Bangarang
Backgrounder1
February 2014
How to Lunge Feed
(Expert tips from the greatest whales)
Eric Keen
Abstract
The lunge feed is among the most innovative and outlandish behaviors exhibited by the ocean’s great predators.
But its glory comes at great energetic cost. This fact informs almost everything about the largest whales: their
dive behavior, their size, their range, their reproductive strategy, and most other dimensions of their natural
history.
Contents
Orientation
Rorqual Filtration
The Lunge
The Approach
Opening
Closure
Filtration
Energetics
Efficiency
Costs
Implications
Feeding rates - Dive depth and duration - Prey assessment - Behavioral
innovation - Ecological entrapment - Imperative of expanse - Scale Voracious vagrancy - Dynamic immunity - Selection for size - Sacrifice of
maneuverability - Prey specialization - Tautological selection
Conclusion
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Bangarang Backgrounders are imperfect but rigorous reviews – written in haste, not peer-reviewed – in an effort to organize and
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Orientation
2
The overall size and shape of the great rorquals point to their lifestyle as long-distance, high-speed wanderers.
But look closer and the details of the mouth and throat betray alternative designs (Fig. 1). The mouth is
enormous (Pivorunas 1977, Lambersten et al. 1995), and many dozen gular grooves (60-80 on the blue whale)
of ventral groove blubber (VGB) span the body from the rostral tip to the naval (Goldbogen et al. 2010). The
VGB is reversibly extensible up to several times its resting length (Orton and Brodie 1987), fully capable of
converting a fin whale from a hydrodynamic torpedo to a bloated tadpole. But when her mouth is closed, the
mandibles and maxillary suborbital plate are held in a water-tight seal. This prevents the VGB from expanding
(Goldbogen et al. 2006) and preserves the rorqual’s hydrodynamic fusiform shape (Goldbogen et al. 2011). In
this way, the rorqual’s feeding equipment accommodates a grossly plastic form. This is their key to reconciling
two otherwise exclusive foraging strategies into one: that of a far-ranging trans-oceanic predator who is also an
intermittent ram-engulfment feeder specializing on aggregated micronekton.
Neither dimension of this foraging strategy is feasible without the other. At their size, the fin and blue whales
cannot be either a trans-oceanic wanderer or an eccentric engulfment feeder; they must be both. Filter-feeding
upon lower trophic levels is the only way to support the body size needed for hemispheric home ranges.
Megascale mobility is required to exploit secondary production that is patchy in both space and time. Even
though the maneuverability needed for the former opposes the sleek, large form needed for latter, the two
nevertheless go hand in hand, and the rorqual body plan has somehow accommodated both. In one sense, this
dual nature is evolutionarily self-reinforcing; when selective pressures align, the appearance of such a favorable
trait becomes all the more probable. However, more conditions afford fewer solutions; adaptations would have to
satisfy both selective constraints at once, backing the largest whales even more tightly into an ecological corner
where only rare bounties in an otherwise desolate ocean will suffice. Such evolutionary trappings are wellsprings
of biological creativity, bringing about such outlandish innovations as the feeding machinery of the rorqual.
Figure 1. The blue whale’s body frame accommodates two seemingly exclusive designs: the hydrodynamic fusiform of a
trans-oceanic wanderer and the enormous maw and buccal cavity of an intermittent ram engulfment feeder.
Rorqual Filtration
All baleen whales filter feed. The evolution of filter feeding was driven by an interaction of (1) the spatial
dynamics of prey, (2) the aggregative tendencies of prey on small scales, and (3) the selection for large body
size (Croll et al. 2008). Large marine vertebrates forage in an ever-changing environment in which food
resources are patchy and ephemeral (Croll et al. 2008, Goldbogen et al. 2012b). Despite the complications
associated with subsisting upon these highly unstable resources (Croll et al. 2008, Williams 2006), the energy to
be gained from exploiting low trophic levels is a compelling selective pressure (Werth 2000). Filter feeding has
enabled mysticetes (among other large vertebrates) to exploit prey from lower trophic levels (Berta et al. 2006),
2
The term rorqual generally refers to the mysticete family Balaenopteridae, which has recently been found to be paraphyletic. The grey
whales (currently in the Eschrictidae) are sister to balaenopterid humpback whales. For the purposes of this paper, I use rorquals to refer
to lunge-feeding mysticetes. And by the “largest whales”, I am referring primarily to fin and blue whales even though bowhead and right
whales are not much smaller than fins.
