A year in the thermal life of a free-ranging herd of springbok Antidorcas marsupialis
1 School of Physiology, University of the Witwatersrand Medical School, 7
York Road, Parktown 2193, South Africa
2 Physiology, School of Biomedical and Chemical Science, University of
Western Australia, Crawley, Perth, 6009, Australia
3 National Zoological Gardens Lichtenburg Game Breeding Centre, PO Box 716,
Lichtenburg 2740, South Africa
4 Department of Zoology and Physiology, University of Wyoming, Laramie, WY
82071, USA
* Author for correspondence (e-mail: fullera{at}physiology.wits.ac.za)
Accepted 27 May 2005
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Summary |
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Key words: springbok, Antidorcas marsupialis, circadian rhythm, body temperature, homeothermy, thermoregulation
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Introduction |
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Although earlier studies yielded valuable insights, recent investigations
employing miniature data loggers (Fuller et al.,
1999,
2000
;
Jessen et al., 1994
;
Lehmer et al., 2003
;
Maloney et al., 2002
;
Mitchell et al., 1997
;
Mzilikazi and Lovegrove, 2004
)
or radiotelemetry (Ostrowski et al.,
2003
; Zervanos and Salsbury,
2003
) to record body temperatures in free-living mammals have
demonstrated the importance of studying thermoregulatory responses in the
natural environment, where animals are subjected to complex stressors that
alter their behaviour and thermoregulatory mechanisms. Indeed, no African
ungulate studied so far, free-living in its natural habitat, has employed
adaptive heterothermy (for a review, see
Mitchell et al., 2002
). The
ungulates had low-amplitude 24 h rhythms of body temperature (13°C)
with mean body temperatures at night higher than those during the day. We have
suggested that the absence of wide swings in body temperature can be
attributed to the animals employing behavioural thermoregulation, both to
maintain body temperatures at night and to escape solar radiation during the
day (Fuller et al., 2004
;
Mitchell et al., 2002
).
All the published studies on free-ranging African ungulates have been
carried out over periods ranging from as little as 6 days (for one springbok,
Antidorcas marsupialis; Mitchell
et al., 1997) to 2 months, much shorter than necessary to detect
seasonal changes, and the consequences of life events like births. For some
species, including springbok (Mitchell et
al., 1997
), data were not obtained during the hot summer months,
when adaptive heterothermy is most likely to be employed. In the absence of
long-term studies allowing comparison of body temperature fluctuations across
seasons, the finding that core body temperature of large African arid-zone
ungulates is independent of environmental thermal load
(Mitchell et al., 2002
) may be
attributed, at least in part, to limited fluctuations in nychthemeral heat
load that happened to occur during the study periods. To obtain continuous and
long-term data and to investigate whether heterothermy is evident in
springbok, a small antelope that occupies semi-arid regions of Southern
Africa, we used implanted miniature data loggers to record abdominal
temperatures of free-ranging animals in their natural habitat for a period of
1113 months.
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Materials and methods |
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Procedures
All experimental procedures were approved by the Animal Ethics Screening
Committee of the University of the Witwatersrand (protocol no. 99/28/4). On
the day of surgery, springbok were herded into a mass crate, and transported
to a nearby temporary surgical theatre. They were removed individually from
the crate and anaesthetised with 12% halothane (Fluothane, Zeneca,
South Africa) in oxygen, administered via a face mask. Using aseptic
techniques, we implanted miniature thermometric data loggers into the
abdominal cavity of each animal. Average duration of surgery was 6 min.
