| dc.description.abstract | The physiological capacity for thermoregulation has been
examined in the laboratory for two sub-species of Heteronyrax
brucei. Together with measurements of microclimatic conditions
of the hyrax habitats, the results are used to suggest the animals' scope for and the significance of behavioural thermoregulation
under less controlled, more natural conditions and in
the field.
For both species TB is remarkably labile and varies directly
with TA• Over an ambient range of 0° - 42.5°0. the TBls
observed span 7°0.
The range of minimum variation of TB with TA is narrow or h.b. hindei TB is 36.5 ° + 0.4 °O. at TAls between 20° -
25°0. For h.b. rudolfi TB is 37.3° + 0.5°0. at TA's between
20° and 30°0. Above or below these zones of constant TB, TB
rises or falls linearly with TA at similar rates in each subspecies.
Diurnal fluctuations of TB are slight and seem restricted
to a relatively narrow range of TA• The amplitude of these
fluctuations is probably dependent upon TA•
It is suggested that the metabolic reserves of Heterohyr8X
brucei are inadequate to maintain its TB within normal limits
in prolonged exposure to TAt s much lower than its LOT. Basking
in the sun doubtless augments this limitation since 1t-hour
periods in direct sun can result in 4°0. increases in TB•
Under simulated "natural" conditions (TA'S of 16° - 31°0.)
TB 1s°°for h.b. hindei averaged 36.1 + 1 0., ranging from 34.5 °
to 38.3 °0., and appeared to follow the course of fluctuations
The lower critical temperature of H. b. hindei is ca. 25°O.
Below this, at 0°0., the animal has the capacity to increase
its resting metabolic rate three-fold. For -H.-b. rudolfi the
the lower critical temperature is ca. 300e. It has the same
capacity for increasing resting metabolism at low temperatures.
The LOT in both sub-species is higher than expected on the basis
of body weight and is similar to that shown by animals from
warm or tropical environments.
Limited experiments with huddled animals show that at 10°0.
paired animals lose only 1.4 times more heat. Exposure to low
temperatures is infrequent and of short duration for these hyraxes.
It is suggested that huddling under such conditions
provides a more immediate energy savings than could be supplied
by entering torpor. Torpor was not observed.
At TA' s above the LOT the oxygen consumption of H. b. hindei
agrees with that expected on the basis of body weight. How-
ever, at•0TA-''s above 30 O. there is a steady decline to values which at 42.50c are only 50% of this predicted level. It is
shown that this reduction results in a considerable water economy
at high temperatures.
At all TA's above the LOT the measured oxygen consumption
of H. h rudolfi is only 50% of that predicted on the basis of
body weight. The principal environmental demands on both subspecies
are high ambient temperature and water scarcity. The
savings of energy and water possible because of these abnormally
low resting metabolic rates demand the low levels of activity
that typifies hyrax behaviour.
It is shown that the pattern of homeothermy below the
Let in each hyrax conforms to the usual mammalian model based
on Newton's law of cooling since both TB and oxygen consumption
are linear functions of TA and the extrapolated value of TA at
zero oxygen consumption is equivalent to 'the extrapolated value
of TB at that TA•
Evaporative water loss accounts for 100% of metabolic heat
production at high temperatures. This is only possible because
of the low metabolic rates of both species at these temperatures.
The sweat glands of hyrax are limited to the feet and appear
well adapted for protecting the feet and minimizing heat
flow into the animal when it moves across hot rock surfaces.
Tl].edemands of water economy and the fact that ENL from the
feet is only 22% of total water-loss suggest that ENL from the
feet is not a major avenue of heat loss from these animals.
shown that this reduction results in a considerable water economy
at high temperatures.
At all TAIs above the LOT the measured oxygen consumption
of H.b. rudolfi is only 50% of that predicted on the basis of
body weight. The principal environmental demands on both subspecies
are high ambient temperature and water scarcity. The
savings of energy and water possible because of these abnormally
low resting metabolic rates demand the low levels of activity
that typifies hyrax behaviour.
It is shown that the pattern of homeothermy below the
CT in each hyrax conforms to the usual mammalian model based
on Newton's law of cooling since both TB and oxygen consumption
are linear functions of TA and the extrapolated value of TA at
zero oxygen consumption is equivalent to the extrapolated value
of TB at that TA•
Evaporative water loss accounts for 100% of metabolic heat
production at high temperatures. This is only possible because
of the low metabolic rates of both species at these temperatures.
The sweat glands of hyrax are limited to the feet and appear
well adapted for protecting the feet and minimizing heat
flow into the animal when it moves across hot rock surfaces.
The demands of water economy and the fact that EWL from the
feet is only 22% of total water-loss suggest that EWL from the
feet is not a major avenue of heat loss from these animals.
High respiratory rates (approaching 200 breaths/min.) are
observed only at high TB's (i.e., greater than 400C.). Neither
open-mouthed panting nor licking were observed, again suggesting
the importance of water conservation.
Mean minimal heart rates are only slightly lower (117
beats/min.) than expected on the basis of body weight. The
rate increases regularly above and below an ambient temperature
Below the LCT thermal conductances are reasonably stable.
They are high and similar to those of mammals adapted to warm
or tropical climates.
Above the Let in H. b. hindei the thermal conductance increases
to three times its minimum value until the point at
which TB = TA' and then returns to below this minimum when TA
exceeds TB• These changes in conductance are linked to savings
in the amount of water needed for heat dissipation below and
above the point of reversal of heat flow (Tn = TA).
Similar changes in thermal conductance are apparent in
H. b. rudolfi, but here minimum conductance is lower and the
port of reversal of heat flow (41°C.) is one degree higher
than in h.b. hindei. Thermal conductance was not measured beyond
this point. It is likely that H.b. rudolfi is at least
as well adapted to high temperature living as is H. b. hindei. Further studies are needed to verify this.
Observations of a group of Heterophyrax on a single outcrop
for an eight-day period showed that the movement of animals
on the rock and their daily pattern of activity follow a
definite sequence dependent on changes in ambient temperature
conditions.
Feeding appears limited to periods of declining temperature
or of limited solar radiation through overcast skies or
shade.
It is suggested that by altering their degree of body
contact with the rock surfaces on which they spend the day,
and by selecting surfaces on which to rest depending on their
need to lose or to gain heat, hyraxes can maintain body temperatures
within a much narrower range at less cost than if they
were dependent solely on physiological mechanisms of temperature
control.
Temperature gradient studies and simultaneous measurements
of TB are needed to quantify the role of this behavioural thermore-
lation. | en |