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which is a critical precondition for attaining enormous size (Alexander 1998, Friedman et al. 2010). Predators
that feed consistently upon lower trophic levels are generally larger than those that eat fewer meals of larger
prey (Williams 2006). This can be due to both the energetic benefits conferred (McNab 1986) and the need to
accommodate the feeding equipment (Croll et al. 2008) that best captures and digests plankters (e.g. long
gastrointestinal tracts and large heads to hold baleen racks)(Goldbogen et al. 2010).
Again, all baleen whales filter feed -- but only a select subgroup of them lunge feed. Lunge feeding is a foraging
tactic, unique among vertebrates (Sanderson & Wasserburg 1993) to certain (not all) member of the mysticete
family Balaenopteridae (Mitchell 1974). The evolution of lunge feeding conferred a novel competitive advantage
for crown balaenopterids (Lockyer 1981, Tershy 1992, Berta et al. 2006). The foraging tactic admitted the clade
into unexplored ecological space that, evidently, could accommodate the diversity of rorqual species extant
today which range widely in both their prey specialties and size (from the 7m minke whale to the 30m blue
whale; Lockyer 1981, Tershy 1992). The suite of adaptations involved in a lunge feed results from the exaptation
of pre-existing traits that were shared by all early mysticetes (Lambertsen et al. 1995, Kimura 2002, DemEre et
al. 2005) – including gular grooves, baleen racks, and enormous heads (Werth 2000, 2004) – and the derivation
of myriad new traits unique to the balaenopteridae (Goldbogen et al. 2010).
Because rorquals target discrete patches of euphausiids rather than individual plankters (Croll et al. 1998),
lunge feeding is functionally more similar to how odontocetes capture prey items than the filter-feeding methods
of other mysticetes. Rorquals pursue individual super-organisms, which are less maneuverable than their
constituent euphausiid individuals (Webb & de Buffrenil 1990, Domenici 2001; Stephens & Krebs 1986).
The Lunge
In a lunge feed, the whale attacks a swarm of prey, engulfs the prey-laden water en masse, and then purges the
water through its baleen to retain the prey within (Pivorunas 1979). Simple enough to summarize, sure, but the
adrenaline is in the details.
A single lunge feed takes place on relatively small scales in space and time – ~6 fluke-strokes and ~6.6 seconds
(Goldbogen et al. 2011). Half of this time is spent opening the mouth and the other half is spent closing it
(Goldbogen et al. 2011 krill density). The sequence of events within this timeframe involves many body systems
interacting under extreme strains from both intrinsic and extrinsic forces, all of which must be executed with
controlled precision (Brodie 1975). I herein depict the lunge feed as a four-step sequence (after Goldbogen et al.
2011 and Pyenson et al. 2012): 1) the approach (prey detection and acceleration); 2) mouth opening (buccal
expansion and prey capture), 3) mouth closure, and 4) filtration.
The Approach
As a fin whale enters a prey field, visual cues (Goldbogen et al. 2012b) and tactile vibrissae lining her snout
(Ogawa & Shida 1950, Slijper 1979) register the qualities of the krill patch (Ling 1977, Pyenson et al. 2012). If
conditions suffice (discussed below), the whale accelerates abruptly (Goldbogen et al. 2011).
Approach speed can be as high as 5 m/s (~10 knots), but is most typically 3.7+-0.4 m/s (Goldbogen et al. 2011).
The escape response of krill individuals (O’Brien 1987) is a concern for rorquals large and small. The proportion
of a patch that is successfully captured by a lunging remain likely remains the same if larger whales are
targeting proportionally larger catches (it would be interesting to know if they in fact do). All lunge feeding
species therefore need to take the same measures to maximize capture rate: maximize speed (Potvin et al.
