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MARINE MAMMAL SCIENCE, 28(4): E516–E527 (October 2012)
C
2012 by the Society for Marine Mammalogy
DOI: 10.1111/j.1748-7692.2012.00567.x
Abdominal fat pads act as control surfaces in lieu of dorsal fins in the
beluga (Delphinapterus)
ALEXANDER J. WERTH,1Department of Biology, Hampden-Sydney College,
Hampden-Sydney, Virginia 23943, U.S.A. and THOMAS J. FORD,JR., Ocean Alliance,
209 Harvard Street, Brookline, Massachusetts 02446, U.S.A.
Almost all cetaceans have a dorsal fin, which acts as a surface to control attitude and
prevent roll, yaw, and side-slip (Fish 2002, 2004). Among odontocetes, only right
whale dolphins (Lissodelphis), the finless porpoise (Neophocaena), and both members
of Monodontidae, the beluga or white whale (Delphinapterus) and narwhal (Monodon),
lack a dorsal fin. Both Delphinapterus (whose generic name refers to its absence
of a fin) and Monodon possess, like Neophocaena, an irregular, notched dorsal ridge
running along the caudal portion of the back (Struthers 1895, Brodie 1989, Hay and
Mansfield 1989), but the low (4–5 cm high) dorsal ridge of monodontids (also called
a cuticular crest by Kleinenberg et al. 1964) appears insufficiently large to act as a
suitable hydrodynamic control surface on these 4–5 m long odontocetes.
Based on kinematic analysis of locomotion in captive and wild beluga whales
and dissection of abdominal musculature of Delphinapterus, we present a previously
undescribed morphological feature that we believe allows belugas to enhance vertical
stabilization and especially to control roll during turns and during swimming while
in a partially rotated or wholly inverted position. This stabilization involves a pair of
distinct fat (blubber) pads running longitudinally along the ventrolateral aspect of
the abdomen, parallel to the dorsal ridge. These pads form large, elevated ridges that
can be seen clearly (Fig. 1–3) in various views of Delphinapterus. The ventrolateral fat
pads are in line with the pectoral flippers and extend from just caudal of the axilla to
the level of the pelvic bones. The abdominal fat comprising the elevated ridges does
not consist of discrete, paired bodies of isolated, encapsulated adipose tissue (as in
the mandibular fat body of odontocetes). Rather, it involves a pair of unusually thick,
longitudinal blubber deposits; it entails more than mere bulging of uniformly thick
abdominal blubber. We contend that during some swimming maneuvers (inverted
swimming, longitudinal rolls, and whole body turns), the fat pads formed by these
markedly thickened blubber deposits are tensed and raised by abdominal muscles
to form a pair of large, rounded erect structures (elevated 10–14 cm higher than
the adjacent trunk and tailstock; 107–119 cm in length and 8–12 cm in width,
averaging roughly 128 cm2in area), often including a focal bulging of the ridges
near their caudal terminus which results in fin-like protuberances surrounding a
crater-like, semicircular depression on the ventral surface of the animal (estimated
at up to 16 cm deep; Fig. 3). Based on our qualitative and quantitative kinematic
1Corresponding author (e-mail: awerth@hsc.edu).
E516
NOTES E517
Figure 1. Three views of the ventrolateral abdominal fat pads in captive Delphinapterus.The
pads can be seen in lateral view (ventral to and in line with the dorsal ridge) in the animal
in the foreground, in full ventral view in the lower left background, and inverted and fully
distorted (with fin-like protuberances) as the whale in the upper right background swims
upside down.
analysis, we suggest that these fat pads, when elevated, form a pair of prominent
vertical stabilizers that (perhaps in conjunction with the dorsal ridge) enhance an
animal’s ability to control heading and limit roll.
Our kinematic analysis involved digitally recorded sequences (323 min of total
footage recorded from 2005 to 2010) of nine captive adult belugas at three sites (the
Georgia Aquarium in Atlanta, Georgia; Mystic Aquarium in Mystic, Connecticut;
and Vancouver Aquarium in Vancouver, British Columbia), plus 36 min of videotaped
sequences of an undetermined number of wild beluga whales observed off Point Lay
and Point Barrow, Alaska. The captive belugas were recorded engaging in typical
locomotion, resting, and other normal activities, alone and in groups of 2–4 animals
(male and female), through clear viewing windows, and in some cases from above
the surface of the water, almost always (>95% of sequences) during daylight hours
when aquarium visitors were present. For some recordings a handheld camera (a
Sony DCR-SX63 or JVC GR-D270U, with up to 60×optical and 2,000×digital
zoom) was moved along with animals; for other recordings the camera was fixed to
a tripod placed in front of the viewing window behind which animals swam, with a
fixed or variable focal length to capture locomotor behaviors at different distances.
