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First published online October 18, 2006
Journal of Experimental Biology 209, 4283-4294 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02503
Water conservation in fasting northern elephant seals (Mirounga angustirostris)
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95064, USA
* Author for correspondence (e-mail: cwlester{at}comcast.net)
Accepted 21 August 2006
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1 min in duration, followed
by eupneic recovery, compared with a breathing mode with no apneas longer than
20 s and resembling typical breathing patterns in other mammals (normative
breathing). Overall REWL fell 41% from 0.075±0.013 g min-1
(mean ± s.d.) during normative breathing to 0.044±0.006 g
min-1 during apneic breathing. The decline in REWL is attributed to
a decrease in overall ventilation rate, made possible by a decline in
metabolic rate along with an increase in oxygen extraction that would occur
during apneic breathing. Data on the range of ambient humidity conditions at
the local breeding site were collected and used to bound the range of
environmental conditions used in laboratory measurements. Our data showed that
the observed variations in ambient humidity had no significant effect on REWL.
A combination of apneic breathing and the complex nasal turbinates allows
fasting elephant seals to reduce REWL well below the rate of MWP so that they
can maintain water balance during the fast.
Key words: fasting physiology, Mirounga angustirostris, Pinnipedia, respiratory evaporative water loss, water conservation
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;
Renouf et al., 1990), or those
species that venture into estuarine and riverine environments, these animals
must satisfy their needs for water with metabolic water production, the
preformed water in their food, or seawater ingestion
(Irving et al., 1935). Whereas
seawater ingestion has been measured or observed in a variety of marine
mammals (Costa, 1982;
Costa, 2002;
Costa and Gentry, 1986;
Costa and Trillmich, 1988;
Gentry, 1981;
Hui, 1981;
Lea et al., 2002), most
earless or true seals (Phocidae) either do not or cannot drink seawater, and
thus it is not considered a significant positive contributor to their overall
water budget. This therefore leaves preformed water in prey and metabolic
water production (MWP) as the primary sources of water for these species
(Costa, 2002;
Ortiz, 2001;
Storeheier and Nordøy,
2001).
The challenge of maintaining water balance becomes even more severe in
those species of marine mammals, such as the northern elephant seal,
Mirounga angustirostris (Gill), that endure prolonged fasts in arid
terrestrial environments (Ortiz et al.,
1978). In most phocid species, the female does not leave the pup
at all during nursing, but fasts during the entire period of lactation
(Bonner, 1984). Likewise, males
in the haremforming phocid species must remain on land in order to defend
their breeding territories (Le Boeuf,
1974). Both the males and females are thus prevented from leaving
their territories or pups even to drink, eliminating seawater ingestion as a
potential contributor to water balance in most cases.
Phocid pups undergo prolonged fasting as well
(Bowen et al., 1987;
Burns et al., 1999;
Crocker and Costa, 2002;
Lydersen et al., 1997;
Oftedal et al., 1989). In
northern elephant seals this post-weaning fast lasts for 9-12 weeks
(Ortiz et al., 1978;
Reiter et al., 1978), and it
takes place in often arid or semi-arid terrestrial environments, ranging from
central California to the southern Baja Peninsula
(Stewart et al., 1994). Like
the adults, elephant seal pups do not drink during the fast
(Ortiz et al., 1978), and must
therefore rely on the catabolism of body stores as their sole source of
water.
The northern elephant seal thus endures a severe challenge to water
balance: prolonged terrestrial fasting, without access to fresh water and
abstaining even from seawater consumption during the first few weeks after
weaning. It is also a well-studied system, having long been the subject of
active research covering aspects of water balance
(Adams, 1991;
Adams and Costa, 1993;
Blackwell, 1996;
Blackwell and Le Boeuf, 1993;
Ortiz et al., 1978;
Ortiz et al., 1996),
cardiovascular and respiratory physiology
(Hammond et al., 1968;
Castellini et al., 1994a;
Castellini et al., 1994b;
Kohin et al., 1997;
Milsom et al., 1996), diving
and foraging behavior (Crocker et al.,
1997; Le Boeuf et al.,
2000; Le Boeuf et al.,
1996), energetics (Crocker et
al., 2001; Houser and Costa,
2001; Noren,
2002a; Ortiz et al.,
1978; Ortiz et al.,
1984), protein catabolism
(Adams and Costa, 1993;
Crocker et al., 1998;
Houser and Costa, 2001),
thermoregulation (Bartholomew,
1954; Noren,
2002b), etc. The elephant seal is thus ideally suited for study of
the adaptive limits of mammals with regard to water conservation.
