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MARINE MAMMAL SCIENCE, 7(4):369-384
0 1991 by the Society for Marine Mammalogy
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(October 1991)
DIVING PATTERNS OF NORTHERN
ELEPHANT SEAL BULLS
ROBERTL. DELONG
National Marine Mammal Laboratory, NMFS, NOAA, Bldg. 4,
7600 Sand Point Way, N.E.,
Seattle, Washington, USA 98 115
BRENTS. STEWART~
Hubbs-Sea World Research Institute, 1700 South Shores Rd.,
San Diego, California, USA 92109
AEWRACT
We used small microprocessor-based, time-depth recorders to document the
diving patterns of six adult male northern elephant seals (Mirounga angustirostris)
from San Miguel Island, California. The recorders stored measurements of hydrostatic pressure every 30 or 60 set while the seals were at sea for 107 to 145
d in spring and early summer; collectively, over 36,000 dives were recorded.
Seals dove continually while at sea, most often to depths of 3 50-45 0 m although
two seals had secondary modes at about 700-800
m; maximum depths for
two seals of 1,3 3 3 m and 1,5 29 m are the deepest yet measured for air-breathing
vertebrates. Seals were submerged about 86% of the time they were at sea,
rarely spending more than 5 min at the surface between dives; 99% of all postdive surface intervals were shorter than 10 min. Dives averaged 21-24 min,
the longest was 77 min. The uninterrupted patterns of long dives punctuated
by brief surface periods suggest that most if not all dives were well within these
seals’ aerobic limits. Dives of bulls were, on average, about 18% longer than
those published earlier for cows, evidently because of the substantially greater
body mass of bulls and allometric scaling of dive endurance. Dive depths and
dive durations varied seasonally; depths were greatest in spring, durations greatest
in early summer. During each season dives were deepest during the day and
shallowest at night except for the sixth seal whose consistently shallow dives
(50-150 m) in spring were independent of time of day. Prey remains recovered
by lavage from seals’ stomachs were primarily of vertically migrating, epi- and
meso-pelagic squid. The die1 patterns in dive depths suggest that five seals dove
to and foraged in the offshore mesopelagic zone, pursuing those vertically migrating prey. The sixth seal behaved similarly in early spring and early summer
but may have foraged in nearshore epibenthic habitats in spring.
Key words: pinnipedia, northern elephant seal, Mirounga
depth recorder, diving patterns, foraging ecology.
i Correspondingauthor: Brent S. Stewart.
369
angustirostris, time-
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Adult northern elephant seal males fast while ashore for up to three months
in winter when breeding and about one month in summer when molting (Stewart
1989). Bulls have been well studied during these terrestrial phases (e.g., Le
Boeuf 1974, Sandegren
1976, Cox 1981, Stewart 1989) but knowledge
of
their behavior and ecology when at sea for eight to nine months each year has
been limited to a few brief encounters with scientists and fishermen in coastal
waters (Condit and Le Boeuf 1984; C. L. Hubbs, unpublished
notes; S. Leatherwood, personal communication;
B. S. Stewart and R. L. DeLong, personal
observation). The annual terrestrial and aquatic schedules of adult females differ
somewhat from those of males (Stewart 1989; Huber et ul., in press) but recent
studies have shown that when at sea for about two months in spring, cows dive
and presumably
forage continually
until they return ashore to molt (Le Boeuf
et a[, 1989, Stewart and DeLong, unpublished
data). Here we describe the
diving patterns of six bull elephant seals from San Miguel Island, the largest
rookery in the species’ range (Stewart 1989), when they were at sea for four to
five months in spring 1988 and compare these patterns with dive data for cows.
MATERIALS
AND
METHODS
We immobilized
eight northern elephant seal bulls in late February at San
Miguel Island, California (34”02’N,
120”23’W) with intramuscular
injections
of ketamine hydrochloride
(1.5-3.5
mg/kg
body mass) and then glued microprocessor-based,
time-depth
recorders (TDRs) and VHF transmitters to their
dorsal hair with quick-setting
epoxy. The transmitters
permitted us to relocate
seals when they returned ashore to molt in summer. The TDRs are described
in detail by Stewart et al. (1989). Briefly, they were 15 cm long, 2.9 cm in
diameter and weighed 196 g. A two-stage pressure transducer measured depth
to 2,000 m with about 6 m accuracy and stored the readings in electronic
memory with reference to an internal clock. We programmed
the TDRs to
sample depth at 60 set intervals for four seals (440, 560, 640 and 666). For
the other two seals (5 16 and 618), data were collected at 30-set intervals for
the first 30 d; then, periods of 4 d when no measurements
were made alternated
with 4-d periods during which depth measurements
were made every 30 sec.