Respiratory rate, heart rate and rectal temperature were monitored throughout
surgery. Wounds were sutured with nylon and treated with a topical antiseptic
spray (Necrospray, Centaur Labs, Pretoria, South Africa), and each animal was
given prophylactic long-acting penicillin (3 ml i.m., Duplocillin, Intervet,
Johannesburg, South Africa) and an analgesic and anti-inflammatory medication
(1 ml s.c., Dexa-Tomanol, Centaur Labs). A coloured ear tag was fixed to each
animal for identification. After recovery from anaesthesia the animals were
released into a study enclosure, a 62 ha fenced section of the reserve, where
they became ambulatory within 5 min, and subsequently ranged freely with other
ungulate species. On the day of release two male springbok (males 2 and 3)
escaped from the enclosure, into a larger section of the reserve (1470 ha),
which is open to tourist vehicles. We could not readily identify these males
for recapture; they returned to their original territory with a springbok
population of 250, and several other large African mammal species. All
animals had access to water ad libitum and grazed on natural
vegetation of the reserve.
The six springbok in the study enclosure were recaptured 1 year or more after surgery, by chasing them into game capture nets. Each animal was sedated with haloperidol (10 mg i.m.) before being carried to a mass crate. Data loggers were removed the following day during surgery (using the surgical and anaesthetic procedures as described above). The animals' surgical sites had healed and there were no signs of infection. After surgery, springbok (with ear tags) were released back into the main reserve; intermittent observations of the animals in the following year revealed that all animals remained healthy. All females gave birth to healthy lambs during the study period and were pregnant again at the time of logger retrieval.
Loggers from the two male springbok that escaped from the study enclosure were retrieved serendipitously after the animals were captured, more than 1 year after surgery, during routine game management practices at the centre.
Body temperature measurement
The miniature data loggers (StowAway XTI, Onset Computer Corporation,
Pocasset, USA) had outside dimensions of 50 mmx45 mmx20 mm
and a mass of
40 g, when covered in wax. These loggers were
custom-modified for us, to have a storage capacity of 32 kb, a measurement
range from +34 to +46°C, and a resolution of 0.04°C. Before
implantation, loggers were calibrated against a high-accuracy quartz
thermometer (Quat 100, Heraeus, Hanau, Germany) in an insulated water bath, to
an accuracy of better than 0.05°C. The scan interval of the loggers was
set at 30 min, to allow recording for up to 677 days. Loggers retrieved from
animals were in perfect order and all data was retrieved. However, four
loggers had stopped recording after
11 months because of premature
battery failure. Recalibration of loggers revealed no calibration drift.
Climate measurement
Climatic data were obtained from a portable weather station (Mike Cotton
Systems, Cape Town, South Africa) at the study site and from the nearby
Lichtenburg meteorological station (South African Weather Bureau). Data on air
temperature, globe temperature and wind speed are given in more detail below
and in Table 1. Solar noon was
between 12:00 h and 13:00 h; times of sunrise and sunset for each season are
given in Table 1. Total annual
rainfall was 514 mm in 1999, similar to that of previous years, but was
significantly higher at 753 mm in 2000, when rainfall occurred atypically in
all months of the year.
|
Statistical analyses
The relationship between air temperature and body temperature of each
animal was assessed using Pearson's linear correlation and regression.
Repeated measures analysis of variance (ANOVA), with
StudentNewmanKeuls post hoc test, was used to determine
changes in body temperature of the eight animals across seasons. For analyses
of seasonal rhythms, data were grouped and averaged in four 3-month periods
(seasons). The three coldest months (May, June and July) were termed `winter',
and successive three-month periods then named `spring' (AugustOctober),
`summer' (NovemberJanuary) and `autumn' (FebruaryApril). Other
comparisons of body temperature were made using paired or unpaired Student's
t-tests, where appropriate. For analysis of nychthemeral patterns of
body temperature, actual values of minimum body temperature and maximum body
temperature, and the time they occurred, were used. A curve-fitting procedure
was not used because a biphasic 24 h bodytemperature pattern was not always
evident. Values of P<0.05 are considered significant. All data are
reported as means ± S.D.