2012) and minimize the duration of the lunge. There is an energy implication here: because lunge duration (~6
seconds) is roughly constant regardless of species or size (Potvin et al. 2012), much more power (work per unit
time) is exerted by the largest whales during a lunge. As we will see, this power load may be the governing
constraint on rorqual body size (Potvin et al. 2012).
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Opening
The maw
As the blue whale reaches her peak velocity, she opens her mouth to 80 - 90º (Goldbogen et al.
2006, Potvin et al. 2012). This is the largest biomechanical action in the animal kingdom (Brodie 1993), and the
many drag forces in play quickly decelerate the whale to a near halt (Goldbogen et al. 2006).
Pivoting the mandibles (Brodie 1993) requires the skull to accommodate both a wide rotational range and
enormous musculature. To maximize the former, the mandibles are posteriorly joined to the skull only by fibrous
joints (Brodie 2001, Orton & Brodie 1987), but they are stabilized by a complex of enlarged muscles. The lower
jaw bears a broad coronoid process, unique to the rorquals, that serves as an attachment site for their large
temporalis muscle. A tendinous part of this muscle, the frontomandbular stay, enhances the mechanical
leverage between the skull and the lower jaw. It thus serves to optimize the maximum gape of the mouth
(Lambertsen et al. 1995).
As the unfused mandibles are lowered, their dorsoventral curvature causes them to rotate and swivel open into
a “hoop” that is larger than when the mouth is closed (Lambertsen et al. 1995, Arnold et al. 2005). This
maximizes the open area of the mouth during a lunge (Lambertsen et al. 1995). The rostrum itself may also be
lifted as the mouth opens, further increasing mouth area (Arnold et al. 2005, Goldbogen et al. 2007).
The lower jaw is steadied further by a maxilla-manibular cam articulation, unique to balaenopterids, that locks
the mouth tightly when it shuts renders the head highly streamlined (Lambertsen & Hintz 2004). Thanks to the
superficial masseter, a tongue muscle which links the mandibles to a unique cranial process, this cam
articulates the jaw in three dimensions to close the mouth tightly and streamline the head. The mandibles are
locked in an upturned position flush to the maxilla, sealing the oral cavity and providing stability for the entire jaw
at high speeds and in turbulent conditions. This cam system may have been the adaption that instigated the
divergence of the Balaenopteridae (Lambertsen & Hintz 2004).
At the front end of the mandibles are other novel features. Unlike most mammals, the two mandibles in a rorqual
do not fuse. The dense connective VGB tissue where they come together, known as the mandibular symphysis,
houses a structure called the Y-Shaped Fibrocartilage (YSF)(Pivorunas 1977). The two branches of the YSF run
along the interior wall of each mandible like a wishbone wreathed in nerves. A newly discovered sensory organ,
absent in all other mysticete families and still without a name, bears enervated mechanoreceptors linked directly
to the YSF (Pyenson et al. 2012). Together the YSF and its sensory organ are sensitive to rotational and
extensional shear between the jaws and the VGB (Pyenson et al. 2012). They detect changes in the expansion
of the VGB and rotation of the jaws as they are lowered, providing precise nervous control to the engulfment
process (Pyenson et al. 2012).
Buccal expansion
As the mouth opens, the buccal cavity expands. It does so passively, solely by
hydrodynamic pressures both within and external to the whale (Orton & Brodie 1987). Drag acts not upon the
mandibles -- which are streamlined to minimize boundary layers that might repel their prey -- but upon the inner
and outer surfaces of the ballooning buccal wall. Passive forces, therefore, instigate the VGB’s expansion
(Goldbogen et al. 2007).
Buccal expansion is aided and abetted by the whale’s weakly muscularized, eversible tongue (Lambertsen
1983). As the mouth opens, the tongue invaginates into an enormous hollow sac (the “oropharyngeal cavity”) to
increase the capacity of the buccal cavity (Lambertsen 1983, Arnold et al. 2005). The backward and downward
eversion of the tongue, combined with the forward motion of the lunge, creates negative pressure in the mouth.