Wild animals were recorded from the ice edge at distances of 3–30 m as they swam
and surfaced in polynyas and leads in shorefast ice. Digital sequences (total n=128,
ranging from 3 s to 40 s for captive animals and n=17, 2–29 s for wild belugas) were
E518 MARINE MAMMAL SCIENCE, VOL. 28, NO. 4, 2012
Figure 2. Ventrolateral abdominal fat pads, with dorsal margin indicated by arrows: (top)
lateral view, pads in relaxed position; (bottom) oblique lateral view, with fat pads extended,
generating protuberances, on beluga rotated left 90◦.
analyzed on a Dell Optiplex 745 or Dimension D10 computer using Kinovea 0.8.15
video chronometer and motion analysis software. Sequences were analyzed mainly
for amounts of time spent in various postures (e.g., rolls) and concomitant presence
or absence of elevated fat pads (Fig. 4). Other kinematic variables include locomotor
velocity, turning radius, turning and rolling rate, pitch, and roll angle, all tracked
relative to observational references (fixed or on subject’s body), and with playback
at 10%–100% of original speed or frame-by-frame, synchronized to time coding.
NOTES E519
Figure 3. Ventral view, showing funnel-like ventral concavity between abdominal fat pads
fully extended to create fin-like protuberances. Note outlines of paired, contracted muscles,
likely rectus abdominis to left, cranial to umbilicus, and pyrimidalis (dashed arrows) to right
of umbilicus.
The software allowed for magnification, plane perspective, tracking of path distance,
and velocity measurement. These were applied to whole animals and used to detect
and measure elevation of the abdominal fat pads. Unless specified, all data include
sequences of captive and wild animals analyzed together; ANOVAs and t-tests of
kinematic data from captive vs. wild animals revealed no statistically significant
differences.
Analysis (Fig. 4) revealed that the ventrolateral fat pads are not consistently present
in footage of captive and wild animals; however, they are clearly apparent in 36.7%
of all sequences showing normal forward locomotion (swimming in normal upright
position, with dorsum of whale pointing toward water surface) and in 86.3% of all
locomotor sequences in which the animal had rotated along its longitudinal axis. For
sequences in which the animals were totally inverted during swimming (surprisingly
common: n=56 or 44.2% of all swimming sequences of captive animals), with the
ventral surface pointing upward (180◦roll), the ventrolateral fat pads were clearly
visible in 96.1% of these sequences. The fat pads were frequently observed (89.1%
of total sequences) in locomotion during which a whale made a “tight” turn, which
we define as having a turning radius less than half the animal’s body length (turning
radius measured directly from video sequence using known body length and/or
external landmarks). Such turns were common (90.6% of all turns) when the whales
were swimming in a laterally rolled position, and in all such cases the whale’s dorsum
is on the “outside” of the turn (i.e., ventral surface facing into turn). Most tight turns
observed in captivity occurred along a barrier (a wall or window of an enclosure);
E520 MARINE MAMMAL SCIENCE, VOL. 28, NO. 4, 2012
Figure 4. Results of the kinematic analysis of captive and wild belugas showing relation
between abdominal fat pad elevation and different behavioral patterns and postures. Tight
turns involve turning radius measured at less than one half body length.
they were seen only twice, at the ice edge, in wild animals. Elevation of the fat pads
from their normal (nondeployed) condition was not found to have any significant
correlation with turning velocity, turning radius, roll rate, or pitch.
