson et al. 2014). ...
... A more complete answer to this question will require biophysical analyses and quantification of radiant conditions within the roost during heat stress events that jointly consider the combined influences that fur has on radiant heat gain through solar radiation penetrance and heat loss by conduction and radiation through the fur (e.g. Conley and Porter 1986;Dawson and Maloney 2004;Mathewson and Porter 2013). These analyses will also need to consider the role of fur properties in influencing the evaporative cooling mechanisms that these species use, such as licking their wings, fur and skin (Welbergen et al. 2008). ...
Article
Fur properties play a critical role in the thermoregulation of mammals and are becoming of particular interest as the frequency, intensity, and duration of extreme heat events are increasing under climate change. Australian flying-foxes are known to experience mass die-offs during extreme heat events, yet little is known about how different fur properties affect their thermoregulatory needs. In this study, we examined the differences and patterns in fur properties among and within the four mainland Australian flying-fox species: Pteropus poliocephalus, P. alecto, P. conspicillatus, and P. scapulatus. Using museum specimens, we collected data on fur solar reflectance, fur length and fur depth from the four species across their distribution. We found that P. poliocephalus had significantly longer and deeper fur, and P. alecto had significantly lower fur solar reflectivity, compared with the other species. Across all species, juveniles had deeper fur than adults, and females of P. alecto and P. conspicillatus had deeper fur than males. The biophysical effects of these fur properties are complex and contingent on the degree of exposure to solar radiation, but they may help to explain the relatively higher mortality of P. alecto and of juveniles and females that is commonly observed during extreme heat events.
... Plumage thickness determines the probability that a ray propagating to the skin will be intercepted by a feather element, and so affects the mode of heat transfer from the feather surface to the skin (Walsberg 1988). Indeed, in some species, such as the Emu Dromaius novaehollandiae, the dense plumage provides almost complete protection from solar radiation by preventing heat transmission to the skin (Maloney & Dawson 1995, Dawson & Maloney 2004. Sunbirds (Nectariniidae) have less dense plumage and the differential temperatures between feather surface and skin decrease with plumage reflectivity (Rogalla et al. 2021a). ...
... Although extremely dense plumage might in some cases prevent solar radiation from penetrating deeply enough to contribute to radiative heating of the skin (Maloney & Dawson 1995, Dawson & Maloney 2004, the direct exposure of dark skin to the sun that has been observed in other species suggests a thermoregulatory function of skin colour (Ohmart & Lasiewski 1971, Downs et al. 2000, McKechnie et al. 2004). However, more research is needed to fully understand the role of avian skin coloration in influencing solar heat loads and the metabolic cost of maintaining a constant body temperature. ...
Article
Plumage coloration can have substantial effects on a bird's energy budget. This is because different colours reflect and absorb light differently, affecting the heat loads acquired from solar radiation. We examine the thermal effects of feather coloration on solar heat gain and flight performance and discuss the potential role of plumage colour on a bird's energy budget. Early investigations of the effects of plumage colour on thermoregulation revealed complex interactions between environmental conditions and physical properties of the plumage that may have led to diverse behavioural and physiological adaptations of birds to their thermal environment. While darker feather surfaces absorb more light, and heat more, than light‐coloured surfaces under exposure to the sun, this relationship is not always straightforward when considering heat transfer to the skin. Heat transfer through plumage varies depending on multiple factors, such as feather density and transmission of light. For instance, higher transmissivity of light‐coloured plumage can increase heat loads reaching skin level, while conduction and convection transfer heat from the surface of dark feathers to the skin. Solar heating can affect the metabolic costs of maintaining a constant body temperature, and depending on environmental conditions, colours can have either a positive or negative effect on a bird's energy budget. More specifically, solar heating can be advantageous in the cold but may increase the energetic costs associated with thermoregulation when ambient temperature is high. More recent studies have further suggested that the thermal properties of feather coloration might reduce the energetic costs of flight. This is because surface heating can affect the ratio between lift and drag on a wing. As concluding remarks, we provide future directions for new lines of research that will aid in improving our understanding of the thermal effects of feather coloration on a bird's energy budget, which can potentially explain factors driving colour evolution and distribution patterns in birds.