Into this vacuum prey-laden water accelerates (Berta et al. 2006). Together, tongue eversion and VGB
compliance increase the time it takes for engulfed water to accelerate up to the speed of the whale, meaning
that most of the lunge can take place before a bow wave can establish and push away prey (Brodie 2001,
Goldbogen et al. 2007).
Again, neither the jaws nor the VGB are actively distended. In fact, the converse is the case: buccal expansion
is a controlled relaxation of both the mandibular muscle complex and the VGB matrix (Potvin et al. 2009, Potvin
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et al. 2010, Goldbogen et al. 2011, Goldbogen et al. 2012). Controlling the flood of water into its mouth and
eventually stopping the jaw from being ripped open 180 degrees are where the energy costs ramp up. Because
gape angle is an important determinant of the overall energetic cost of a lunge, the ability to control the rate and
degree of buccal cavity expansion enables the whale to maximize the volume captured while minimizing
energetic cost (Pyenson et al. 2012). This control is achieved by close coordination between signals from rostral
vibrissae and the sensory organ of the anterior symphysis (Pyenson et al. 2012).
Because buccal expansion is actively resisted by ventral groove musculature, the engulfed water mass is
accelerated forward to some degree. This introduces a novel form of drag from “within” the whale, termed
“engulfment drag” (Potvin et al. 2009, Goldbogen et al. 2011 scaling lunge-feeding). This is added to the
enormous “shape drag” generated by opening the mouth, which destroys the formerly efficient water flow over
the blue whale’s body (Goldbogen et al. 2011 krill density). Only halfway through a single lunge, the energetic
costs are already mounting.
Capture
As the whale’s mouth approaches its maximum gape of 80 degrees (Goldbogen et al. 2006) to 90
degrees (Pyenson et al. 2012), the combined forces of shape and engulfment drag quickly decelerate the whale
to a near halt (Goldbogen et al. 2006, Goldbogen et al. 2011 krill density). At this denouement, the VGB is
stretched to 300-400% its resting state (Orton & Brodie 1987), the unsuspecting krill should be encompassed by
the buccal cavity. The volume of prey-laden water in the whale’s throat can be equal to or greater than the
volume of the whale itself (Goldbogen et al. 2007).
All that remains is closing the mouth before any micronekton are able to escape (Brodie 1993). It is at this point
that the whale may employ an acoustic tactic to maximize its capture rate. This hypothesis was posed by Brodie
(1993) and to this author’s knowledge has been neither confirmed nor denied. Brodie based his argument on
personal repeated manipulations of freshly killed fin whale jaws on the flensing floor of whaling ships. These
data corroborate anecdotal results of acoustic tag deployments.
“On many occasions during similar operations, and prior to cutting any joint tissue, a familiar sequence of
sounds was observed to originate from the jaw apparatus as it was opened: a growl or rumble, a low hydraulic
suction noise, followed by a powerful knock, the latter seeming to emanate from the tip of the jaw… The plan
view of the jaw changes to a wider parabolic arch as the gape is opened, the knock occurring as the jaw
seemingly dislocates and settles into this new configuration.” (Brodie 1993)
The dislocation he observed seemed to occur in a flexible but inelastic ligament in the inner mandibular
symphysis (what Brodie calls the “synovial tissue”), which is snapped into a new position as the jaws swivel out
into their widest position at maximum gape (Brodie 1993). The rapid separation of this ligament from its previous
articular surface caused a pseudocavitation, producing a large impulsive pressure “accompanied by noise
ranging from low rumbles to loud knocks” (Brodie 1993, citing Myers 1969). This noise would reverberate
throughout the jaws, which are saturated with whalebone oil and shaped as resonant recurved beams. The
mandible’s posterior joint with the skull is covered in thick connective tissue that ensures that this noise will not
propagate into the whale’s cranium, but the mandibles’ interior walls are thinly lined with epithelium so that the
noise will transmit strongly into the buccal cavity, bombarding the krill from all sides. The distended walls of VGB
would also reverberate the sound. Brodie argues that this fracas could induce an escape response of prey away
from the sound source and further into the deep interior of the mouth, thereby minimizing the escapement of
engulfed prey. Brodie also mentions that such knocking noises would have a social utility as well, serving to
orient peers in a group-feeding scenario.