We used the punctuated
sampling protocol with the 30 set rate to examine the
influences of the longer 60 set sampling rate which was required for uninterrupted, five-month recordings. When the seals returned to land to molt in July
we chemically immobilized
them and removed the instruments;
to assess seals’
diets we lavaged their stomachs using techniques described by Antonelis et al.
(1987). Prey species were identified by comparing the recovered fish otoliths,
cephalopod mouth parts, invertebrate
exoskeleton, and tunicates with reference
materials at the National Marine Mammal Laboratory (Seattle, WA).
Statistical analyses of dive parameters were performed using Systat (Wilkinson
1988); we used one way analysis of variance (ANOVA)
with a Tukey multiple
range test (Sokal and Rohlf 1981) except where noted in the text. Parameter
values of sequential dives were correlated (first-order Markov process) but the
correlation became insignificant beyond the third dive in a series. Therefore, for
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each seal’s records we created a second data set by randomly sampling 20% of
the dives from the complete record and assumed that dives in that sub-sample
were independent of one another. We report sample statistics (i.e., 2 = sample
mean, SD = standard deviation of the sample mean) as summary statistics for
all dives of each seal and population statistics (i.e., P = population mean, SEM
= standard error of the population mean) as summary statistics for all dives of
all seals. Statistical tests were performed on the sub-samples.
RESULTS
All seals left San Miguel Island by 11 March (Julian date = 70). We recovered
TDRs from six of the eight instrumented bulls in July when they returned to
San Miguel Island to molt. The instruments contained dive data for the seals’
entire periods at sea; collectively 36,233 dives were recorded. We analyzed six
variables for each dive: dive depth (m) = the maximum depth reached during
a dive; dive duration (min) = elapsed time between submergence and surfacing;
rate of descent (m/set) = depth at bottom of a dive/time elapsed to reach
that depth; rate of ascent (m/set) = depth at bottom of a dive/time elapsed
to reach the surface; bottom time (min) = the amount of time spent at the
bottom of a dive; surface interval (min) = the amount of time spent at the
surface following a dive. After visual inspection of printed records of seals’ dive
patterns we divided each record into three Julian-date periods as follows: early
spring = 60-100;
spring = 101-160;
early summer = 161-200.
Seals were at sea for 130 d, on average (SD = 13 d, range = 107-145
d).
Each began diving immediately after entering the water in March and dove
continually with only rare interruptions until returning to shore in July. Time
spent at the surface between dives was brief; only 0.39 to 3.2% (a = l.O%,
SD = 1.1%) of each seal’s surface intervals were greater than 5 min. On average,
seals were submerged about 86% (SD = 2.5%, range = 82.5~86.9%)
of the
time they were at sea; bottom time accounted for about 25% (SD = 4.9%,
range = 19.2-34%)
of the time seals were at sea. Thus seals were submerged
about 2 1 of every 24 h, 15 h of which were spent descending to and ascending
from depth and the other 6 h spent at the bottoms of dives.
Depth-Dive
depths of all seals averaged 388.8 m (SEM = 16.5 m; Fig.
1); the deepest dives ranged from 978 (seal 440) to 1,529 m (seal 666) with
an average of 6.2% (SEM = 2.3%, range = 0.3-12.6%)
of dives exceeding
700 m. There was significant variation among seals in their diving patterns (P
< 0.00 1). Five of the seals dove most frequently to depths of 3 50-450
m but
two of those (618 and 666) had deeper secondary modes at 700-800
m; seal
560 usually dove only to 100-l 50 m although he had a secondary mode at
about 400 m (Fig. 2). Dive depths of all seals varied seasonally (P < 0.001)
and were greatest in spring (Fig. 3) with two exceptions: seal 560’s dives were
shallowest in spring whereas dives of seal 440 did not differ between spring and
early summer (P = 0.27). Dives in early summer were deeper than those in
early spring except for seal 6 18 which dove slightly deeper in early spring (P
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372
50
T
_
40
T
30
20
T
10
0
I
o-1
I
l-2
I
2-3
POST-DIVE
_
T
I
3-4
I
4-5
5-6
I
6-7
I
-
7-8
SURFACE INTERVAL (MN)
Figure 1.