... change body shape and size, posture, and fur properties) (Scholander 1955 or more specialised functions such as waterproofing (Dawson & Fanning 1981) and sensory receptors (Diamond & Arabzadeh 2013). Importantly, one of the main roles of fur is to aid in thermoregulation (Scholander et al. 1950a;Scholander et al. 1950b;Scholander et al. 1950c;Hammel 1955;Dawson & Maloney 2004). There are different properties of fur that are responsible for mammalian thermoregulation (Figure 3.1). ...
... In addition to the fur properties considered in this study, fur density (Hutchinson & Brown 1969;Dawson & Maloney 2004;Rymer, Kinahan & Pillay 2007) and fur colour (Maloney & Dawson 1995;Walsberg & Wolf 1995;Hetem et al. 2009) are regarded as important traits relevant for thermoregulation. All these different fur traits play an important role in flying-fox thermoregulation. ...
Thesis
Heatwaves have increased in intensity, duration and frequency in the recent years as a result of climate change. Australian flying-foxes (Pteropus spp.) have been gravely affected by these extreme heat events with mass die-offs resulting in thousands of dead bats and abandoned pups. In order to prevent or control the death toll, it is important to predict when, where, and how these deaths might occur. I first tested the ability of a simple air temperature threshold to predict when and where flying-fox heat stress related mortality might occur. I used the ACCESS-R meteorological forecast from the Bureau of Meteorology and the flying-fox colony location data from the national flying-fox monitoring program to assess the accuracies of using forecast air temperature to predict die-offs. More than 70% predictions were correct when using 48 hour and 24 hour forecasts. In an effort to better understand the mechanistic basis of heat die-offs, I examined fur properties, thermophysiology and behaviours of flying-foxes relevant to their heat budgets, to gain more insight into how these variables and traits may affect different species, sexes and age classes during extreme heat events. Using museum specimens, I explored the impact of fur depth, fur length and fur solar reflectance on flying-fox heat stress. Grey-headed flying-foxes (Pteropus poliocephalus) had longer and deeper fur, while the black flying-fox (P. alecto) had lower fur solar reflectivity compared to other species. Moreover, females and juveniles had deeper fur compared to males and adults, respectively. I conducted thermophysiological experiments to gain insight on the effect of the natural variation in water vapour pressure on the body temperature, metabolic rate, water loss rate and behaviour of the grey-headed flying-fox at high air temperatures. The body temperature did not significantly vary at the different vapour pressure levels. However, the metabolic rate and the intensity of wing-fanning and panting (thermoregulatory behaviours) were significantly higher at higher vapour pressure levels. By means of these results and other parameters affecting the heat budget of a grey-headed flying-fox, I created and validated a biophysical model to better predict flying-fox heat stress and related mortality, using the NicheMapR mechanistic modelling framework. Analyses revealed that flying fox energy expenditure was most sensitive to the variation in fur depth and incoming solar radiation. Furthermore, the biophysical model showed greater accuracy at predicting flying-fox die-offs compared to the forecast air temperature models (88.0% vs. 72.0%). This indicated the importance of incorporating all environmental variables and animal characteristics to predict heat stress, and not only air temperature. Importantly, this mechanistic approach to predicting die-offs helps to identify key driving forces of heat stress conditions in flying-foxes, and provides a framework to understand the effects by habitat and unusual weather combinations. This in turn will allow authorities to use evidence-based strategies to manage and conserve the flying-fox colonies and species as a whole. Moreover, as flying-foxes are regarded as bioindicators of extreme heat events, these results will help us to gauge the impact of extreme heat events and climate change on other more cryptic taxa.
