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

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.

Full text

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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