Ortiz et al. hypothesized that fasting weaned elephant seal pups could
derive sufficient water from metabolic water production (MWP) and from the
release of preformed water from catabolized tissue if water losses from
excretion and respiratory evaporation were minimal
(Ortiz et al., 1978). Huntley
et al. (Huntley et al., 1984)
documented the presence of complex nasal turbinates, convolutions in the air
passages that act as a countercurrent heat exchanger and conserve both heat
and moisture (Jackson and Schmidt-Nielsen,
1964; Schmidt-Nielsen et al.,
1970). These turbinates would thus reduce respiratory evaporative
water loss (REWL), which results from the evaporation of water across the
epithelium of the respiratory tract and is a major route of water loss for all
air-breathing endotherms. Adams and Costa
(Adams and Costa, 1993) found
that the urinary output of pups was high at first but decreased substantially
over the course of the post-weaning fast, from 
430 ml day-1 in
the early stages to 70 ml day-1 by the end of the fast. Water
loss through feces production is small, having been measured at 20 g
day-1 in fasting pups (D.P.C., unpublished data), and loss through
cutaneous evaporation is generally considered negligible in phocids due to a
lack of eccrine sweat glands (King,
1983; Renouf et al.,
1990). Together these studies confirm that fasting weaned elephant
seals pups have a series of physiological and anatomical mechanisms that allow
them to survive without an exogenous source of water by reductions in
respiratory evaporative and urinary water loss.
Huntley et al. documented the presence of a nasal counter current heat
exchanger (Huntley et al.,
1984), but they did not directly measure REWL. REWL can be further
reduced through apneustic breathing, a distinctive respiratory pattern seen in
phocids in which periods of extended breath-hold are interspersed with periods
of rapid, deep breathing (Bartholomew,
1954; Blackwell and Le Boeuf,
1993; Castellini et al.,
1994a; Castellini et al.,
1994b). Apneustic breathing is thought to increase the efficiency
of oxygen extraction, allowing the seal to take fewer breaths overall
(Costa, 2002;
Ortiz et al., 1978); this
reduces the total volume of air humidified by the lungs in taking up a given
quantity of oxygen, thereby reducing REWL. Blackwell
(Blackwell, 1996) studied the
breathing behavior of fasting pups and estimated total respiratory water loss
of a hypothetical weanling with or without apneustic breathing. However, like
Huntley et al. (Huntley et al.,
1984), she was unable to measure water loss empirically, nor was
it possible to characterize the impact of increasing apnea duration on water
economy. Furthermore, while it is possible that ambient humidity will affect
water conservation, it was unknown to what extent the differences in humidity
actually experienced by fasting pups affected REWL.
In order to determine whether increasing apnea duration leads to a reduction in water loss, and thus whether the activity of apneustic breathing is itself a behavioral strategy for water conservation, we directly measured respiratory water loss in fasting elephant seal pups and examined the relationship between REWL and apnea duration. To determine whether humidity has a noticeable effect on water conservation, we performed these measurements under a range of humidity conditions consistent with what the animals experience at the rookery. If there were variations in local climate between sites at the rookery, and if humidity could be demonstrated to have an effect on REWL, then site selection could also be an important behavioral strategy for maintaining water balance. Finally, the collected data were used, in conjunction with the previous research on other aspects of water balance, to create a water budget for an early-stage weanling elephant seal.
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); the exact ages of pups were unknown, but regular censuses
of nursing pups at the rookery revealed that all pups were weaned by 12 March
2003 and only three pups remained un-weaned by 9 March 2004 (P. Morris,
personal communication). It can therefore be concluded that all pups were at
least 6 weeks into the post-weaning fast when our trials began in 2003, and
were almost certainly at least 20 days into the fast in 2004. The experimental setup is shown in Fig. 1. At the beginning of each trial the seal was secured on a fiberglass board. A collar made of PVC pipe, approximately 30 cm in diameter, was secured to the board in order to reduce head movement. The animal's nose typically protruded 1-5 cm beyond the collar; this prevented accumulation of moisture on the collar and in the system. This was tested by using a variety of funnel-and- collar configurations such that there was no lag in the response time for relative humidity (RH) while the animal breathed or during entrance into apnea. Moisture accumulation in early iterations of the system was accompanied by a noticeable lag in the response of RH during apnea, and the design was adjusted until this lag no longer occurred.
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REWL was determined by measuring relative humidity in an open-flow
respirometry system (Withers,
1977). Exhalations were captured using a Teflon funnel (Welch
Fluorocarbon, Dover, NH, USA) and drawn by vacuum into a 1 m-long Teflon tube
(Harrington Industrial Plastics, Fremont, CA, USA), connected to a Singer dry
gas meter (American Meter Company, Horsham, PA, USA). A sample line made of
Bev-a-Line (Thermoplastic Processes, Inc., Warren, NJ, USA) carried a fraction
of the incurrent air to a Sable Systems RH-100 relative humidity analyzer
(Sable Systems, Las Vegas, NV, USA) 30 cm anterior to the funnel. Excurrent
air from the RH-100 was carried to an oxygen sensor (I) (AEI Technologies,
Sunnyvale, CA, USA). The use of Teflon and Bev-a-Line was necessary in order
to minimize adhesion of water vapor and ensure maximum accuracy in humidity
measurements. The accuracy of this technique for determining respiratory water
loss has been previously verified, with a correlation coefficient of 0.999
(Hammarlund et al., 1986). The
RH-100 was calibrated before data collection using dry compressed
N2 gas for the zero-point, and the saturation-point for a known air
temperature was set by passing air through an aquarium aerator stone immersed
in water. Air flow through the main line varied between 95 and 121 l
min-1. Air flow through the sample line in 2003 was 300 ml
min-1 in 2003, 150 ml min-1 of which passed through the
O2 sensor; and in 2004 was 400 ml min-1, all of which
passed through the O2 sensor. These flow rates were necessary to
ensure complete capture of exhalations and rapid clearance of water vapor from
the system, and to allow coincident measurements of RH and O2.