Closure
At the moment of full expansion, the drag forces acting upon the inside of the mouth are transmitted to the YSF
and then the Pyenson et al.’s (2012) sensory organ, triggering the onset of mouth closure. The mouth will be
3
closing on a volume of prey-laden water equal to or greater than that of the whale itself: 71 – 82 m (70 – 80
tons) for fin whales (Goldbogen et al. 2007, whose models suggest higher volumes than earlier estimates from
Pivorunas (1979) of 70% of the animal’s volume).
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As with mouth opening, a huge mandibular musculature is needed to handle these gulps. The special orientation
of the temporalis muscle with the mandible’s coronoid process amplifies the mechanical leverage required to
raise the jaws (Berta et al. 2006). The frontomandibular tendon prevents the hyperextension of this muscle and
its elastic property acts like a spring, storing kinetic energy during mouth opening and providing recoil leverage
to initiate closure (Schulte 1916, Lambertsen et al. 1995, Brodie 2001, Sanderson & Wassersug 1993).
Filtration
The mouth is closed. Over the course of 6 seconds (Goldbogen et al. 2011), the whale decelerated from 5m/s to
a near halt, combated tens of kiloNewtons of drag to open and close its mouth (Goldbogen et al. 2007 big gulp
high drag), and it has doubled in volume. Now, in a gliding creep, the whale moves to purge the water through
its baleen racks and retain its prey.
Within its sealed buccal cavity the engulfed water seisches back and forth in a cross-flow fashion (Kot 2005),
inducing passive and energy-efficient filtration of the prey-laden water through the whale’s baleen racks
(Goldbogen et al. 2007). Keeping her mouth cracked in a sneer, the whale everses her tongue back to its resting
state, the elastic properties of the VGB facilitate the efficient contraction of the buccal wall, and water is forced
through the baleen (Lambertsen et al. 1995, Goldbogen et al. 2006, Croll et al. 2008). When feeding vertically
or at the surface, hydrostatic or gravitational forces may assist the whales by passively compressing their buccal
cavities (Lambertsen et al. 2005, Potvin et al. 2012). Prey is caught in the frayed margins of her baleen racks.
The problem of how whales remove their prey once they have been compacted into the baleen is still without an
answer, but it is likely that the whale licks along the inner walls and coerces the crustacean mass towards its
gullet (Werth 2001).
Once filtration is complete, the lunge is over. The process can then be repeated. The only difference is that,
starting from a near-stagnant repose, energy is needed to accelerate the whale again to lunging speed
(Goldbogen et al. 2011). Negative buoyancy during filtration can help whales to descend and allows them to
regain both momentum and a strategic attack angle without expending energy (Croll et al. 2001).
Energetics
Clearly a single feeding lunge is energetically intensive (Goldbogen et al. 2007, Potvin et al. 2009), but lunge
feeding can be demonstrably efficient (Goldbogen et al. 2007). Despite the enormous tolls inherent to their
feeding method, the largest blue whales accrue lipids, grow enormous and remain competitive planktivores in
marine habitats (Goldbogen et al. 2011). The energetic costs of both lunging and filtering can apparently be
dwarfed by the calories gained from the engulfed krill (Goldbogen et al. 2011).
This efficiency is due to the rorqual’s unmatched engulfment capacity, rapid filter rate, low mass-specific
metabolic rate, and, most importantly, the whale’s ability to target anomalously high-density krill patches
(Goldbogen et al. 2011). A blue whale’s lunge feed can be up to an order of magnitude more efficient than in
other marine mammals, but – again -- only if extremely high-density prey patches are targeted (Goldbogen et al.
2011). Efficiency hinges entirely upon prey density.
Efficiency
The efficiency of a lunge is governed first by prey concentrations and second by allometric relationships
(Goldbogen et al. 2010). In the Animalia, the efficiency of metabolism, morphology, behavior, and locomotion
change as body size grows (“allometry”, as opposed to size-dependent scaling: “isometry”)(Biewener 2005). To
compare the efficiency of an action among rorqual species, we must speak in terms of relative or mass-specific
cost: not just bang, but rather bang per buck.