... Due to the essential role that feathers play, other collateral costs of feather growth and wear also exist. Thermoregulation is affected when birds begin feather molt (Dawson and Maloney 2004) and flying during molt or on worn feathers also increases energetic demands of flight (predicted to be more costly for migratory individuals; Swaddle and Witter 1997;Chai et al. 1999;Hedenström and Sunada 1999;Hedenström 2003). Thus, the timing of feather molt rarely overlaps with migration, often occurring during a short period at the end of the breeding season (e.g. , Foster 1975;Jenni and Winkler 1994;Rappole 2013;Mumme 2018). ...
... Molting is not only an energetically costly process, but also one that produces a great impact on other biological functions. For instance, body molt can affect body thermoregulation (Dawson and Maloney 2004), which can in turn determine when and where the molt occurs. Flight feather molt can influence flight performance, as the replacement feathers reduce wing area (Hedenström 2003), which has been associated with a reduced escape capacity in some species (Williams and Swaddle 2003), potentially leading to survivorship costs during molt. ...
Chapter
Bird migration in South America is highly diverse, with some species migrating solely within the tropical latitudes, others migrating between tropical wintering grounds and south-temperate breeding grounds, and yet other migrating within south-temperate latitudes of the continent. Until recently, bird migration in South America was virtually ignored; however, a growing body of research is shedding new light into the timing, routes, and overall migratory strategies employed by birds that migrate within this vast continent. The unique geographic and climatic context of South America (i.e., no oceanic or mountain barriers and a buffered oceanic climate) likely results in a distinct set of tradeoffs between seasonal life-history demands of migration, reproduction, and molt. Nevertheless, we still understand very little about the mechanisms underpinning such relationships throughout the annual cycle of migratory birds on the continent, and how they ultimately influence the evolution of bird migration in South America. In this chapter, we (1) provide a brief description of bird migration patterns in South America, (2) review the current state of knowledge about the drivers of songbird migration on the continent, particularly those resulting from research on the Fork-tailed Flycatcher (Tyrannus savana), and (3) suggest future avenues for research to understand the mechanisms driving these patterns.
... Due to the essential role that feathers play, other collateral costs of feather growth and wear also exist. Thermoregulation is affected when birds begin feather molt (Dawson and Maloney 2004) and flying during molt or on worn feathers also increases energetic demands of flight (predicted to be more costly for migratory individuals; Swaddle and Witter 1997;Chai et al. 1999;Hedenström and Sunada 1999;Hedenström 2003). Thus, the timing of feather molt rarely overlaps with migration, often occurring during a short period at the end of the breeding season (e.g. , Foster 1975;Jenni and Winkler 1994;Rappole 2013;Mumme 2018). ...
... Molting is not only an energetically costly process, but also one that produces a great impact on other biological functions. For instance, body molt can affect body thermoregulation (Dawson and Maloney 2004), which can in turn determine when and where the molt occurs. Flight feather molt can influence flight performance, as the replacement feathers reduce wing area (Hedenström 2003), which has been associated with a reduced escape capacity in some species (Williams and Swaddle 2003), potentially leading to survivorship costs during molt. ...
Chapter
Research on the fruit-taking and fruit-handling behaviors of birds flourished in the 1980s when empirical observations and experimental studies revealed that the feeding techniques of birds affect which fruits they eat and, ultimately, which seeds they disperse. Recent calls to incorporate natural history aspects into empirical studies and theoretical models to improve our understanding of the seed dispersal process bring some fresh air to the topic, given its potential to influence fruit choice by birds. Here we compile information scattered in the literature, some of which hard to access, to reveal broad patterns of fruit-taking and fruit-handling behaviors observed in the wild for 197 bird species representing a wide spectrum of the bird phylogeny (117 genera, 26 families). We tested the influence of phylogeny on fruit-taking and fruit-handling behaviors, and also investigate the previously unexplored relationship between the flexibility in fruit-taking and fruit-handling behaviors with body size and degree of frugivory (i.e., a measure of the importance of fruits in the diet). We are ultimately interested in stimulating further studies that incorporate the often neglected behaviors used by birds to take and handle fruits to investigate the reciprocal ecological and evolutionary consequences between frugivorous birds and plants.