Relative humidity and O2 depletion were recorded using Datacan data
acquisition software (Sable Systems, Las Vegas, NV, USA).
Each seal was used in at least two trials, 1-5 days apart, typically lasting 3 h. Trial duration was dictated by the animal's behavior, which was monitored throughout the trial. Data were only used when the animal was calm and exhalations were being fully collected.
Defining breathing intervals
Two distinct modes of breathing were identified, and these were broken into
discrete intervals for analysis. Each such interval is hereafter referred to
as a `measurement interval', and constitutes a single data point in
analysis.
In the first mode, the animals alternated between apneas of 1 min and
subsequent recovery periods of eupnea. During these recovery periods, rate and
depth of breathing and the rate of oxygen consumption were increased with
respect to periods of breathing that did not immediately follow a period of
apnea. We designated this behavior as S-phase breathing, since this was
principally observed during sleep
(Castellini et al., 1994a;
Castellini et al., 1994c;
Huntley, 1984). Each interval
of S-phase was measured from the beginning of an apnea to the end of the
eupneic recovery period that followed it; the metabolic water production and
respiratory water loss calculated for the recovery period were averaged over
the entire cycle. We considered a recovery to be completed under either of two
conditions: (1) if breathing ceased entirely and the animal returned to apnea,
or (2) if deflections in the O2 trace were reduced suddenly by more
than 25% and then persisted at a steady intermediate level for at least 1 min
before returning to apnea. This latter condition was interpreted to be a
temporary shift to the second mode of breathing described below. However, if a
period of breathing that followed an apnea was less than half the length, and
the O2 deflection was less than half as great as for the other
eupneic periods following apneas during the same experimental trial, then the
recovery was judged not to be complete and this apnea/eupnea period was
combined with the next such period in the recording. The times for both apneas
were then added together to determine the total apnea duration for the S-phase
interval in question, and both eupneic periods were considered part of the
recovery from apnea. Other cues to the end of a recovery period included a
sudden reduction in the rate and depth of breathing, and/or resumed awareness
and movement by the animal. Although the methods used to identify changes in
breathing mode were largely qualitative, subsequent analysis showed a clear
quantitative difference in breathing rate between the two modes that upheld
the observations taken during the trials (see Results).
The second commonly observed mode of breathing resembled `normal' breathing
in other mammals. In this mode there were no periods of breath-hold longer
than 1 min, and depth and rate of breathing were variable but always less than
was seen in a post-apnea recovery period during S-phase breathing. Because
this behavior was commonly associated with wakefulness
(Castellini et al., 1994a;
Huntley, 1984), we have
designated it as W-phase breathing. Sometimes an interval of W-phase breathing
followed the recovery period of an S-phase interval, when the seal awoke
briefly before returning to apnea and sleep; these instances were easily
identified both by the change in breathing pattern and by the reduction in
oxygen consumption mentioned above. At other times the animal would remain in
W-phase for extended periods, sometimes in excess of 1 h, until the trial was
concluded or the animal returned to S-phase once more.
Extended eupnea was often accompanied by activity and it was rare to record a long `bout' of W-phase breathing without interruption. Therefore, data for W-phase breathing were only used when there were no vocalizations, no missed breaths (which could occur if the animal withdrew its nose from the funnel), and no post-apnea recovery (determined as described above), and when the interval of breathing was at least 1 min in duration (if following a recovery period and ending in a return to apnea) or at least 3 min in duration (if part of a longer period of wakefulness).
Humidity and temperature
RH data were transformed to measurements of absolute humidity (g
H2O min-1), and polynomial baseline correction was used
to subtract out ambient humidity, thus giving g H2O enrichment
min-1 (i.e. REWL). The method was modeled after that used to
calculate ml O2 consumption from continuous %O2
measurements taken in respirometry
(Withers, 1977), with the
added complication that RH must be converted to absolute humidity by
multiplying the measured RH by the saturation vapor density (SVD), which is
temperature-dependent. Polynomial baseline correction is a partially automated
feature included in the Datacan analysis software (Sable Systems, Las Vegas,
NV, USA), and is described in detail in its operations manual. Briefly, the
technique uses points throughout the data record that have been defined by the
user as baseline (i.e. points at which ambient conditions are represented
without added input from the test animal) to generate a polynomial curve that
is then subtracted from the raw data curve, producing an output in which
deviations from baseline are represented as fluctuations above and below zero.
