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First published online November 14, 2008
Journal of Experimental Biology 211, 3712-3719 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.023366
Foraging behavior of humpback whales: kinematic and respiratory patterns suggest a high cost for a lunge
1 Department of Zoology, University of British Columbia, 6270 University
Boulevard, Vancouver, British Columbia, Canada V6T 1Z4
2 Cascadia Research Collective, 218.5 W. 4th Avenue, Olympia, WA 98501,
USA
3 Department of Ecology and Evolutionary Biology, Center for Ocean Health, 100
Shaffer Road, University of California, Santa Cruz, CA 95060, USA
4 Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA
95039, USA
5 Scripps Institution of Oceanography, University of California, San Diego, La
Jolla, CA 92093, USA
* Author for correspondence (e-mail: jergold{at}zoology.ubc.ca)
Accepted 22 September 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: diving, foraging, whale
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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): (1) skim or
continuous ram feeding (Balaenidae), (2) suction feeding (Eschrichtiidae) and
(3) intermittent ram or lunge feeding (Balaenopteridae). Lunge feeding only
occurs in Balaenopteridae (rorquals), a group that is characterized by a
reduced tongue and a series of longitudinal grooves of highly extensible,
elastic blubber located on the ventral side of the body
(Orton and Brodie, 1987).
During a lunge, rorquals accelerate toward prey and lower their mandibles,
exposing the oral cavity to oncoming flow. Drag is generated, causing
expansion of the ventral groove blubber around a large volume of water and
prey (Goldbogen et al., 2007).
The high drag generated during engulfment dissipates the kinetic energy of the
body, and as a result, the next lunge requires acceleration from rest. The
forces required to repeatedly accelerate the body demands more energy compared
with maintaining constant speed; therefore, the number of lunges executed
during a dive is predicted to have a significant effect on the energetic cost
of foraging.
Lunge feeding occurs not only at the sea surface, but also apparently at
any depth where prey is particularly abundant
(Calambokidis et al., 2008).
However, regardless of depth, rorqual foraging dives are limited to very short
durations despite their large body size
(Croll et al., 2001;
Croll et al., 2005;
Dolphin, 1988;
Goldbogen et al., 2006;
Panigada et al., 1999), a
characteristic that typically enables longer diving in a wide range of
air-breathing vertebrates (Halsey et al.,
2006; Schreer and Kovacs,
1997). The energetic cost of lunge feeding is hypothesized to be
the cause of low dive durations observed among larger rorquals, such as blue
and fin whales (Acevedo-Gutierrez et al.,
2002; Croll et al.,
2001). This limited diving capacity contrasts with the longer
dives of bowhead whales (Krutzikowsky and
Mate, 2000), which are nearly as massive but ram feed continuously
– a feeding strategy that has been considered more efficient
(Acevedo-Gutierrez et al.,
2002). Dive profiles of blue and fin whales (Balaenoptera
musculus, B. physalus) provide support for this hypothesis, demonstrating
an increase in post-dive recovery time when more lunges are performed at depth
(Acevedo-Gutierrez et al.,
2002). Further support is provided by the detailed kinematics of
these lunges at depth, which indicate a rapid deceleration of the body due to
the high drag experienced during engulfment
(Goldbogen et al., 2006;
Goldbogen et al., 2007).
However, it is unknown whether respiratory rate is increased during these
extended post-dive surface periods, and previous methods to detect lunges have
relied on subjective analysis of dive profiles
(Acevedo-Gutierrez et al.,
2002; Blix and Folkow,
1995) or have assumed that whales perform only one lunge per dive
(Dolphin, 1987b). The methods
to determine the number of lunges during a foraging dive have since been
developed (Goldbogen et al.,
2006), and in this study we build on those efforts by recording
breathing events for tagged humpback whales from kinematic and acoustic data.
Because rorquals breathe once upon surfacing
(Brodie, 2001), this serves as
a way to determine the number of breaths between dives. The number of breaths
taken after a dive is important because it provides information on the oxygen
deficit and carbon dioxide build up that has occurred during a dive
(Boutilier et al., 2001;
Kooyman et al., 1971). Thus,
if lunge feeding is energetically costly, we would expect respiratory
compensation when this activity is superimposed on apnea.
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), foraging whales
exhibited severely limited dive durations and increased respiratory rates.
These data suggest that the high foraging costs associated with lunge feeding
in blue and fin whales also occur in intermediate sized rorquals. |
MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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; Oleson et al.,
2007). Once tagged, researchers on the R/V John Martin
would begin tracking the whale to collect hydroacoustic data (see
`Hydroacoustic prey-field mapping' below). The high-resolution digital tag
(Bioacoustic Probe; Greeneridge Sciences, Goleta, CA, USA) contains a pressure
transducer, hydrophone and a two-axis accelerometer
(Burgess et al., 1998;
Goldbogen et al., 2006). The
tag is equipped with silicon suction cups for attachment and a flotation
device to facilitate tag recovery after the tag falls off the whale. Depth,
flow noise and two-dimensional body acceleration (body pitch and swimming
strokes) were recorded by the tag
(Goldbogen et al., 2006).