... Due to the essential role that feathers play, other collateral costs of feather growth and wear also exist. Thermoregulation is affected when birds begin feather molt (Dawson and Maloney 2004) and flying during molt or on worn feathers also increases energetic demands of flight (predicted to be more costly for migratory individuals; Swaddle and Witter 1997;Chai et al. 1999;Hedenström and Sunada 1999;Hedenström 2003). Thus, the timing of feather molt rarely overlaps with migration, often occurring during a short period at the end of the breeding season (e.g. , Foster 1975;Jenni and Winkler 1994;Rappole 2013;Mumme 2018). ...
... Molting is not only an energetically costly process, but also one that produces a great impact on other biological functions. For instance, body molt can affect body thermoregulation (Dawson and Maloney 2004), which can in turn determine when and where the molt occurs. Flight feather molt can influence flight performance, as the replacement feathers reduce wing area (Hedenström 2003), which has been associated with a reduced escape capacity in some species (Williams and Swaddle 2003), potentially leading to survivorship costs during molt. ...
Book
This book covers central aspects of behavioral ecology, including sexual selection, social and genetic mating systems, cooperative breeding, brood parasitism, brood reduction, migration, personalities and communication. Over the past several years, Neotropical bird species from temperate to tropical latitudes of South America have been extensively studied, yielding valuable insights into the evolutionary mechanisms that drive their behavioral traits. In this book, international experts provide a general overview of main behavioral aspects. They also present the main findings of their work, including experimental approaches to testing the most accepted behavioral theory in their model systems. In closing, they propose new theoretical frameworks and future research directions. As such, the book provides a comprehensive and updated guide for all researchers, students and professionals whose work involves the study and management of birds across the Neotropical region.
... An underappreciated role of feathers in thermoregulation is in deflection of solar radiation to keep birds cool (Medina et al., 2018). For example, unlike kangaroos who must seek shade during the hottest parts of the day, in the harsh Australian desert, emus do not need to seek shade, because feathers are superior to fur at reflecting solar radiation (Dawson & Maloney, 2004). ...
Full-text available
Article
The ability of feathers to perform many functions either simultaneously or at different times throughout the year or life of a bird is integral to the evolutionary history of birds. Many studies focus on single functions of feathers, but any given feather performs many functions over its lifetime. These functions necessarily interact with each other throughout the evolution and development of birds, so our knowledge of avian evolution is incomplete without understanding the multifunctionality of feathers, and how different functions may act synergistically or antagonistically during natural selection. Here, we review how feather functions interact with avian evolution, with a focus on recent technological and discovery‐based advances. By synthesising research into feather functions over hierarchical scales (pattern, arrangement, macrostructure, microstructure, nanostructure, molecules), we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into an astounding array of environments and life‐history strategies. We suggest that future research into avian evolution involving feather function should consider multiple aspects of a feather, including multiple functions, seasonal wear and renewal, and ecological or mechanical interactions. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.
... Thermoregulation, either behavioral or physiological, can allow animals to regulate body temperature to become thermal specialists, optimizing performance within a narrow temperature range (Angilletta Jr et al., 2002, 2010aLogan et al., 2019;Neel et al., 2020). Morphology can also affect the body temperatures of organisms, with integumental boundaries impeding heat transfer with the environment (Best, 1982;Cena et al., 1986;Dawson and Maloney, 2004), body size altering thermal inertia (Stevenson, 1985;Paladino et al., 1990), and the surface area to volume ratio regulating rates of core temperature flux (Bell, 1980;Phillips and Heath, 1995). For obligate ectotherms, loss of endogenously generated heat is not an issue, and so they have evolved sizes and shapes (elongate, flattened) that are not possible for endotherms (Pough, 1980). ...