Frequency distributions of dive depth, dive duration, and surface interval
for 36,233 dives of six northern elephant seal bulls (error bars are standard errors of the
mean).
< 0.001). About 0.4% (140) of. all seals’ dives exceeded 1,000 m; most
(89.7%) of those occurred in spring.
During each season dive depths varied with time of day (Fig. 4; P < 0.001)
except for seal 560 which dove to similar depths at all hours in spring. Daytime
dives were about 200 m deeper than night dives in early spring and early
summer; the die1 patterns were less pronounced in spring and there was greater
variability in dive depths at all hours when compared with the other periods
(Fig. 4).
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ELEPHANT
SEAL - 440
SEAL - 560
SEALS
373
SEAL - 516
.
SEAL - 618
SEAL - 666
n - 7317
1000
1
500
0 ldd
0 .\oo200 300 ,
DEPTH (M)
Figure 2.
Frequency
distributions
of dive depth for six northern
elephant
seal bulls.
Dive duration -Dive
durations varied among seals (P < 0.001) but averaged
22.6 min overall (SEM = 0.38 min; Fig. 1); the longest dive recorded was 77
min (Table 1). About 1.1% (4 10) of all dives were 40 min or longer. These
long dives occurred at all hours but most (66%) were between 0500 and 1600
-
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VOL. 7, NO. 4, 1991
SPRING
EARLY SPRING
EARLY SUMMER
i+
MEAN - 406.0
1
SD - 109.5
n - 854
20
40
iIs
MEAN - 567.0
YEAN
- 346.7
SD - 110.9
loo
.
.
MAXIMUM
,
,
- 952 m
,
,
,
,
,
YAXIMUU
.
- 1333
,
II)
MAXIMUY
- 710
m
MAXIMUM
- 909
m
MEAN - 513.3
MAXIMUM - ,523
m
too0
NUMBER
OF
Figure 3.
Seasonal variation in dive depths
standard deviation, n = number of dives).
1woo
100
200
300
4w
600
coo
DIVES
of northern
elephant
seal bulls (SD =
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375
EARLY SUMMER
a’0I SEAL = 640
HOUR
Figure 4.
Die1 variation
sample sizes).
OF DAY
in dive depth of northern
(PST)
elephant
seal bulls (see Fig. 3 for
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and nearly all (92%) were in early summer; most (75%) were to depths of
350-600 m.
Dive durations varied seasonally (Table 1; P < 0.001); they were longest in
early summer for all seals (p = 29.5 min, SEM = 1.14 min). Dives were shortest
in early spring for three seals (5 16, 640, 6 18) and in spring for the other three
(Table 1). Dive durations also varied with time of day (P < 0.01) and those
made during the day were longer than those at night. The differences among
seasons in these die1 patterns in dive duration were similar to those in dive depth,
being least pronounced in spring.
Dive duration was positively correlated with dive depth for all seals (P <
0.05) but dive depth accounted for only 16% or less (r2 = 0.11-o. 16) of the
variability in dive duration for all but one seal (560; r2 = 0.8 1).
Surface interval-Post-dive
surface intervals averaged 3.6 min (SEM = 0.26
min; Fig. 1) but they varied significantly among seasons (P < 0.001); they
were longest in spring (p = 3.9 min, SEM = 0.3 min) and shortest in early
spring (/.L= 3.1 min, SEM = 0.2 min). On average, 99.1% (SD = 0.99%) of
seals’ dives were followed by surface intervals of less than 10 min. Between
0.08% (seals 618 and 440) and 0.44% (seal 560) of seals’ surface intervals
were 3 0 min or longer and the longest ranged from 1.7 h (seal 5 60) to 5.7 h
(seal 640). Most (83%) of these long surface intervals were in spring but their
occurrences were unrelated to time of day (P > 0.50). They followed dives that
were, on average, 205 m deep (SD = 124.4 m) and 15 min long (SD = 4.0
min). Dive duration accounted for 2% or less (r2 = 0.0003
- 0.19) of the
variation in surface interval.