Integration of data over a baseline-corrected curve yields values of
H2O enrichment or O2 depletion in which the baseline
drift common in respirometry has already been taken into account. We used the
animals' own periods of apnea as the primary points for baseline correction,
since both O2 concentration and RH returned to ambient levels while
the animal was in breath-hold.
In 2004, absolute ambient humidity was modified to vary between 7.00 and
11.21 g m-3 using a dehumidifier (Sears, Roebuck and Co., Hoffman
Estates, IL, USA). Temperature was monitored during the trial and used
together with RH to determine absolute humidity. The rate of oxygen
consumption was determined according to Withers
(Withers, 1977), assuming a
respiratory quotient (RQ) of 0.711, calculated for the low protein catabolism
of fasting elephant seals (Pernia et al.,
1989; Houser and Costa,
2001).
Oxygen consumption and metabolic water production
Data were integrated over the measurement intervals to give REWL and ml
O2 consumed. Metabolic water production (MWP) was calculated from
O2 consumption using a conversion factor of
5.23x10-4 g H2O ml-1 O2,
following the equation:
 | (1) |
where W is the mass of metabolic water produced per gram of lipid
or protein, 
O is the volume
of oxygen consumed per gram of lipid or protein, and % lipid and % protein are
the relative contributions of each to metabolism. We used the values obtained
by Costa and Williams (Costa and Williams,
2000) for these metabolic constants and assumed a contribution of
protein catabolism of 6%, based on the mean mass losses from lean and adipose
tissue reported by Rea and Costa (Rea and
Costa, 1992) and Houser and Costa
(Houser and Costa, 2001); see
`Estimating water budget' in the Discussion for a more detailed description of
the calculations used.
Data analysis
Data were analyzed using SYSTAT 10.2 (Systat Software, Richmond, CA, USA).
Unless otherwise noted, data are summarized as mean ± s.d. REWL was
divided by MWP to standardize for differences in metabolic rate, and the
resulting water loss ratio was examined in an analysis of covariance (ANCOVA)
against the following factors: individual seal (fixed effect variable), apnea
duration and absolute humidity. All interaction terms were examined, and
removed from the model if not significant. Additional analysis was performed
on total water lost by respiratory evaporation for a measurement interval
(EW_Total) in a multiple linear regression against apnea duration and the
total oxygen consumed over the same interval (O2_Con).
Measuring ambient conditions
Weather stations were placed at two sites at Año Nuevo State
Reserve, 30 km north of Santa Cruz, CA. These sites reflected the extremes
likely to be observed within the habitat used by the fasting pups. The station
at Mid-Bight Beach (MBB) was located atop a small dune on a broad, sandy shelf
approximately 3 m above sea level, directly exposed to wind coming in from the
sea. The second station, at Año Point Dunes (APD), was more sheltered,
sitting at the edge of a small tussock of arroyo willows (Salix
lasiolepis) northeast of a broad sandy expanse that sloped gradually down
toward the water. Monitor II weather stations (Davis Instruments, Hayward, CA,
USA) recorded temperature and relative humidity from 23 March to 1 June 2002
(for APD) or 8 June 2002 (for MBB). Data were collected continuously, averaged
and stored every 15 min, and downloaded every 7 days. Data were grouped into
equivalent 15 min blocks of the 24 h cycle and were averaged together to
create an `average day' for each site that was divided into 15 min
increments.
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When data for all breathing intervals were combined for each seal, mean values for the water loss ratio REWL/MWP ranged from 0.309 to 0.538. Mean water loss ratios were significantly higher in 2004 than in 2003 (t=4.656, P<0.01), and males had significantly higher mean water loss ratios than females (t=5.09, P <<0.001).
An analysis of covariance indicated that water loss ratio was significantly related to apnea duration; the interaction term between individual seal and absolute humidity was also highly significant (Table 2). There were no other significant interaction terms. Absolute humidity for all lab trials ranged from 7.00 to 11.21 g m-3; in 2004, when humidity was experimentally manipulated, the absolute difference in humidity between trials for any individual seal ranged from 0.5 to 3 g m-3.
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Water loss ratios showed high variance at short apnea durations, but became
less variable and declined as apneas became longer
(Fig. 2A). This pattern was
similar to the trend in metabolic rate
(Fig. 2B). A plot of total
respiratory water loss (EW_Total) against total oxygen consumption
(O2_Con) revealed a strong linear trend
(Fig. 2C). EW_Total increased
linearly with both O2_Con and apnea duration
(Durationap), following the equation:
 | (2) |
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The ratio of apnea duration to interval time during S-phase breathing was notably consistent across all seals; on average, 67.5%±5.5% of time during S-phase breathing was spent in apnea, with 32.5% of time being spent in recovery breathing. Post-apnea recovery was associated with a markedly higher breathing rate than breathing intervals not associated with apnea (t=7.66, P <<0.001): mean breathing rate during eupnea, BRE, averaged 9.57±1.89 breaths min-1 during the recovery periods of S-phase breathing but only 5.15±0.86 breaths min-1 during W-phase breathing.