Using flow noise to determine lunges and breaths
The flow noise recorded by the hydrophone generates information on the
whale's speed at any given point of a dive
(Goldbogen et al., 2006) and
also pinpoints when a lunge occurs
(Calambokidis et al., 2008). We
established a relationship between flow noise and speed by: (1) measuring the
body velocity from kinematic data (vertical velocity divided by the sine of
body pitch angle) during steep glides (–30 deg.
pitch30 deg.),
and (2) calculating the root-mean-square sound pressure (50 Hz 1/3 octave
band). We used this relationship to calculate the speed of the whale
throughout each dive (Fig. 2).
This is advantageous because the speed calculated from body kinematics is
inaccurate when body pitch is close to zero
(Goldbogen et al., 2006),
which is the typical orientation of the whale during lunges. Speed profiles
were low-pass filtered (0.2 Hz finite impulse response filter) to remove any
noise associated with lift production by the fluke.
An excursion below a depth greater than one body length (>10 m) was
considered a dive. A dive was considered a foraging dive if a lunge was
detected. The presence of a lunge was confirmed by the following criteria
(Goldbogen et al., 2006;
Goldbogen et al., 2007): (1) a
bout of fluking associated with a distinct speed maximum (determined from flow
noise), and (2) continued swimming throughout the lunge, particularly during
the deceleration phase. The rapid deceleration during continued fluking is
characteristic of the high drag experienced during lunge feeding.
Following each dive, the amount of time the whale spent at the surface was
recorded, defined as the time between the whales' first and last breath. A
breath could be detected in two ways (Fig.
3): (1) an acoustic signal when the tag breaks the water surface,
and (2) a phase relationship between undulations in the dive profile and body
pitch angle. We determined the number of breaths taken before and after a
dive. Following previously described methods
(Goldbogen et al., 2006),
other diving parameters were recorded during each phase of a dive, including
dive duration, maximum dive depth, body angle, gait transition depth, and
glide time.
Prey-field distribution and relative density
When a whale surfaces and dives a `footprint' is left on the water surface
because of the water displaced by the moving body. When possible, we navigated
the R/V John Martin directly from one surface location (`footprint'
series) to the next. Along this route, acoustic backscatter by depth was
recorded using a Simrad EK60 (Strandpromenaden, Horten, Norway) digital
scientific echosounder operating at 38, 120 and 200 kHz. The echosounder
operated at a pulse length of 1024µs pinging every 2 s along the route.
These data allowed us to generate a prey–field map that shows the
relative density and distribution of zooplankton as a function of time and
depth (Croll et al., 2005). We
then superimposed the synchronized dive profiles onto the corresponding
prey–field maps. We calculated relative density of krill aggregations as
a function of depth by integrating nautical area scattering coefficient
(m2 target nautical mi–2) values every 15
sx10 m along the path of the foraging whale
(Croll et al., 2005). We also
determined prey type with targeted zooplankton net tows that consisted of 333
micron nets on a tucker trawl.
Statistics
All parameters were tested for normality and homoscedacity before
performing statistical tests. An overall significance level of 0.05 was used.
We used least-squares linear regression to determine the relationship between
diving parameters. We used analysis of covariance (ANCOVA) to test whether
lunge frequency has a significant effect on the relationship between dive
duration and respiratory frequency or surface recovery time.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). These characteristics included a gliding descent,
several lunges at depth, and an ascent powered by steady swimming
(Fig. 4). Speed of the body
gradually increased throughout these gliding descents, which is indicative of
negative buoyancy. On ascent, speed was relatively constant and similar in
magnitude with respect to a variety of swimming animals of different sizes
(Sato et al., 2007). The
integration of dive profiles and prey distribution maps showed how lunges were
directed towards the upper boundary of dense aggregations of prey
(Fig. 5). Prey was identified
as krill (94% Euphausia pacifica,6% Thysanoessa spinifera)
by zooplankton net tows. The foraging behavior of each whale appeared to be
serially correlated, where deeper, longer dives occurred in bouts
(Fig. 6).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Dolphin, 1987b;
Dolphin, 1987c;
Dolphin, 1988) tracked diving
humpback whales and their prey with an echosounder. He found that deeper dives
resulted in an increase in post-dive surface time and respiratory frequency
(Dolphin, 1987c). For some of
these deep dives, the echosounder trace indicated feeding behavior when whales
swam through patches of krill in a sinusoid-like fashion [see
figure 1C of Dolphin
(Dolphin, 1987c) and also
figure 1b of Dolphin
(Dolphin, 1987b)]. These
vertical undulations at the bottom of each dive are reminiscent to the
patterns in the dive profile that we observe here
(Fig. 4). From his data,
Dolphin concluded that the energetic cost of foraging was determined by dive
depth, and therefore dive duration, which was influenced by the spatial
distribution and density of target prey patches
(Dolphin, 1988).