Article
Organismal performance is strongly linked to temperature because of the fundamental thermal dependence of chemical reaction rates. However, the relationship between the environment and body temperature can be altered by morphology and ecology. In particular, body size and body shape can impact thermal inertia, as high surface area to volume ratios will possess low thermal mass. Habitat type can also influence thermal physiology by altering the opportunity for thermoregulation. We studied the thermal ecology and physiology of an elongate invertebrate, the bark centipede (Scolopocryptops sexspinosus). We characterized field body temperature and environmental temperature distributions, measured thermal tolerance limits, and constructed thermal performance curves for a population in southern Georgia. We found evidence that bark centipedes behaviorally thermoregulate, despite living in sheltered microhabitats, and that performance was maintained over a broad range of temperatures (over 20 ◦C). However, both the thermal optimum for performance and upper thermal tolerance were much higher than mean body temperature in the field. Together, these results suggest that centipedes can thermoregulate and maintain performance over a broad range of temperatures but are sensitive to extreme temperatures. More broadly, our results suggest that wide performance breadth could be an adaptation to thermal heterogeneity in space and time for a species with low thermal inertia.
... Birds' contour feathers can be considered as a porous medium comprising feathers and the surrounding air. Empirical studies have been conducted on heat transfer phenomena by the contour feathers of such birds as penguin (Kooyman et al., 1976;Mccafferty et al., 2013;Taylor, 1986), emu (Dawson and Maloney, 2004), crow (Marder, 1973), pigeon (Walsberg et al., 1978), sparrow (Ward et al., 1999), and penduline (Wolf and Walsberg, 1996). Dawson et al. (1999) and Du et al. (2007) modeled heat transfer in penguin feathers. ...
Article
Contour feathers of birds consist of layers of plumulaceous and pennaceous feathers. Differences in the structures of plumulaceous and pennaceous feathers lead to differences in such functions as heat transfer and air penetration. In this study, samples were collected from plumulaceous and pennaceous feathers of chickens, pigeons, magpies, and ostriches. Measurements were carried out to determine the density and diameter of the plumulaceous barbules as well as layer thicknesses of the pennaceous feathers. Permeability coefficients of the plumulaceous and pennaceous feathers were obtained using an air permeability tester and compared to investigate the effects of fiber diameter, porosity, and number of layers. The highest permeability coefficients were obtained for the plumulaceous and pennaceous contour feathers of the ostrich and chicken while pigeon and magpie recorded the lowest values. The permeability coefficients measured for the plumulaceous contour confirmed the validity of the Carman–Kozeny Relation.
Article
To investigate the radiative heat transfer in birds' coatings necessitates knowing the radiative properties such as absorption coefficient, scattering coefficient, and phase function. In the present work, by simulating an arrangement similar to the structure of the birds' feathers, the radiative properties were investigated using Monte Carlo method for fibers with diameters of 5, 10, 15, and 20 μm at fiber volume fractions of 1%, 3%, 5%, and 7% and at different angles for fibers in two dark and light colors. According to the results, by increasing the fiber's diameter from 5 to 20 μm, the attenuation coefficient was reduced by 75%. Considering the studied diameters, an increase in the fiber's volume fraction from 1% to 7% led to an increase of 531% in the average attenuation coefficient. In fibers with light color, by increasing the fiber's diameter from 5 to 20 μm, the average albedo (the scattering coefficient-to-attenuation coefficient ratio) at volume fractions of 1%, 3%, 5%, and 7% was reduced by 10%, 12%, 13%, and 14%, respectively. Considering the studied diameters in the light-colored fibers, increasing the fiber's volume fraction from 1% to 7% led to a reduction of 12% in the average albedo. However, in the dark-colored fibers, the albedo showed slight changes. As indicated by the results, although the average attenuation coefficient was independent of the fibers' arrangement, the changes in the arrangement of the fibers significantly affected the angular distribution of the attenuation coefficient.