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Results |
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Body temperature as a function of climate
The means of core body temperature, calculated over the entire period of
recording but excluding days when animals experienced fevers or exercise
hyperthermia, were 39.32±0.20°C (female 1, 384 days),
39.51±0.21°C (female 2, 346 days), 39.44±0.16°C (female
3, 258 days), 39.26±0.11°C (female 4, 334 days),
39.44±0.22°C (female 5, 341 days), 39.26±0.16°C (male 1,
359 days), 39.30±0.13°C (male 2, 306 days) and
39.39±0.12°C (male 3, 315 days). Mean body temperature of female
springbok was not significantly different (t=1.16, P=0.29)
to that of the males. Mean body temperature for the springbok, weighted by the
reciprocal of the variance for each individual animal, was
39.34±0.003°C.
In each springbok, mean 24-h body temperature was correlated significantly with mean 24 h air temperature (P<0.0001; female 1 shown in Fig. 2A). Although correlation coefficients were modest (0.30.7), upper and lower 95% prediction intervals (around a fitted regression line; not shown in Fig. 2A) never separated by more than 1°C. Thus, at any mean air temperature between 0 and 30°C, body temperature usually varied by no more than 0.5°C above or below the mean body temperature expected for that air temperature. Slopes of linear regression lines fitted to each animal's data ranged from 0.01 to 0.03, so that, on average, mean daily body temperature of springbok increased 0.022°C per 1°C increase of mean daily air temperature.
|
The association of body temperature with air temperature is depicted in another way in Fig. 3, which shows, as a function of time of day, mean air temperature (Fig. 3A) and mean body temperature for all springbok (Fig. 3B) over 1 month with high air temperatures (November) and 1 month with low air temperatures (June). The parallel downward shift in air temperature at all times of day in June was accompanied by an almost parallel downward shift in body temperature. For both months, body temperature was at its lowest in the early morning after sunrise, and started to rise about 2 h after the morning rise in air temperature to reach a maximum in the early evening, several hours after air temperature had peaked. Despite large fluctuations (more than 10°C) in mean air temperature over the 24 h, the mean nychthemeral amplitude of body temperature usually was no more than 1.2°C (see also Table 1).
|
Seasonal variations in body temperature
Table 1 shows mean (±
S.D.) daily air temperature, globe temperature and wind
speed, and mean (± S.D.) monthly rainfall, during
each season. Air temperature, as measured each hour, reached a maximum of
34°C (in November) and dropped as low as 6°C (in July). The
average 24 h range of air temperature was about 16°C in autumn and winter,
and 12°C in summer and spring. Peak black globe temperature was 54°C
(in November); globe temperatures were not obtained in winter because of
equipment failure. Air temperatures were unusually low in January because of
anomalously high rainfall (154 mm, compared to annual January average of
50 mm).
Mean daily body temperature, maximum and minimum body temperature, and mean amplitude of daily body temperature for the eight springbok over each 3-month period are also shown in Table 1. Mean daily body temperature in winter was significantly lower (F=19.39, P<0.0001, N=8) than it was in summer (by 0.28°C; P<0.001), spring (0.21°C, P<0.001) and autumn (0.17°C, P<0.001), and was lower in autumn than in summer (by 0.11°C, P<0.05). These seasonal reductions in mean body temperature were accompanied by decreases of similar magnitude in both minimum daily body temperature and maximum daily body temperature, so that the mean nychthemeral amplitude of body temperature did not differ across the four seasons (F=1.48, P=0.25).
The times at which the minimum body temperature and the maximum body temperature (acrophase) occurred in each season also are given in Table 1. The time at which body temperature reached a minimum was significantly later in winter (F=6.57, P=0.003) than in summer (by 1.2 h; P<0.01) and autumn (by 0.8 h; P<0.05). Similar shifts in the acrophase occurred (F=6.49, P=0.003); in winter it occurred 1.2 h later than in summer (P<0.01), and in autumn it was delayed by 0.9 h (P<0.05). Because both phases of the nychthemeral cycle were shifted in the same direction and by similar amounts, there was no significant change in the time elapsed between the minimum and maximum body temperature, over seasons.