The fat pads were visible but prominently elevated during only 15.6% of se-
quences when belugas were deemed “resting” (i.e., not making forward progress;
Fig. 4). During movements that involved obvious lateral rolls but no forward swim-
ming, elevated fat pads were visible in 27.3% of sequences. Elevation of the paired
ventrolateral pads could be sustained for long durations (in one extended sequence,
for 9.5 min or 566 s). Elevated pads were visible during ascents, descents, and level
swimming at all levels of the water column. Additional results of the kinematic
analysis are that: (1) the pads are either clearly apparent (prominently elevated) or
not, with no instances visible in videotaped sequences in which fat pads are partially
elevated; (2) there are no sequences in which a fat pad is visible on only one side
of the body (i.e., one of the pair); (3) the presence or absence of the fat pads during
locomotion is independent of a whale’s forward velocity (20–210 cm/s, with bursts to
365 cm/s), as measured by stationary landmarks in the pool or in front of the viewing
window, controlling for the plane of the camera; (4) the presence or absence of fat
pads is independent of the size or sex of a beluga; and (5) the fat pads were slightly
more evident in captivity when more than one beluga was present in the enclosure
(n=95, or 74% of sequences, in which pads were visible) as opposed to alone
(n=33, or 26%), possibly due to the fact that these situations yielded interactions
between animals that involved more swimming and turning than resting.
NOTES E521
Table 1. Mean blubber thickness (cm) in adult specimens (excluding skin; all measurements
taken post mortem; samples taken at genital slit, midway between umbilicus and anus).
Species nLateral (mean ±SD) Ventral (mean ±SD)
Delphinapterus leucas 30 4.71 ±1.29 10.80 ±1.93
Monodon monoceros 44.39±1.86 6.17 ±2.65
Globicephala melas 30 3.25 ±0.41 3.36 ±0.52
Grampus griseus 63.12±0.35 3.62 ±0.41
Tursiops truncatus 30 1.66 ±0.24 1.58 ±0.30
Lagenorhynchus acutus 30 1.74 ±0.28 1.94 ±0.33
Delphinus delphis 30 1.72 ±0.33 1.85 ±0.39
Phocoena phocoena 30 1.16 ±0.28 1.19 ±0.34
Because the elevated fat pads can be seen in all beluga orientations except a
pure dorsal view (and even then, whales often rotated or changed direction during
swimming), they were clearly visible and easily noted. The marked semicircular
ventral concavity at the caudal terminus of the paired fat pads, just cranial to the
genital slit and medial to the fin-like protuberances (Fig. 3), is best seen in a
ventral view, but was clearly visible in nearly half (46.1%) of all sequences in which
a beluga was swimming and/or turning while rolled (from 0◦to 180◦) along its
longitudinal axis. In such cases, the ventral concavity was present during the entire
sequence; the concavity was most apparent during the turn, but this was not because
it suddenly appeared during or prior to the turn—as long as the fat pads were
prominently elevated in these sequences, the concavity was present. It may be that
the more marked bulging of the fat pads (creating the distinct ventral depression)
occurs during tight turns due to a dorsoventral compression of the trunk and tail
stock and/or due to contraction of trunk and abdominal musculature. Although
corrugations or wrinkles are frequently seen in beluga skin and blubber, the elevated
fat pads we observed were consistently smooth in external appearance: no rippling,
undulating, flexing or other movements were observed along these bulging, bulbous
lines, which are taut but not inflexible and unyielding. This is not a deformable
surface, and it is dynamic only in the sense that it is raised (elevated) or not.
The next portion of this study involved examination of anatomical specimens, with
dissections of beluga whales and other odontocete species for comparative analysis
(Table 1). Access to beluga carcasses captured for subsistence hunting (Frost and
Suydam 2010) was graciously provided by Inupiat people of Point Lay and Point
Barrow, Alaska (1992–2007). Thirty adult beluga specimens were examined for
this study (13 male, 17 female), along with two juvenile female belugas and two
(male and female) late term fetuses. Digital photographs and videotape sequences
recorded these dissections, as sketches were made and measurements taken of blubber,
abdominal musculature and other anatomical features, including the pelvic bones
and adjacent structures, especially adipose and connective tissues. Three narwhals
(one adult male, one adult female, and one late term female fetus) were also dissected,
all near Pond Inlet, Nunavut. Comparative data for other adult specimens dissected
for this study (all necropsy of stranded animals, at the New England Aquarium,
E522 MARINE MAMMAL SCIENCE, VOL. 28, NO. 4, 2012
Figure 5. Schematic illustration (ventral view) of superficial dissection of caudal abdominal
and trunk musculature of an adult beluga, Delphinapterus leucas, showing the paired pyrimidalis
abdominis muscles which originate on the pelvic bones and insert onto the superficial surface
of the rectus sheath and via terminal tendons along the linea alba ventral and medial to rectus
abdominis muscle fibers. Note that the deep transversus abdominis is partially reflected over
the internal oblique.