Whereas breathing during the recovery period itself was greatly increased,
breathing rate as averaged over total time interval declined during S-phase
breathing. We averaged the breaths taken in a given interval over the entire
time period, including apnea, to produce mean overall breathing rate,
BRO. BRO declined as apnea duration increased
(R2=0.386, F=250, P<0.01). During
S-phase, mean BRO across all seals was 3.04±1.36 breaths
min-1; during W-phase, it was 5.96±2.15 breaths
min-1. There was a positive linear relationship between
BRO and the rate of evaporative water loss, described by the
equation:
 | (3) |
where REWL is in g H2O min-1 and BRO is in breaths min-1 (R2=0.482, F=370, P<0.01).
Ambient weather conditions at breeding site
Temperature and humidity showed substantially different diurnal patterns at
the two weather station sites. Temperature at APD ranged from a minimum of
4.7°C (06:15 h, 8 May) to a maximum of 26.5°C (13:15 h, 22 April);
absolute humidity ranged from 5.19 g m-3 to 14.59 g m-3.
At MBB, temperature ranged from 2.7°C (05:00 h, 8 May) to 25.5°C
(16:15 h, 24 April); absolute humidity ranged from 5.95 g m-3 to
15.49 g m-3. Absolute humidity was approximately normally
distributed for both sites; means were 7.84 g m-3 for APD and 9.53
g m-3 for MBB, a highly significant difference (Student
t-test, t=7.31, P<0.01).
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There appear to be two mechanisms by which S-phase breathing reduces
evaporative water loss. First, S-phase breathing leads to, or at least is
strongly associated with, a reduction in metabolic rate
(Fig. 2B). Metabolic depression
in conjunction with extended breath-hold has long been characterized as part
of the dive response in marine mammals
(Butler and Jones, 1982;
Castellini et al., 1992;
Hurley and Costa, 2001;
Scholander, 1940), and the
same correlation is evident here. Furthermore, the pattern observed here, in
which variance is high during W-phase breathing and becomes constrained during
extended apnea, is comparable to that observed in free-diving Weddell seals
(Williams et al., 2004) and in
California sea lions performing trained submersions
(Hurley and Costa, 2001). In
both diving animals and fasting elephant seals the hypometabolism induced by
apnea results in fewer breaths being taken, which leads to decreased water
loss as fewer lungfuls of air are humidified and expelled from the body.
The second way in which S-phase breathing reduces water loss is by
facilitating increased oxygen extraction efficiency. Increasing apnea duration
leads to an increase in oxygen extracted per breath
(Irving et al., 1941;
Kramer, 1988;
Ridgway, 1972). This can be
explained by both an increase in the time allowed for oxygen exchange in the
lungs and by the low partial pressure of oxygen in the blood after a long
apnea, which results an increased partial pressure differential between the
blood and the inspired air and a faster rate of oxygen diffusion. As oxygen is
extracted more efficiently from the lungs, the seal can take fewer breaths,
and again the amount of water lost by the animal is reduced.
We found no evidence that changes in ambient humidity affected respiratory
water loss. The range of humidity conditions we were able to produce in the
lab reflected the average range of absolute humidity measured at the
Año Nuevo rookery, and over this range of values there was no
relationship between humidity and the water loss ratio. The significant
interaction term observed in the analysis of covariance between individual
seals and absolute humidity (Table
2) resulted from the fact that different seals showed markedly
different responses to the low- and high-humidity trials: some seals showed
lower REWL/MWP during the high-humidity trials, but some showed higher values,
and others showed little or no change. Therefore, we conclude that the
interaction term is a statistical artifact, and that changes in REWL/MWP
between trials were driven more by random variations in individual seal
behavior than by differences in humidity. Given the known effect of ambient
humidity on the performance of the nasal turbinates, this may appear
counterintuitive (Huntley et al.,
1984). However, the absolute humidity inside a mammal's lungs
(assuming 37°C core body temperature and 100% saturation) is 44 g
m-3. Thus, the humidity differential between the lungs and the
ambient air over the range of conditions measured in the field is only about
9.5% greater at 7 g m-3 than it is at 11.2 g m-3; over
that small a range, it is not surprising that there was no effect of changes
in humidity on water loss. These results suggest that elephant seals are
well-adapted to the near shore coastal environment in which they raise and
wean their pups, and that even the driest conditions in this maritime climate
will not adversely affect their overall water economy.