Bottom time-Bottom
time accounted for about 29.1% (SEM = 3.5%) of
the durations of each seal’s dives but bottom time was proportionately greatest
in spring (W = 33%, SEM = 6%) and least in early spring (F = 22%, SEM =
2%). Seals remained at relatively constant depth once they reached the bottom
of a dive; after maximum depth was reached there was less than one vertical
movement per dive, on average (seasonal range in p = 0.6-0.9,
and in SEM
= O.l-0.2),
for average vertical distances of 7.7 m (SEM = 1.1 m) in spring
to about 10 m in other periods. Of the 140 dives that exceeded 1,000 m, 73%
had bottom times of one minute or longer (Z = 3.1 min, SD = 3.3 min). Of
the 40 dives that lasted 40 min or more, bottom time accounted for about 25%
of their durations (SD = 19.9%).
Rates of descent and ascent-Rates
of descent varied among seasons (P <
0.001); they were greatest in spring (p = 1.22 m/set, SEM = 0.13 m/set)
whereas those in early summer (/.L= 0.79 m/set, SEM = 0.05 m/set) were
slightly greater than those in early spring (p = 0.72 m/set, SEM = 0.04
m/set>. In early spring rates of ascent (p = 0.79 m/set, SEM = 0.06 m/set)
were slightly greater than rates of descent, whereas rates of ascent in spring (y
= 1.02 m/set, SEM = 0.10 m/set> and in early summer (I* = 0.73 m/set,
SEM = 0.30 m/set> were less than rates of descent in those seasons (P < 0.00 1,
paired sample t-tests).
Seals’ rates of descent and ascent increased with dive depth during each season
(P < 0.00 1 in each case). Dive depth accounted for 28% of the variation in
.
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Table 1. Seasonal variation in dive duration in minutes of northern elephant seal
bulls (SD = standard deviation, 1z= number of dives).
Season
Seal
Early spring
Spring
Early summer
21.4
3.3
2,010
33.0
20.0
4.9
3,443
51.0
27.8
4.9
1,684
49.0
:D
n
maximum
19.7
2.9
1,647
29.5
20.3
4.4
1,791
63.5
27.7
4.8
854
55.0
560
R
SD
n
maximum
26.3
4.2
922
44.0
16.3
3.9
3,677
45.0
33.9
8.7
1,362
77.0
618
R
SD
n
maximum
21.4
4.1
1,364
36.0
23.8
4.4
1,638
45.0
27.6
5.1
810
47.0
ED
n
maximum
19.8
3.6
2,465
29.0
22.3
4.9
3,625
68.0
32.2
5.4
1,624
49.0
666
.f
SD
n
maximum
21.3
4.2
2,060
46.0
20.9
5.1
3,775
60.0
27.9
5.6
1,482
71.0
440
f
SD
n
maximum
516
640
rates of descent in early spring, 54% in spring and 23% in early summer; depth
accounted for 25%, 6 l%, and 28% of the variation in rates of ascent in early
spring, spring, and early summer, respectively. Dive duration accounted for only
5% or less of the variation in rates of descent (r2 = 0.01-0.02)
and rates of
ascent (r2 = 0.0002-0.05).
Diet-Fish
and cephalopod flesh was present in the lavaged materials from
all seals, indicating recent feeding. We recovered the identifiable remains of 58
individual of 16 species of cephalopods, two Pacific whiting (Merluccius productus) and one tunicate (Pyrosoma atlanticum).
The number of prey taxa
recovered from each seal varied from one (seal 618) to 13 (seal 440). Cephalopods were eaten by five seals, Pacific whiting by one and Pyrosoma atlanticurn
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Table 2. Prey species recovered by lavage from six northern elephant seal males at
San Miguel Island, July 1988 (frequency of occurrence = the number of seals from which
the prey was lavaged).
Frequency of
occurrence
Prey species
Cephalopods
Histioteuthis heteropsis
Moroteuthis robusta
Histioteuthis dofleini
Histioteuthis sp.
Octopoteuthis deletron
Gonatopsis borealis
Chiroteuthis calyx
Taonius pavo
Number of
prey items
:
2
3
3
1
2
1
Galiteuthius sp.