Boston, Massachusetts, from 1989 to 2004) include six species (Table 1): the long-
finned pilot whale (Globicephala melas), Risso’s dolphin (Grampus griseus), bottlenose
dolphin (Tursiops truncatus), common dolphin (Delphinus delphis), Atlantic white-sided
dolphin (Lagenorhynchus acutus), and harbor porpoise (Phocoena phocoena).
Dissections and manipulation indicate that elevation of the fat pads in Delphi-
napterus is a consequence of the contraction of ventral abdominal muscles (Fig. 5)
that tense the abdomen and its overlying blubber, and especially of muscles that
attach to the pelvis, creating the bulbous, “erected” folds. Although various abdom-
inal muscles likely contribute to this action (Cotten et al. 2008), and the rectus
abdominis is the largest such muscle (measured by cross-sectional area and total
length of excursion), we believe that the paired pyrimidalis abdominis muscles of the
lower abdomen (Evans and Christensen 1979, Schaller et al. 2007) is an especially
important contributor. The pyrimidalis (also known as the pyramidalis, NAV 2005)
is not consistently seen in all odontocetes; it is rarely described in myological ac-
counts of cetaceans. Of the dissections conducted for this study, we found a distinct,
well defined pyrimidalis abdominis muscle in all specimens of Delphinapterus and
Monodon, and in a majority of specimens of Grampus and G. melas. An unequivocal
NOTES E523
pyrimidalis muscle was observed in 1 of 10 Tursiops specimens, 2 of 10 L. acutus
specimens, and in no other specimens dissected for this study.
In Delphinapterus, the pyrimidalis abdominis (Fig. 5) originates on the cranial
end of the pelvic bone and inserts on the linea alba (a fibrous structure composed
mostly of collagen running down the abdominal midline) at its ventral, caudal
border, approximately midway between the pelvic bones and umbilicus (Howell
1930a); its fibers resemble a caudal continuation or head of the rectus abdominis,
but the pyrimidalis has separate, shorter fibers with a discrete epimysial sheath.
The pyrimidalis is dorsoventrally flattened, approximately 22 cm in mean length
in adults (n=30, SD =1.4) and 7 cm in mean width (n=30, SD =0.6), with
a cross-sectional area (mid-belly) of roughly 36.7 cm2(n=10, SD =3.5). The
caudal portion of the rectus abdominis sheath splits (near its origin on the pelvis)
and encloses the pyrimidalis muscles. Few fibers of either the pyrimidalis or rectus
abdominis attach to the overlying blubber. The prime motion of both the pyrimidalis
and rectus abdominis is to tense (stiffen) the abdominal wall and draw the pelvic bones
ventrally and cranially. Because the cetacean os pelvis is a single, rod-like element not
fused to the vertebral column, contraction of this musculature causes the abdomen
to shorten and the skin and blubber to tighten, elevating the paired ventrolateral fat
pads of the abdominal blubber. Although we performed no histological analysis, the
dense, vascularized layer of hypodermis in these paired structures appeared (upon
gross anatomical examination) indistinguishable from adjacent blubber apart from
its greater thickness. The hypodermal fat in the pads feels, to the touch, the same as
other blubber, with similar pliability, and qualitatively appears to behave similarly
when deformed in both tension and compression, though it is somewhat looser than
surrounding blubber due to its greater thickness and abundance of fascia ventral to
the pads (just beneath the skin). There is no notable innervation (though a nerve
which may be the iliohypogastric nerve was often seen; Schaller et al. 2007), nor any
subcutaneous muscle fibers in the prominent fat pads, which average roughly 130
cm long, 12 cm wide, and 12 cm high in a full-grown adult.