There are no clear explanations for why mean water loss ratios were higher in 2004 than in 2003, or why males showed higher water loss ratios than females. Given that the majority of our subjects were female in 2003, whereas in 2004 the majority were male, these two factors may well be intertwined. Given that there were no significant differences in mass either between years or between sexes, it seems unlikely that body size played a major role in the observed differences, nor can we identify any other single factor that seems likely to be responsible. Given the small sample sizes involved when comparing years or sexes (N=5 for 2003, N=8 for 2004; N=5 for female, N=8 for male), we would caution against assigning any great significance to the differences between these groups, as these differences could simply be the result of random variation in the study population. The differences observed within each of our individual seals, due to S-phase versus W-phase breathing, are far better attested and can be interpreted with much more confidence than the differences between sexes and study years; additional study of a much larger sample size would be required to confidently ascertain whether there is a sex bias in water conservation during the post-weaning fast.
Role and origins of apnea
The central importance of S-phase breathing in improving water conservation
in fasting seals leads to the question of when and how this behavior
originated. Terrestrial apnea is common to all phocids
(Knopper and Boily, 2000),
including those that do not engage in prolonged fasting; since fasting is
considered a derived trait in seals rather than the ancestral condition
(Costa, 1993), it can be
concluded that S-phase breathing did not originally evolve as an adaptation to
this life history pattern.
Given the similarity in metabolic responses between terrestrial apnea and
breath-hold while diving (Castellini,
1996), it seems evident that this behavior developed in response
to the need for increased dive duration to improve foraging efficiency. A
reduction in metabolic rate while diving reduces the rate at which oxygen
stores are depleted, increasing foraging time while avoiding the long
post-dive surface intervals associated with anaerobic diving
(Costa et al., 2001;
Hurley and Costa, 2001).
Furthermore, the increase in oxygen extracted per breath caused by prolonged
apnea allows for more rapid replenishment of oxygen at the surface and a
reduction in the post-dive surface interval, so long dives are doubly
effective in increasing foraging efficiency
(Kramer, 1988). The reduction
in water loss is a useful ancillary benefit that was probably exploited later
to enable prolonged fasting behavior. This would make terrestrial S-phase
breathing a classic example of exaptation
(Gould and Vrba, 1982).
Although it is true that phocids dive on exhalation, whereas terrestrial apnea begins on inhalation, this need not mean that the two behaviors are unrelated. The ancestors of modern pinnipeds may well have begun by diving on inhalation, with their descendants only beginning to dive on exhalation when they expanded their range to forage at greater depths. This additional adaptation to the effects of pressure at depth need not have carried over to terrestrial apnea, where it is advantageous to hold breath on inhalation and thus extract as much oxygen from the lungs as possible. Terrestrial apnea might therefore represent a throwback to the ancestral pattern of diving, before the challenges of deep-water foraging necessitated a transition to diving on exhalation.
Nasal turbinates
We can estimate the effects of nasal turbinates by comparison with one of
the seal's close phylogenetic relatives, the domestic dog (Canis lupus
familiaris). Although there may be other differences in the respiratory
systems of seals and dogs that could contribute to differences in water
economy, the exceptional complexity of the phocid turbinates is the most
striking difference between them. It must be cautioned that phocid lungs have
a number of other important anatomical differences that set them aside from
the typical mammalian structure, such as reinforced airways, a unilobular lung
and unique alveoli (Drabek and Kooyman,
1984; Kooyman,
1973); however, these differences are not likely to contribute to
differences in water conservation, as they occur within the lungs themselves
where, in both dogs and seals, the air is already warmed to body temperature
and completely saturated with water
(Schmidt-Nielsen et al.,
1970).
In a study of breathing behaviors in dogs at varying levels of ambient
temperature and exercise intensity, Goldberg et al.
(Goldberg et al., 1981) found
that the temperature of the air exhaled through the nose at rest
(TE) followed a positive linear relationship with ambient
temperature (TA) that was elevated above and shallower in
slope than the line TE=TA. Mean
temperature for our experimental trials was 19.4°C; at 20°C, the dogs
in the aforementioned study had a mean expired air temperature of about
26°C (as judged by fig. 3 in the referenced paper). Given this, and
assuming that expired air is effectively 100% saturated
(Schmidt-Nielsen et al.,
1970), percent recovery of respiratory water will follow the
equation given by Collins et al. (Collins
et al., 1971):
 | (4) |
where WI is the water content of the inspired air, WE is the water content of the exhaled air, and WB is the water content of the air in the lungs (i.e. at body temperature). With TB=38°C, TE=26°C and an absolute humidity of 9.0 g m-3 (the mean value for our experimental trials), the percentage recovery by the canine nasal passages is 58.7%. Based on our findings - using empirical measurements of water loss, rather than the estimate given by Eqn 4 - the percentage recovery by the phocid nasal passages under the same conditions is 92.5% for W-phase breathing. Water loss per breath is thus only 7.5% of the water vapor added to the air in the lungs, whereas the dog's loss per breath (41.3%) is proportionately much higher. The use of S-phase breathing slightly increases water loss per breath (recovery is 91.2%), but the mean overall breathing rate (BRO) is also reduced by 49%, and thus the efficiency of the entire breathing process is nearly doubled. If the dog's breathing rate while resting is comparable to that of the seal in W-phase, then a dog's REWL will be 9.6 times that of a seal in S-phase.