Full-text available
Article
1.1. The emu (Dromaius novaehollandiae) is a large (40 kg) diurnal bird that inhabits the arid inland of Australia where solar radiation levels can exceed 1000 W/m2 for many hours of the day.2.2. We measured the solar heat load at skin level below plumage samples from wild emus. At low wind speeds the heat load was less than 10% of the incident radiation load. This fell to less than 1% at wind speeds above 6 m/s.3.3. Application of a simple model shows that the radiation is absorbed close to the surface of the plumage. The resultant heat is prevented from flowing to the skin by the coats' insulation.4.4. On an average summer day in the arid zone an emu will require less than 330 g of water to vaporate the solar heat load.
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Article
Two large kangaroos, the red kangaroo (Megaleia rufa) and the euro (Macropus robustus) live in the arid regions of Australia. On hot summer days they are found in entirely different microenvironments: red kangaroos in the shade of small desert trees in relatively open country and euros around rocky outcrops in caves and under rock ledges. Measurement of the temperature and radiation characteristics of these microhabitats showed that the euro avoided much of the heat load of the desert, particularly the radiation heat load. While the solar radiation influx reaching the red kangaroo resting site was only about 20% of that incident on open ground it still resulted in the radiation temperature of this microenvironment exceeding the animal's body temperature by as much as 30@?C when air temperature also exceeded the body temperature. Considerable evaporative water loss would be expected under these circumstances. Radiation heat loss from the red kangaroo microenvironment to the sky, possible in the late afternoon, may be used by the animal to reduce the amount of water required for temperature regulation.
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Article
The effects of temporal (time of day and season) factors and size, sex, female reproductive state and group size on the diel time-use of free-ranging red kangaroos (Macropus rufus) was examined. Particular emphasis was given to the effects on their foraging behaviour, with foraging divided into cropping, chewing and searching components. The study was conducted in semi-arid western New South Wales from July 1991 to March 1992, a time of deepening drought conditions in New South Wales. Group size had very little influence on the time-use of M. rufus. It was negatively but only weakly correlated with the proportion of foraging time spent chewing (chewing intensity). No significant differences in time-use were found between size classes of adult males (large and medium-sized males), females with or without pouch young, or females with different-sized pouch young (no visible young, small pouch young or large pouch young). Differences occurred between adult males, adult females and subadult kangaroos. These differences were mainly associated with their chewing and searching behaviour and were related to body size; as body size increased the proportion of time spent chewing and the intensity of chewing increased while the proportion of time searching and the proportion of foraging time spent searching (searching intensity) decreased. Neither the proportion of time spent cropping or foraging nor the proportion of foraging time spent cropping (cropping intensity) or the proportion of active time spent foraging (foraging intensity) differed between any size/sex/reproductive class. Temporal effects had a considerable influence on time-use. M. rufus were most active at night and in the few hours after sunrise and sunset. Seasonal changes in time-use were largely a result of changes in daytime behaviour. M. rufus foraged less and rested more during the day in winter than in spring or summer. There was no increase in the intensity or proportion of time spent foraging or cropping at night to compensate for the reduction in diurnal foraging. It is hypothesised that temporal variations in time-use were related to variations in weather and vegetation conditions.
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Article
The pattern of activity of Emus Dromaius novaehollandae in western New South Wales has been examined and contrasted with that of the other large native animals in the area, Red Kangaroos Megaleia rufa and Euros Macropus robustus. A detailed behavioural analysis indicated that Emus were diurnal and speni a large part of theaay feeding in both summer and winter. During hot days in summer they occasionally sheltered among trees from the radiation heat load. The water requirements of adult Emus measured by tritiated water turnover do not appear high but intake may be limited by the size of the simple gut, resulting in a relatively high frequency of drinking, once per day and occasionally twice per day during hot summer conditions. The water use of chicks, especially young chicks, was much greater than that of the adults. Water losses from an incubating bird, however, were one fifth of those of adult birds in similar conditions. The Emus were omnivorous, relying on insects, seed heads, berries and succulent vegetation. The Emus successfully make their living in the arid zone very differently from the marsupials and the basis of this is discussed in relation to recent findings about their physiological adaptations.