Onychoteuthis borealijaponicus
Architeuthis japonica
Megalocranchia sp.
Gonatus sp.
Ocythoe tuberculataa
Loligo 0palescensa
Octopus rubescensa
13
4
4
16
7
1
2
1
2
1
1
1
2
1
1
1
Teleosts
Merluccius productus
2
2
1
1
Tunicate
Pyrosoma atlanticum
a Probable
secondary
occurrence.
by one (Table 2). The most common prey were the squid Histioteuthis
Moroteuthis robusta, and Octopoteuthis deletron.
heteropsis,
DISCUSSION
Recent studies have documented
the breathhold
diving abilities of many
species of seals and sea lions (e.g., Kooyman 1981, Gentry and Kooyman 1985,
Feldkamp
et al. 1989, Le Boeuf et al. 1989, Stewart and Yochem
1989).
Northern elephant seals are superlative divers among pinnipeds with regard to
dive depth, duration, and repetition. Our results indicate that northern elephant
seal bulls are relentless divers while at sea following their winter breeding season
fasts of up to 3 mo. They typically descenced rapidly to depths greater than
300 m, spent 5-10 min at maximum
depth with little vertical movement there
and then ascended quickly for a brief visit to the surface before diving again.
Although there were significant differences among seals and among seasons in
the values of each dive parameter that we measured, some generalizations
about
their diving and foraging patterns are possible. Seals’ repetitious
15-77 minlong dives, punctuated
by brief periods of 2-4 min at the surface, continued
with rare interruptions
for 4-5 mo until they returned ashore in July to molt.
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These patterns of continual, deep, long-duration dives punctuated by brief
periods at the surface are similar to those of northern elephant seal cows (e.g.,
Le Boeuf et al. 1989) which are at sea for about half as long in spring. However,
dives of the bulls were significantly longer (P < O.Ol>, by about 18%, than
those of cows (I* = 19.2 min, SEM = 0.7 min, n = 15; statistics calculated
from data presented in Le Boeuf et al. 1986, Le Boeuf et al. 1988, and Le
Boeuf et al. 1989). This difference in dive duration is evidently accounted for
by the large differences in body mass between bulls (l,OOO-1,600
kg.; Stewart
and DeLong, unpublished data) and cows (250-500
kg; Le Boeuf et al. 1988
and Stewart and DeLong, unpublished data). Body oxygen stores increase isometrically with body mass in phocids (Kooyman 1985), as in other mammals
(e.g., Scholander 1940, Calder 1984), whereas metabolic rate should increase
allometrically (power coefficient < 1). Aerobic dive endurance, which is determined by the ratio of a seal’s body oxygen stores to its diving metabolic rate,
should therefore also increase allometrically (power coefficient < 1) with body
mass. The greater apparent dive endurance of northern elephant seal bulls
compared with cows that we describe here is similar to observations of allometric
scaling of dive endurance with body mass for Weddell seals, Leptonychotes
weddellii (Kooyman et al. 1983).
Kooyman et al. (1983) identified the aerobic dive limit (ADL) as the longest
dive possible for an animal of a given mass that would not lead to an increase
in blood lactic acid concentration during or after the dive. Castellini et al. (1988)
argued that a diving seal can exceed its ADL but still continue to dive, while
accumulated lactate is processed, if subsequent dives are shorter than the ADL.
Otherwise the seal will either have to spend an extended period at the surface
ventilating or terminate the diving bout. We found that surface intervals of
northern elephant seal bulls were brief and remained relatively constant regardless
of dive duration (for similar data on elephant seal cows see Le Boeuf et al.
1988). Further, dive bouts of bulls lasted several months or more and extended
surface periods were virtually non-existent. This means that these seals rarely
exceeded their ADLs even though long series of dives of 30 min or longer were
common. Even extraordinarily long dives did not appear to constrain the patterns
of subsequent dives. This is perhaps best illustrated by seal 560. After making
a 77 min dive, he spent 3 min at the surface and then made dives of 39, 56,
36, 43, 38, 35, 49, 48, 41, 47 and 42 min with no more than 3 min at the
surface between dives. This pattern continued for several more weeks. These
data (and other data for dives >45 min of all seals) indicate ADLs of 40-50
min or more. The facts that surface intervals between dives were brief and
unchanging, and that dives that followed exceptionally long dives were not
significantly shorter than typical dives, indicate that seals remained well within
their aerobic dive limits, permitting them to dive continually for nearly 5 mo
without accumulating large amounts of lactate from anaerobic metabolism. The
substantially greater dive durations of all seals in early spring, just before returning
to shore, indicate that ADL may increase while seals are at sea resulting, perhaps,
from exercise-induced aerobic conditioning. Determining the longest dives that
can be supported by aerobic metabolism alone will require analyses of post-dive
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lactate concentrations,
as was done for Weddell seals (Kooyman
et al. 1980,
Kooyman et al. 1983, Castellini et al. 1988).