The shifts in the phases of the nychthemeral rhythm across seasons were coincident with changes in the time of sunrise. Time of sunrise was 1 h later in autumn, and 1.5 h later in winter, than it was in summer (Table 1). Time of sunrise was significantly correlated with the time at which body temperature reached a minimum (r=0.98, P=0.02, N=12). Time of sunset was earliest in winter and latest in summer (Table 1), resulting in a photoperiod (length of day light) that was shortest in the winter months (by an average of 2 h and 53 min, compared to summer; Table 1). Comparisons of mean body temperature for each month of the year with the mean photoperiod for that month revealed that mean body temperature was very tightly correlated with photoperiod (r=0.9, P<0.0001, N=12; Fig. 4).
|
|
Exercise-induced hyperthermia
In addition to the fevers, which lasted days, the springbok also
sporadically experienced short-duration episodes of elevated body temperature
(for example, see Fig. 1).
Fig. 6 shows the body
temperature of one springbok (male 2) on 2 days in December. On one occasion
on day 1, and two on day 2, body temperature rose by more than 1°C within
1 h, reaching a level of 41°C. The high body temperatures resolved
rapidly. These elevations of body temperature probably reflect episodes of
moderate to intense activity. Elevations were not always evident in the
records of all animals at the same time and no unusual climatic conditions
prevailed. Short-duration episodes of elevated body temperature were evident
at any time, but particularly during the night.
|
What we presume to be exercise episodes may have been spontaneous or induced by a disturbance of which we were unaware. The animal shown in Fig. 6 was one of the males resident in the main reserve and may have fled from vehicles (during the day), or from predation by a resident population of jackals Canis mesomelas at night, or may have been involved in territorial disputes with other males. Both those males (males 2 and 3) were members of small bachelor herds at the time of their capture. Indeed, episodes of elevated body temperature occurred much more frequently in these two springbok in the main reserve than in any animal housed in the study enclosure, including one male animal, where the animals were disturbed less often and where there were no other males.
Examination of our records revealed that our occasional observations (by
vehicle) of animals in the study enclosure indeed elicited moderate
hyperthermia of short duration (less than 1 h), typically in all animals in
the herd. However, attempts to catch the animals at the end of the study
period using capture nets erected in the enclosure invoked much higher
elevations in body temperature. Fig.
7 shows the body temperatures of three female springbok, the
abdominal loggers of which still were recording at the time, during attempted
capture by high-intensity chasing with two vehicles (for 45 min). During
and after the chase, body temperature rose precipitously, reaching 43°C, a
level much higher than that measured at any other time during the year of
recording (see Fig. 1, for
example). The hyperthermia, however, resolved rapidly after the unsuccessful
capture attempt, with no apparent sequelae for the animals.
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Discussion |
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Although our springbok exhibited extraordinary homeothermy from week to
week, and month to month, there were sporadic episodes when body temperature
deviated outside of its normal range, sometimes exceeding 41°C or dropping
below 38°C (for example, see Fig.
1). The hyperthermic episodes presumably were associated with
animals engaging in exercise or, when sustained over days, developing
spontaneous fevers. We previously have observed similar sustained elevations
in body temperature with maintenance of the normal circadian rhythm in
free-ranging impala (Kamerman et al.,
2001). There also was a period when springbok in the herd, like
the impala, developed fevers consecutively, possibly reflecting the spread of
a contagious pathogen. Body temperature also rose precipitously during
capture, consistent with previous findings that springbok reach rectal
temperatures in excess of 42°C in response to capture stress
(Gericke et al., 1978
). Rapid
decrements of abdominal temperature, which occurred sporadically, were not
associated with unusually cold climate, and typically occurred at different
times for each animal, may reflect drinking
(Mitchell et al., 1997
).
Because we deliberately wanted to limit human interactions with the animals we
did not observe behavioural patterns, except on rare occasions when we entered
the study enclosure. Further studies with other remote-sensing miniature
devices (Cooke et al., 2004
) to
record, for example, activity, heart rate or skin temperature, are necessary
to discover what thermoregulatory mechanisms unrestrained animals exhibiting
voluntary behaviour in the absence of human observers actually employ. Further
studies are also needed to investigate if patterns of thermoregulation differ
in detail between genders, age classes and animals in different social groups.