True described the beluga’s blubber distribution in 1910:
“Beginning opposite the anterior end of the dorsal fin, the body, seen from above,
assumed a form resembling a pillar consisting of three attached columns, laid hori-
zontally. It was made up of a median dorsal rounded ridge, with a similar rounded
mass below it on either side.” However, True (1910) did not speculate on the po-
tential functional significance of these structures. Extensive review of publications
on beluga whale morphology yields no prior mention of this morphological feature,
aside from reference to abnormally thick blubber (to 25 cm) “on the venter near the
milk and anal glands” by Kleinenberg et al. (1964) in their book-length monograph
on Delphinapterus. Although Reeves et al. (2002) noted that in belugas “the belly and
sides may be lumpy, with folds and creases of fat,” the fixed anatomical distribution
of the adipose pads as revealed by necropsy and their characteristic position during
swimming (Fig. 1–3) reveals a highly uniform rather than variable arrangement. The
presence of elevated abdominal ridges in deceased specimens indicates that they can
appear without need for muscular contraction by the abdominal musculature if the
animal is laid out properly (lying on its side); their presence in animals hauled out im-
E524 MARINE MAMMAL SCIENCE, VOL. 28, NO. 4, 2012
mediately after death indicates that elevation is not due to post mortem contraction
and stiffening of the pyrimidalis and rectus abdominis muscles or other structures.
However, we contend that muscular contraction plays a role in the appearance of
these pads in vivo. Further, the presence of the ventrolateral fat pads in wild belugas
demonstrates that these are not an artifact resulting from obesity in captivity.
Based upon our kinematic and morphological investigation, and especially the
finding that these structures are not uniformly present but become visible during
nearly all locomotion in which an animal adopts a rotated or fully inverted body
position, we suggest that they act as vertical stabilizers. Our novel hypothesis is that
in the absence of a dorsal fin, the paired ventrolateral fat pads of Delphinapterus offer
a previously undescribed surface to control roll. An alternative hypothesis is that
rolling is common in Delphinapterus, and is especially manifested in inverted (upside
down) swimming, precisely because this species has no vertical stabilization due to
its lack of a dorsal fin. However, our data show that rolling in belugas is in fact
not uncontrolled; whales clearly were able to maintain attitude (without roll, pitch,
or yaw) for long distances and durations during locomotion, even during sharp and
wide turns and changes in depth. Whales in which the fat pads were raised (elevated)
were still observed to continue rolling (rotating along the longitudinal body axis),
but most often (>70% of sequences analyzed) did not do so. Further, the presence
of elevated ventrolateral fat pads during almost all (90% of) rolls and tight turns,
coupled with their rarity during periods of rest, lends credence to our functional
inference, which is in turn bolstered by evidence from our comparative anatomical
survey. We do not dare surmise the functional significance, if any, of the crater-like,
semicircular depression at the caudal terminus of the fat pads; this may be a mere
artifact of the stiffened abdominal musculature and blubber, although it is possible
that it somehow controls hydrodynamic flow over the body, particularly during
high speed swimming (infrequently observed in this species). When this depression
appears, the caudal portion of the adjacent pads are elevated to such a degree and at
such an angle as to present protuberant structures (Fig. 3, plus Fig. 1 upper right
and Fig. 2 lower) that may act similarly to fins. We propose, based on the shape and
timing of appearance of these fin-like features, that they, like the fat pads generally,
have functional significance as attitudinal control surfaces.
Our preliminary investigation of the abdominal musculature and adipose distri-
bution in Monodon suggests that this sister species may possess a similar system
of paired ventrolateral abdominal fat pads that are elevated during inverted and
other locomotion for attitudinal control. However, the pilot necropsy data we have
gathered indicates that although the abdominal musculature is similar in both mon-
odontid species, in accord with reported findings of Howell (1930a,b, 1935) and
Watson and Young (1880), the ventrolateral abdominal blubber of the narwhal is
not distributed as thickly nor in the pronounced, track-like pads as in Delphinapterus.
Although both monodontids share a similar external form, and both possess thick
hypodermal layers (Table 1), the narwhal exhibits fewer blubber folds and less loose
skin and fat. The blubber and associated subcutaneous adipose layers of adult belugas
(Table 1) is nearly 5 cm (excluding the epidermis) on the lateral surface, but is much
thicker (mean 11 cm) on the ventral surface of the abdomen, where the paired fat
NOTES E525
pads lie. The narwhals we examined had a similarly thick layer of lateral blubber
(over 4 cm thick, second only to belugas in our comparative study) and thicker (>6
cm) ventral blubber, but without an unequivocal pattern of paired pads. In other
odontocetes surveyed there was no significant difference between lateral and ventral
blubber thickness (Table 1).
Inverted and otherwise rolled swimming was frequently seen in juvenile belugas,
captive and wild, but only data from adults was included in our kinematic analysis.