Here we build on these studies with more detailed kinematic data from high
resolution digital tags. These data, combined with a more complete
understanding of how these whales feed
(Goldbogen et al., 2007) allow
us to define actual lunge-feeding events
(Fig. 4) and establish a more
appropriate metric for evaluating the energetic costs of foraging in rorquals.
For example, Dolphin (Dolphin,
1987b) assumed that humpback whales performed only one lunge per
dive, whereas we show that humpbacks are capable of executing up to 15 lunges
per dive (Fig. 3B).
Furthermore, by highlighting where these lunges occur at the bottom of a dive,
we are able to demonstrate how bouts of lunges are directed towards dense
krill patches (Fig. 5). These
results are consistent with video footage from Crittercam studies on foraging
blue whales that show lunges occurring within dense krill aggregations
(Calambokidis et al.,
2008).
Theory predicts that a predator's optimal foraging depth is always
shallower than the depth of highest prey density
(Mori, 1998). Our results
support these predictions because humpback whales executed lunges at the
upper-most boundary of dense krill patches
(Fig. 5), rather than dive
deeper in search of higher density patches. It is not clear how rorquals are
able to detect this increase in prey density with depth. Researchers suggested
echolocation as a possible mechanism after discovering click trains and buzzes
associated with night-time feeding behavior
(Stimpert et al., 2007), but
such signals were not detected during the day time foraging bouts presented
here. Alternatively, rorquals may be able to mechanically sense prey
via tactile hairs or vibrissae located on the rostrum and mandibles
(Ogawa and Shida, 1950;
Slijper, 1979). Thus, a
rorqual may decide to continue descent until it swims into a sufficiently
dense prey patch, as indicated by the number of hits against such sensory
structures.
Our data suggest that lunge frequency may be an indication of prey patch
quality. When krill is abundant, humpbacks should attempt as many lunges as
possible and return to the surface at steep body angles
(Fig. 7E). A steeper trajectory
during a dive should enhance bottom time and the opportunity to execute more
lunges at depth (Fig. 7B). By
contrast, when prey patch quality is poor, the dive is terminated early and
the ascent to the surface, as well as the descent on the next dive, occurs at
shallower body angles (Fig.
7E). For example, note the drop in the depth of the densest krill
layer between the second and third dive of
Fig. 5, which is then followed
by several non-foraging dives. Shallow body angles during diving will expand
the horizontal area covered and thereby increase the likelihood of locating a
better prey patch (Sato et al.,
2004). This may explain why dives that involved fewer lunges were
not relatively longer (Fig.
7A), but instead were terminated early because of poor prey patch
quality. It also suggests that in most cases dive duration is under behavioral
control rather than limited physiologically
(Sparling et al., 2007;
Thompson and Fedak, 2001),
except where prey patch is very good and maximum exploitation of the patch is
desired (i.e. the most lunges possible).
The respiratory patterns associated with lunge frequency for humpback
whales support the hypothesis that lunge feeding is energetically costly.
Foraging dives with more lunges were followed by a longer surface interval
(Fig. 7D) and more breaths
during that interval (Fig. 7C).
Dolphin (Dolphin, 1987c) also
showed respiratory compensation with increasing dive depth and duration
(Dolphin, 1987c), which was
probably related to lunge frequency, based on our observations (Figs
6 and
7). Other diving cetaceans in
controlled experimental conditions, such as bottlenose dolphins and the
beluga, also increase respiratory frequency after longer dives
(Shaffer et al., 1997;
Williams et al., 1999). This
type of respiratory adjustment is a hallmark of increased ventilation that
occurs between dive bouts for a variety of birds and mammals
(Andrews et al., 2000;
Butler and Jones, 1997).
Ventilation is the product of respiratory frequency and tidal volume, and both
of these parameters increase in concert with longer dive durations
(Kooyman et al., 1971).
Increased ventilation is necessary because of the oxygen deficit and
accumulation of carbon dioxide acquired during submergence
(Boutilier et al., 2001). The
rapid replacement of oxygen stores throughout the body is further facilitated
by an increased heart rate during these surface intervals
(Andrews et al., 1997;
Thompson and Fedak, 1993).
If lunge feeding is energetically costly and consequently limits maximum
dive time, there should be respiratory compensation (number of post-dive
breaths) when this type of activity is superimposed on apnea (dive duration).
We can provide indirect evidence for high feeding costs by comparing diving
and respiratory data between singing (Chu,
1988) and foraging humpback whales
(Fig. 8); maximum dive
durations of singing humpback whales were 20 min, approximately twice that for
foraging humpback whales. At the highest lunge frequencies (10–15 lunges
per dive), the number of post-dive breaths is at least triple the value
observed in singing humpbacks that undergo similar dive durations
(Chu, 1988). However, analysis
of covariance does not reveal lunge frequency to be a significant cofactor for
this relationship within each individual whale, which is the result of the
colinearity and asymptotic nature of these dive parameters. Thus more data is
needed to firmly conclude that the increased respiratory rate during foraging
is due to the energetic cost of lung feeding rather than an extended breath
hold.
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Acknowledgments |
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