In our study, episodes of hyperthermia occurred more often in the two males
that escaped from the study enclosure into the main reserve. Whether the
higher incidence of these increments in body temperature was associated with
reproductive behaviour, with more frequent disturbances by tourists, or with
the animals being housed in a much larger area, is unknown.
The short-duration episodes of hyperthermia, in which body temperature
increased by more than 1°C h1 and then decreased at a
similar rate, are likely to reflect high-intensity activity. Springbok,
reported to run at speeds up to 88 km h1 to avoid predation
(Skinner and Louw, 1996),
generate a peak metabolic rate at least 10 times that of basal metabolic rate
(Mitchell et al., 1997
). Their
pelage, which is thinner and has a higher conductance than expected for an
antelope of similar size (Hofmeyr and
Louw, 1987
), together with evaporative cooling from both the skin
and respiratory tract (Hofmeyr and Louw,
1987
), facilitate rapid heat loss at cessation of exercise.
Although the pelage offers the advantage of off-loading excess heat rapidly,
its properties carry the disadvantages of the animals losing heat rapidly even
at moderately low ambient temperatures, and gaining substantial heat under
conditions of high ambient temperature or solar radiation
(Hofmeyr and Louw, 1987
). In
the face of air temperatures that often varied by more than 20°C on a
single day, and minimum and maximum air temperatures that differed by as much
as 40°C across the year, springbok in our study therefore maintained body
temperature within a remarkably narrow range.
The homeothermy of the springbok in the face of substantial nychthemeral
variation in environmental heat load, though remarkable, was not unexpected.
We previously have shown, in a short-term study, that the amplitude of the 24
h rhythm of arterial blood temperature in free-ranging springbok during winter
and spring in similar climatic conditions was less than 1°C, and that core
body temperature was unaffected by day-to-day changes of environmental thermal
loads (Mitchell et al., 1997).
Also, unrestrained springbok in a 4 ha paddock maintained abdominal
temperature between 37.5 and 41.0°C, despite being subjected to summer air
temperatures varying between 16 and 39°C and high solar radiation, and
water deprivation for 17 days (Hofmeyr and
Louw, 1987
). The amplitude of the nychthemeral rhythm of body
temperature of other free-ranging African ungulates, including black
wildebeest Connochaetes gnou
(Jessen et al., 1994
), eland
(Fuller et al., 1999
), zebra
Equus burchelli (Fuller et al.,
2000
) and oryx Oryx gazella
(Maloney et al., 2002
), was
also not related to environmental thermal load. In other words, none of these
African ungulates exhibited the wide swings in body temperature characterising
adaptive heterothermy, an adaptation widely considered to be crucial for the
survival of ungulates in arid-zone habitats.
Although adaptive heterothermy, which relies on thermal inertia, typically
is ascribed to animals of large body mass, it has been reported in Thompson's
gazelle Gazella thompsonii and Grant's gazelle Gazella
granti, antelope species that are similar in size and closely related
phylogenetically to springbok. During water deprivation in a simulated desert
environment, rectal temperature of the gazelles fluctuated by between 2.5 and
3.5°C between morning and evening
(Taylor, 1970). Anomalously,
the increased amplitude of the nychthemeral body temperature rhythm in
dehydrated, compared to hydrated, animals, a feature considered a hallmark of
adaptive heterothermy, was greater in the Thompson's gazelle than in the
larger eland and oryx (Taylor,
1970
). We have argued, however, that the wide nychthemeral swings
in body temperature in captive animals, like those described above, arise
mainly from experimental protocols that deprive the animals of their natural
microclimate and access to behavioural thermoregulation (Fuller et al.,
1999
,
2004
;
Mitchell et al., 2002
) and do
not imply that the same species will employ adaptive heterothermy when living
free. Free-ranging eland, for example, sought shade during the hottest part of
the day and defended body temperature at night, possibly by interactions with
conspecifics (Fuller et al.,
1999
), thermoregulatory behaviour denied to captive animals.