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Article
A study was carried out to examine the effect of posture (standing or lying) on the radiation heat load which would be experienced by a kangaroo resting under a small desert tree during a summer day. Measurements were made to assess the temperature and radiation characteristics in three situations judged to be equivalent to the following: (1) a kangaroo standing in the sun in open country; (2) an animal standing in the shade of a small tree; (3) an animal lying in the same shade. The overall pattern of results was shown by the effective radiation temperature measurements made at midday: 78.3, 57.6, and 52.5C for positions (I), (2), and (3) respectively. The relative contributions of solar and long-wave infrared radiation to the pattern of results are discussed.
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Article
The emu is a large, flightless bird native to Australia. Its habitats range from the high snow country to the arid interior of the continent. Our experiments show that the emu maintains a constant body temperature within the ambient temperature range-5 to 45C. The males regulate their body temperature about 0.5C lower than the females. With falling ambient temperature the emu regulates its body temperature initially by reducing conductance and then by increasing heat production. At-5C the cost of maintaining thermal balance is 2.6 times basal metabolic rate. By sitting down and reducing heat loss from the legs the cost of homeothermy at-5C is reduced to 1.5 times basal metabolic rate. At high ambient temperatures the emu utilises cutaneous evaporative water loss in addition to panting. At 45C evaporation is equal to 160% of heat production. Panting accounts for 70% of total evaporation at 45C. The cost of utilising cutaneous evaporation for the other 30% appears to be an increase in dry conductance.
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Article
1.1. The reflectivity of the furs of two species of desert kangaroo to solar radiation was measured and the effect of season, site on body, and wind speed on their fur insulation was determined.2.2. The red kangaroo Megaleia rufa which lives on the exposed open plain has fur which gives greater protection from solar radiation in summer and from heat loss in winter than does that of the euro or hill kangaroo Macropus robustus, which shelters in caves and under rock ledges.3.3. The fur insulation of both the kangaroos is similar to that of tropical and temperate zone eutherian mammals, even though they have a much lower basal heat production than eutherians.
Conference Paper
The plumage of birds provides a critical thermal buffer between the animal and its environment. Rates of energy expenditure are strongly influenced by the thermal properties of the environment or the microclimates the animal occupies. Current data suggest that the addition of solar radiation is equivalent to three to four-fold changes in wind speed and that solar heat gain can be extremely sensitive to changes in wind speed. Dry heat transfer through the plumage occurs by three avenues 1) conduction and free convection through air 2) conduction along the solid elements of the plumage and 3) radiation. Overall, about 95% of the total heat flow is evenly divided between the first two avenues. Radiative heat transfer accounts for only about 5% of total heat flow, Plumage color, as well as the microstructure and micro-optical properties of plumage elements, when combined with environmental properties (e.g., wind speed), determine the radiative heat loads that birds acquire from solar radiation. Although plumage color or reflectivity determines the fraction of incident solar radiation that is absorbed by the plumage and generates heat, the fraction of this heat that contributes to the thermal load on the animal can vary greatly. In a fibrous coat such as a plumage, there is some variable penetration into the coat, with absorption over a range of coat depths. Factors such as feather microoptics and structure are critical determinants of radiation penetration into avian coats. Significant differences in solar heat loads can also result from behavioral adjustments in plumage thickness.
1.1. Metabolism (Vo2) and evaporative water loss (mwe) were measured in black, male Lark Buntings (Calamospiza melanocorys) at 10, 30 and 35°C with and without supplemental radiation (simulating sunlight).2.2. Vo2 was reduced by radiation at 10°C but not 30° or 35°C.3.3. mwe was unaffected by radiation at 10°C, however; it was increased 36% at 30°C and 150% at 35°C.4.4. I suggest caution should be used in arguing the selective advantages of black pigmentation in environments with incident radiation, for although there may be an energy saving at low ambient temperatures, there is a considerable increase in water loss at high ambient temperatures.