The longest dives reported for mammals are about 70 min for Weddell seals
(Kooyman
1981) and between 45 min and slightly over 2 h for sperm whales
(Watkins
et al. 1985, Clarke 1980) although
data for dives exceeding one
hour are equivocal. The longest dives that we report here for northern elephant
seal bulls (5 l-77 min; Table 1) and a 2-h dive reported for a southern elephant
seal (M. leonina) cow (Hindell et al. 1989) demonstrate
the exceptional breathhold diving abilities of elephant seals.
Although the distributions
of dive depths varied among bulls, they shared a
common mode at 350-450
m (Fig. 1, 2). It was the primary mode for all but
seal 560. Indeed, the shallow mode at 100-l 50 m shown in Figure 1 was due
solely to the dives of seal 560 in spring (Fig. 2) when nearly all of his dives
were to depths of 50-l 50 m. The secondary modes of the other five seals shown
in Figure 2 are due to either bimodal depth distributions
or a deeper single
mode in spring compared to other periods (Fig. 3). Our more recent studies
(DeLong et al. 1989), where the geographic
locations of diving seals were
known, suggest that the seasonal variations
in dive depths and other dive
parameters of bulls in 1988 may have had less to do with season per se than
with the seals’ migratory behaviors. We speculate that seasonal variations in
dive parameters in 1988 were related to bulls’ geographic locations and perhaps
geographic variations in prey depth and behavior.
Except for a single giant squid (Architeuthis japonicus) beak that we recovered
from a bull, the prey that we lavaged from the bulls’ stomachs were similar to
those recovered from females and young males by Antonelis et al. (1987). Those
prey, primarily squid, are almost exclusively epi- and mesopelagic residents that
are first- or second-order
die1 vertical migrators (Roper and Young 1975). Of
course, they represent the seals’ most recent meals. Harvey et al. (1989) found
that nearly all beaks from squid fed to captive elephant seals passed through
the intestines within eight days.
Except for seal 560 in spring, dive depths varied systematically with time of
day in all seasons, being deeper during the day than at night. The die1 cycles
during each seal’s final week at sea did not differ from dive patterns during the
other 14-19 wk of their pelagic existence. These dive patterns and the dietary
information are consistent with the hypothesis that seals were diving and feeding
in the offshore mesopelagic zone, pursuing vertically-migrating
prey as they rose
toward the surface at dusk and descended to depth at dawn. The shallow diving
of seal 560 in spring may represent benthic foraging in epibenthic,
shallow
nearshore habitats.
The die1 patterns of dive duration are evidently due partly to die1 variation
in dive depth but the rather weak correlations between these parameters suggest
that other factors such as die1 variation in prey behavior may be more important
in determining
dive duration. Seasonal differences in rates of descent and ascent
may be artifacts of correlations of these parameters with dive depth, which varied
seasonally. If seals descended and ascended at 90” angles to the surface then the
rates of ascent and descent that we report are true swim velocities. But field
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measurements of angles of descent and ascent or swim speeds are needed before
variations in rates of descent and ascent can be properly interpreted relative to
seals’ foraging tactics.
We find no evidence in the bulls’ diving records to indicate whether or not
seals slept while at sea. Sleep in diving elephant seals may be unihemispheric,
similar to that described for some dolphins and other seals (Mukhametov 1984,
1989), with one hemisphere of the brain alternating with the other between
sleep and wakeful states. This type of sleep would permit elephant seals to swim
and dive continuously during their 4-5 mo foraging excursions. Alternately,
seals may sleep during descent, perhaps while sinking passively, and on the
ocean floor. Data on swimming speeds and brain activity are needed to discriminate between these hypotheses.