Ostrowski et al. (2003)
have argued recently that our studies on eland
(Fuller et al., 1999
) and
springbok (Mitchell et al.,
1997
) failed to demonstrate adaptive heterothermy because they
were conducted over short periods when air temperatures were moderate. Their
intermittent and opportunistic measurements of abdominal temperatures of
free-living Arabian oryx Oryx leucoryx exposed to severe heat (mean
maximum air temperature of 44°C) and water deprivation in summer, revealed
that body temperature fluctuated daily by more than 4°C in summer, but by
less than 2°C in winter. However, the relationship between 24 h
fluctuations in air temperature and body temperature was weak; only 17% of the
variation in heat storage of oryx in summer could be attributed to variation
in air temperature. Only three other comparable studies have investigated
seasonal patterns of body temperature in unrestrained ungulates. Jessen and
Kuhnen (1996
) showed that the
24 h amplitude of arterial blood temperature of goats was greater in summer
than in winter; however, even in summer the amplitude was small (2°C). In
contrast, the 24 h fluctuation of body temperatures of six free-ranging mule
deer Odocoileus hemionus
(Sargeant et al., 1994
) and of
one unrestrained sheep (Bligh et al.,
1965
), like that of our springbok, did not vary across
seasons.
The key to springbok thermoregulation, in the face of nychthemeral and
seasonal changes of ambient temperature, appears to lie not in adaptive
heterothermy but in their ability to employ behavioural thermoregulation.
Although we did not observe our animals' behaviour, Hofmeyr and Louw
(1987) have reported that
springbok counter high intensity solar radiation by seeking shade or, if not
available, by reducing the body surface area exposed to solar radiation by
orienting the long axis of the body parallel to the rays of the sun. Springbok
also exploit the colouration of their pelage, by presenting the white rump,
which has a high reflectance, towards the incident radiation. During cold
nights, when the properties of the pelage make the animals vulnerable to rapid
heat loss, we believe that springbok avoid hypothermia primarily by a
combination of locomotor activity
(Mitchell et al., 1997
) and
peripheral vasoconstriction. Indeed, activity, measured with an
omnidirectional mercury switch attached to a logger on a collar, was greater
overnight for a free-ranging springbok exposed to cold winter nights than for
another springbok exposed to a warmer period in spring
(Mitchell et al., 1997
).
Seasonal patterns in locomotor activity of springbok have not been
investigated. However, time spent foraging increases in winter months, at the
cost of other activities (Davies and
Skinner, 1986
).
Even though the primary thermal problem facing arid-zone ungulates would
conventionally be considered to be that of heat, winter cold may present a
formidable challenge too. One way in which mammals, particularly those of low
body mass (<2 kg), respond to the predictable decrease in food availability
and energetically challenging demands of winter is by reducing body
temperature (for example, Wollnik and
Schmidt, 1995; Lehmer et al.,
2003
). The lowering of body temperature is not simply a passive
response to a fall in air temperature. Hamsters, for example, housed at a
constant 23°C, exhibited a 0.7°C decrease of mean body temperature,
without a change in the nychthemeral amplitude of body temperature, in
response to photoperiod being reduced from 16 h to 8 h
(Heldmaier et al., 1989
).