Comparisons to other homeotherms
Whereas REWL has rarely been measured directly, percentage water recovery
has been estimated from expired air temperature for a variety of mammals and
birds (Table 3). It is clear
that the two phocid species examined, the elephant seal and the grey seal
(Halichoerus grypus), both possess water conservation systems that
are among the most efficient for any homeotherm, rivaling even the
arid-adapted kangaroo rat (Dipodomys sp.). Their water conservation
capabilities are far beyond those not only of dogs, but nearly every large
mammal examined. Remarkably, estimated water recovery in grey seals reaches
80% even at -20°C, when the moisture content of the air is effectively
zero (Folkow and Blix, 1987).
This is particularly telling in light of the fact that the water recovery of
the kangaroo rat under similarly dry conditions is only 56%
(Jackson and Schmidt-Nielsen,
1964).
|
Another marine-adapted species, the herring gull (Larus
argentatus), is also among the most water-efficient animals studied, with
a water recovery of 76% under temperate, relatively dry conditions
(Geist, 2000). Likewise, the
Adelie and Gentoo penguins (Pygoscelis adeliae and P. papua
ellsworthi) displayed water recovery rates of 81.9% at 5°C, when
ambient humidity was less than 5 g m-3
(Murrish, 1973). It would
appear that the pressure to conserve respiratory water in the marine
environment is comparable to that seen in desertadapted species.
Estimating water budget
Together with past findings on sources of water loss and metabolic water
production, the data from this study can be used to model a complete water
budget for a fasting elephant seal pup
(Table 4). For simplicity's
sake, we consider a pup in the early stages of the fast, when these animals
enter the water rarely, or not at all
(Thorson and Le Boeuf, 1994),
and overall activity levels are minimal. Although our own study animals were
older, the benefits of S-phase breathing should be similar for pups at any
point in the fast, as Blackwell (Blackwell,
1996) found that the percentage of time spent in terrestrial apnea
does not change over the course of the fast; only the length of mean apnea
duration increases, from 4.0 min to 8.0 min (fig. 2.8 in referenced volume).
Our results show that there is little change in REWL/MWP over this range, so
we may assume that the benefits of S-phase breathing do not change during the
fast.
|
MWP is dependent upon the quantity and type of body stores used in
metabolism. Houser and Costa (Houser and
Costa, 2001) found that fasting pups lost 600 g day-1
of mass; Rea and Costa (Rea and Costa,
1992) reported a rate of 870 g day-1 in the first 4
weeks of the fast and 510 g day-1 in the second 4 weeks, whereas
Noren et al. (Noren et al.,
2003) reported a rate of 626 g day-1 for a seal with a
weaning mass of 121 kg. To provide a conservative estimate of water available
for our hypothetical seal we used a mass loss rate of 600 g day-1.
Rea and Costa (Rea and Costa,
1992) found that on average seals lost 7.2 kg of lean mass and
14.2 kg of adipose tissue between the sixth and twelfth week of life (i.e.
weeks 2-8 of the post-weaning fast), whereas Houser and Costa
(Houser and Costa, 2001)
reported mean losses of 6.5 kg of lean mass and 14.9 kg of adipose tissue.
Taking the mean of these results, we find that the average mass loss across
both studies was 32.0% lean tissue and 68.0% adipose tissue; we therefore
assumed that 408 g of blubber and 192 g of lean tissue per day were
contributed to metabolism. The values in
Table 4 assume that blubber is
approximately 10% preformed water
(Crocker, 1995), lean tissue
is 73% water (Pace and Rathbun,
1945), and lipid and protein catabolism produce 1.071 g and 0.396
g H2O, respectively, per g of tissue
(Costa and Williams,
2000).
REWL estimates are based on past observations of the activity budgets of
fasting pups (Blackwell, 1996).
From these observations, it can be calculated that a pup spends 38% of its
time, or 9.12 h per day, in S-phase breathing. For an early-stage pup the
remainder of the time, 14.88 h per day, is spent awake but relatively inactive
on land, which equates well to the W-phase breathing measured in the lab.
During S-phase breathing, the seals in this study lost water through
respiration at an average rate of 0.044 g min-1, or 2.64 g
h-1. During W-phase breathing, REWL was 0.075 g min-1,
or 4.5 g h-1. With the time budget listed above, this leads to REWL
of 24 g day-1 during S-phase breathing and 67 g day-1
during W-phase breathing, for a total REWL of 91 g day-1.