Although all seals made dives greater than 900 m (the approximate boundary
between the dysphotic and aphotic regions in many oceans; McFall-Ngai 1990>,
the function of these dives is not clear. Until data are available on the activities
of seals when at depth, we suggest that these dives represent deep exploration
of the water column during descent and ascent in search of aggregations of prey.
Comparisons of dive depths of bulls with those reported earlier for cows, as
arithmetic means (Le Boeuf et al. 1986, Le Boeuf et al. 1988, Le Boeuf et al.
1989), are problematic because of the multimodal distribution patterns that we
recorded for bulls. A grouped distribution presented by LeBoeuf et al. (1988:
fig. 3, 449) and our recent data for cows (Stewart and DeLong, unpublished
data) indicate that the distributions of dive depths of northern elephant seal
cows are also multimodal and that arithmetic means are not appropriate summary
statistics for inter- or intra-sexual comparisons. However, we compared our data
on the percent of bulls’ dives in 50-m intervals with the grouped data presented
by Le Boeuf et al. (1988: fig. 3, 449) and found no significant difference
between the bull and cow depth distributions (P = 0.75, Komogorov-Smirnov
two sample test). A primary mode at 3 50-450
m was common to bulls and
cows and it appears that, overall, cows had a secondary mode at 550-600
m
(Fig. 5). As we discussed above, the shallow mode at loo-150
m that appears
in the overall distribution of dive depths of bulls (Fig. 1) is due to the shallow
diving of one bull in spring (Fig. 2, 3). Additional kinds of data are clearly
needed to determine if the differences in the depths of the deeper secondary
modes of cows (5 50-600
m) ~.r. bulls (650-800
m) are related to physiological
constraints, as are differences in dive durations, or to environmental influences
or prey distribution.
Kooyman (1988) summarized the maximum dive depths for 2 1 species of
diving vertebrates (humans, turtles, kds, and marine mammals); the deepest
was to 1,140 m by a sperm whale (Physeter macrocephalus). LeBoeuf et al.
(1989) found that northern elephant seal cows occasionally exceeded the 1,000
m limitations of their recording instruments but they did not measure maximum
depth. The maximum dive depths that we report here for two northern elephant
seal bulls of 1,333 m (seal 640) and 1,529 m (seal 666) are the deepest yet
measured for an air-breathing vertebrate.
The diving endurance abilities of northern elephant seals to sustain repetitive,
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16,““““““““““,
“~~~~~~~~a~~~~~~~~~~~~~~~~~~~~~,~~l~~~~~~~~~~~
DEPTH (MI
Comparisons of frequency distributions of dive depths for northern elephant
Figwe 5.
seal bulls (this study) and cows (data from Le Boeuf et al. 1986: fig. 3, 449).
long breathhold dives for several months with little time at the surface between
dives, and perhaps without sleep, are clearly remarkable.
Elucidation
of the
general biochemical and physiological mechanisms that permit such performance
in free-ranging phocid seals and other diving mammals will be central challenges
for future studies.
ACKNOWLEDGMENTS
The TDRs were designed and built and the data summary software written by R. and
S. Hill, Wildlife Computers, Woodinville,
WA. We thank R. and S. Hill for their help
and consultation throughout our studies. We also thank W. Ehorn and his staff of the
Channel Islands National Park for their interest and logistic support of our research on
San Miguel Island, P. Boveng and D. DeLong for their assistance in the field during
deployment and recovery of the TDRs, G. Antonelis for consultation about methods of
immobilization
and lavage, Clairol Research Laboratories for supplying hair dye products,
and C. H. Fiscus for identifying squid beaks. BSS was supported in part by a contract
from the United States Air Force and by Hubbs-Sea World Research Institute. We thank
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DELONG AND STEWART: NORTHERN ELEPHANT SEALS
383
G. A. Bartholomew,
H. Braham, H. R. Huber, R. Gentry, J. R. Jehl, L. Jones, S. H.
Ridgway, P. Yochem, and two anonymous referees for reviewing the manuscript and S.
Heller, R. Elsner, S. H. Ridgway, and L. Mukhametov
for discussions of sleep in marine
mammals. The research was authorized under the Marine Mammal Protection Act of
1972, Marine Mammal Permit No. 464.
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Received: August 17, 1990
Accepted: January 21, 1991