Similarly, and surprisingly, considering their substantial body mass, we found
that mean body temperatures of our springbok were correlated strongly with
monthly changes in photoperiod. Mean body temperature in winter, when
photoperiod was shorter by almost 3 h, was
0.3°C lower than that in
summer. Although mean 24 h body temperatures were correlated with mean 24 h
air temperatures, we do not believe that the lowering of body temperature in
winter reflected a failure of thermoregulatory ability. Rather, we think it
represented a regulated, downward shift in the set-point level around which
body temperature was maintained (Heldmaier
et al., 1989
). Nychthemeral fluctuations in body temperature,
which do not vary in amplitude over the year, are superimposed on that
setpoint. The primary environmental cue responsible for triggering the change
in set-point appears to be photoperiod. Seasonal changes in air temperature do
not provide a reliable signal for lowering of body temperature
(Heldmaier et al., 1989
). Our
finding that the times of daily minimum and maximum body temperatures of
springbok were also shifted in parallel with a change in time of sunrise
provides further evidence for the importance of the light:dark cycle in
thermoregulation of springbok. A similar 12 h seasonal shift in the
phases of the nychthemeral body temperature rhythm also has been reported for
mule deer (Sargeant et al.,
1994
).
The downward shift in mean body temperature that we observed in springbok
amounted to 0.3°C, an amount similar to that previously reported for
unrestrained goats (Jessen and Kuhnen,
1996
) and somewhat less than the 0.6 to 0.8°C reported for
sheep (Bligh et al., 1965
;
da Silva and Minomo, 1995
). The
energy saving provided by a 0.3°C lowering of body temperature in winter
may be a critical adaptation for springbok exposed to food scarcity and cold.
We have observed that nutritional stress in African antelope was associated
with an increased 24 h amplitude of body temperature, and episodes where core
body temperatures fell below 36°C (S. A. Leisegang, D. Mitchell, and A.
Fuller, unpublished). Though springbok undoubtedly encounter nutritional
stress in their natural habitats, it is unlikely that our experimental animals
were subjected to significant nutritional stress during our study; rainfall,
the primary determinant of forage availability, was unusually high in 2000
(753 mm; typical annual mean
450 mm) and rain fell in all months of the
study. All female springbok gave birth to healthy lambs 35 months after
implant surgery and were pregnant again, with foetuses of late-gestational
age, at surgery to remove loggers, 13 months after implant surgery. Such a
pattern, where a 6-month gestation period is followed by 4 months of
lactational anoestrus and then another pregnancy, is characteristic of the
springbok reproductive cycle under favourable conditions
(Skinner and Louw, 1996
).
Winter hypothermia of the springbok may well have been more marked in less
favourable conditions.
Another hypothermia, which we expected to observe but did not, was
gestational hypothermia. At least in rodents, a fall in body temperature of
0.5°C is evident in the third trimester of gestation
(Kozak, 1997
). Remarkably,
with the exception of a possible lambing event in one female, we were unable
to detect any obvious changes in body temperature patterns of females during
the approximate periods of pregnancy and lactation. In rats, body temperature
is elevated
0.5°C higher during the energetically expensive period of
lactation as a consequence of increased metabolic heat load and reduced heat
dissipation (Eliason and Fewell,
1997
). If springbok had been faced with inadequate resources, we
predict that they may have forgone precise maintenance of body temperature in
favour of meeting their energetic needs, and those of their offspring.
We cannot rule out that some degree of adaptive heterothermy may be evident
in springbok had they been subjected to food shortage, or indeed to water
deprivation. Water deprivation may also elicit diurnal dehydration
hyperthermia, a consequence of reduced evaporative water loss, which is a
well-characterised phenomenon in dehydrated animals exposed to heat
(Mitchell et al., 2002). Our
animals did not exhibit those phenomena. They maintained body temperature
within extraordinarily narrow limits while being subjected to a wide variety
of stressors over the year. Although springbok exhibited sporadic episodes of
uncontrolled hyperthermia, for example during presumed exercise and during
capture, such episodes resolved quickly. Annual and daily variations in body
temperature, like those of other freeliving antelope (see
Mitchell et al., 2002
),
appeared to reflect an endogenous rhythm rather than a reaction to
environmental thermal load. Our long-term study of body temperatures of
springbok, like others using remote-sensing technology, illustrates once more
the importance of studying thermoregulatory mechanisms in free-living animals
with access to their natural habitat, to conspecifics and to behavioural
thermoregulation.
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Acknowledgments |
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References |
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