All other factors in the water budget are drawn from previous studies
(Table 4). Using these values,
our model seal in the early stages of the fast shows a net water surplus of 51
g day-1. Using higher proportions of adipose tissue consumption
results in a greater surplus, up to 92 g day-1 when 90% adipose
tissue use is assumed, as might be the case in a well-fed pup with a very
large amount of adipose tissue. Even if we assume a minimum of 50% adipose
tissue use, as might be the case in a very lean pup attempting to conserve
blubber for insulation, the model still yields a surplus of 26 g
day-1. In reality, any excess water not lost to evaporation would
result in an increased urine output and a lower urine osmotic concentration;
however, since urinary water loss has been measured and is already
incorporated into the model, it is likely that this additional water is
instead being lost during the small amounts of activity displayed by pups in
the early stages of the fast, since increased activity leads to an increase in
REWL/MWP. It is worth noting that urinary water loss declines greatly over the
course of the fast (Adams and Costa,
1993), while at the same time activity is increasing
(Blackwell, 1996). Because
urine contains the surplus water beyond what is necessary to maintain normal
hydration, it is difficult to tell what proportion of the high urine output
early in the fast represents a constraint on activity (due to obligatory
excretion) as opposed to being simply the result of a lack of activity. Urea
concentration in the urine increases significantly over the course of the
fast, even as total urinary nitrogen excretion declines
(Adams and Costa, 1993). This
suggests that early in the fast pups are excreting more water in the urine
than is necessary for nitrogen excretion, and that at least during this
initial period they have more water than is necessary to maintain
homeostasis.
The complexity of the situation is further increased by the effects of the
hormones used for regulation of urine output. Fasting elephant seal pups rely
on the renin-angiotensin-aldosterone system (RAAS) in regulating conservation
of both water and electrolytes; vasopressin (AVP) remains at relatively low
and constant levels throughout the fast
(Ortiz et al., 2000). Because
the fasting pups are not drinking sea water, retention of electrolytes is
important to prevent hyponatremia, and the sensitivity of the RAAS pathway is
increased in these animals (Ortiz et al.,
2000). However, angiotensin II and AVP are vasoconstrictors,
whereas atrial natriuretic peptide (ANP) is a vasoconstrictor-inhibitor;
during sleep apnea in elephant and Weddell seals, angiotensin II and AVP
levels decline while ANP levels increase
(Zenteno-Savin and Castellini,
1998). This is attributed to the decline in heart rate during
apnea and the consequent increase in cardiac pressure, which stimulates ANP
release (Ortiz, 2001;
Stanton, 1991). Thus, the use
of apnea in S-phase leads to a reduction in respiratory water loss, but also
stimulates hormonal changes that could result in the loss of electrolytes.
This indicates that there may be a tradeoff between respiratory evaporative
water loss, which does not result in electrolyte loss, and urinary electrolyte
and water conservation later in the fast. In the late stages of the fast, the
need to conserve electrolytes may become more important than maintaining a low
REWL/MWP.
The decline in urine production over the course of the fast can thus be seen partly as a result of declining urea production, partly as a means of electrolyte conservation, and partly as a result of more water being lost through respiration as the pup devotes more time to activity. Animals that are more active will have higher REWL/MWP, leading to a decline in the surplus water that would otherwise be lost in the urine; at the same time, hormonal regulation forces a decline in urine output in order to preserve electrolyte balance, and may set an upper limit on the amount of time that a fasting seal can afford to spend in S-phase breathing, since apnea is associated with increased production of hormones that promote electrolyte loss. The addition of diving behavior further complicates matters, as apnea during diving will result in metabolic suppression that will partly counteract the tendency of heightened activity to increase REWL/MWP. It is not possible to tease apart the relative importance of these factors to the overall water budget of a late-stage fasting seal from the available data.
Conclusions
The results of this study reveal that fasting elephant seals have at their
disposal a range of behavioral and anatomical adaptations to restrict
respiratory water loss. The physiological adaptation of the nasal turbinates
provides an impressive benefit to water economy in its own right, whereas the
use of apnea in S-phase breathing has the dual benefit of reducing metabolic
rate and greatly increasing the oxygen extraction efficiency. The combination
of these two factors leads to a drop in mean breathing rate of 49%,
effectively cutting the animal's water costs in half as long as it remains in
this mode of breathing.
If phocids had a respiratory system comparable to that of a terrestrial carnivore, such as the dog, they would lose 41.3% of the water their lungs added to every breath, instead of only 7.3%. Under these conditions, they could not produce adequate water through metabolism alone, and they would not be able to carry out the prolonged post-weaning fast without an exogenous source of water. The challenges of extended fasting in a terrestrial environment are severe, but our results show the importance of physiological mechanisms associated with the dive response - notably the capacity for extended breath-hold coupled with hypometabolism - in pre-adapting these animals to face extended bouts of water deprivation with the behavioral strategies necessary to survive them. At the same time, the capacity for water recovery displayed in their respiratory physiology shows them to be one of the most efficient homeotherms in minimizing REWL. As urine output declines over the course of the fast, more water becomes available for loss through respiration in conjunction with activity. The physiological and behavioral requirements for water conservation must be balanced against the requirements for development, but it is evident that elephant seal pups have the capacity to face them both.
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Acknowledgments |
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Footnotes |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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