Limited examination of fetal beluga specimens indicates that abdominal fat pads
may be present at this stage but are not as conspicuous as in juveniles and adults. In
beluga fetuses, ventral abdominal blubber (=3.35 cm, SD =0.60, n=2) is only
slightly thicker than general body blubber deposits (=2.92 cm, SD =0.88, n=
2). We also found that the dorsal ridge is more conspicuous and narrow and slightly
but not significantly higher in Delphinapterus than Monodon; it is split in belugas
by transverse notches into a series of small bumps, giving it a serrated appearance
(Brodie 1989). Kleinenberg et al. (1964) noted that the dorsal ridge’s prominence
varies widely in beluga populations.
Precise control of orientation is essential in species that navigate and forage in a
complex three-dimensional environment, especially when swimming—as is the case
in Delphinapterus—in varied habitats ranging from shallow river deltas and estuaries
to deep water under an ice sheet that limits movement and breathing. Foraging in
such habitats could be enhanced by a partially rolled or wholly inverted posture,
which might cause echolocation waves to be beamed more toward food resources and
away from a shallow bottom or ice surface, potentially minimizing interfering return
echoes. Heptner (1930) suggested that the lack of a dorsal fin in monodontids (as in
the only finless mysticetes, the bowhead and right whales, Balaena and Eubalaena)
relates to the presence of ice in their habitat, although this does not explain why
subtropical and temperate odontocetes (e.g.,Neophocaena and Lissodelphis)lackafinor
why other species that inhabit icy water (e.g.,Orcinus) have large dorsal fins. Brodie
(1989) asserted that belugas are frequently observed (via aerial observers) to swim
in an inverted position, as was very commonly seen in captive subjects of our study.
DTAG data published by Dietz et al. (2007) indicate that free-ranging narwhals
swim in an inverted posture 80% of the time they are submerged. We contend that
the ability of belugas and possibly narwhals to roll along the body axis during forward
locomotion as well as during sharp turns, readily evident in both species, relates to
their lack of a dorsal fin and apparently corresponding presence of ventrolateral fat
pads. According to Sleptsov (1952), longitudinal ridges or keels might take the place
of a dorsal fin, providing stability in odontocetes that lack a fin. (However, DTAG
telemetry [Madsen et al. 2005] indicates that foraging beaked whales frequently
rotate during dives, and all ziphiids possess a dorsal fin.) We postulate that the
paired ventrolateral fat pads of Delphinapterus (and likely to a much lesser extent in
Monodon) serve the same control purpose but, unlike a dorsal fin, can be manipulated
via muscular contraction for variable deployment, and that this is a morphological
feature unique to Monodontidae. The absence of a dorsal fin might also limit heat
loss in these cold-water odontocetes. Further study should be done to assess the role
of the paired ventrolateral fat pads in locomotor control. In their study of finless
E526 MARINE MAMMAL SCIENCE, VOL. 28, NO. 4, 2012
porpoise (Neophocaena) abdominal myology, Tajima et al. (2004) argued that muscles
attached to the pelvis likely played a key role in cetacean evolution by contributing
to dorsolateral undulation that replaced pelvic limb motion in locomotion. The
kinematics and morphology of other finless odontocetes (e.g.,Lissodelphis) might be
examined for similar features that could potentially serve as compensatory adaptations
to control roll and yaw.
ACKNOWLEDGMENTS
We thank Charles Brower, Robert Suydam, J. Craig George, Gregory O’Corry-Crowe, and
Heather Smith for assistance with this project, and the Inupiat people of Point Lay and Barrow,
Alaska, and the North Slope Borough Department of Wildlife Management for access to beluga
specimens, cooperation and logistical support. Access to narwhal specimens was provided by
David Angnetsiak and the Hunters and Trappers Organization of Pond Inlet, Nunavut, and
to other dissection specimens by the New England Aquarium. We gratefully acknowledge
the Georgia Aquarium, Mystic Aquarium, and Vancouver Aquarium for allowing access to
beluga whales for videotaping of swimming and other behaviors. Special thanks go to Lance
Barrett-Lennard of the Vancouver Aquarium, and to the Vancouver Aquarium Marine Science
Centre for assistance. We are grateful to Dr. D. Ann Pabst, Dr. Frank Fish, Dr. Vadim
Pavlov, and other anonymous reviewers for their valuable suggestions on earlier versions of
this manuscript.
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Received: 24 August 2011
Accepted: 22 January 2012