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As with most mammals, the skull accommodates foraging needs, and the body provides both mobility and
support for the skull (Goldbogen et al. 2010). Larger whales have proportionally larger heads with proportionally
longer mouths and proportionally larger buccal cavities (Mackintosh & Wheeler 1929, Goldbogen et al. 2011),
meaning they have proportionally larger engulfment capacities (Goldbogen et al. 2010). In fact, adult blue
whales have a 70% larger mass-specific engulfment capacity than juveniles; and the engulfment capacity of
individuals 14m or longer is greater than their own volume at rest (Goldbogen et al. 2010). In addition to bigger
mouths, larger rorquals both within and across species have proportionally smaller tails (Mackintosh & Wheeler
1929, Goldbogen et al. 2011). Since the weakly muscularized adipose tissue of the tongue and buccal cavity are
relatively cheap to maintain, exchanging an expensive foot of caudal body material for a low-cost foot of buccal
cavity is energetically frugal (Goldbogen et al. 2010).
Costs
Efficiency may increase with size, but the total energetic costs still climb as whales get bigger (though more
slowly). This is why the blue whale eats both the least and the most of any mammal, depending on how you
think about it; it eats less than 2% of her body weight per day, but eats more pounds of prey per day than any
other animal on earth (Jay Barlow, pers. comm.).
Mass-specific efficiency may increase in larger whales, but mass-specific costs do too (Alexander 1998,
Goldbogen et al. 2010, Goldbogen et al. 2011). Bigger returns are possible, but bigger investments are needed.
Because the work needed to accelerate an engulfed water mass is directly proportional to the water’s mass
(which is proportional to the whale’s mouth size), the work done during a lunge feed is positively allometric: it is
proportionally more work for the largest whales to lunge (Goldbogen et al. 2010, Goldbogen et al. 2011). During
engulfment, blue whale and fin whale metabolism increases to nearly 50 times the basal metabolic rate
predicted for terrestrial mammals of the same body mass (Potvin et al. 2012). Furthermore, the energy required
to filter engulfed water is disproportionately higher in larger whales (Alexander 1998). In both absolute and
mass-specific terms, it is more costly for larger whales to feed (Goldbogen et al. 2010 buccal allometry,
Goldbogen et al. 2011). In both absolute and mass-specific terms, it is more costly for larger whales to feed
(Goldbogen et al. 2010, Goldbogen et al. 2011).
Ceilings
Even if a jetpack went on sale at 95% off, it would still cost way more than you have in your bank account. It is
the same with whales: Even if energy gains outpace these costs, the rorqual Bauplan has nonnegotiable
biomechanical limits that erode the competitive advantage of lunge feeding over a certain body size. That limit
seems to be the required rate of energy delivery (or power) required during mouth opening. In the largest
whales, this power draw can cause significant oxygen deficits with each lunge (Potvin et al. 2012), and may
represent a physiological limit on maximum body size for the lineage (Potvin et al. 2012, Goldbogen et al. 2010).
Rather than being limited by extrinsic conditions such as prey abundance, the body size of the earth’s largest
predator may be limiting itself, confined by the power drain of opening its enormous mouth (Goldbogen et al.
2010, Potvin et al. 2012).
Implications
Croll et al. (2001) were the first to suggest that these inordinate costs had implications for the foraging ecology
and body size of rorqual whales. In the last decade, the advancing science of rorqual energetics has repeatedly
confirmed this hypothesis. The cost of lunging accounts for up to 57% of the total energetic cost of a foraging
dive in tagged whales (Potvin et al. 2009, Goldbogen et al. 2011), and mouth opening seems to be the most
costly component of this process by far (Potvin et al. 2012).
Feeding Rates
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Data suggest that the larger whales lunge fewer times per dive than smaller rorquals (Goldbogen et al. 2010,
Goldbogen et al. 2011). Larger rorquals must filter more water with proportionately less baleen area, requiring
significantly longer filtration times in between lunges than in fin whales (Goldbogen et al. 2008) and humpback
whales (avg. 55 seconds, Goldbogen et al. 2006). Up to a third of a blue whale’s foraging dive is spent filtering
the engulfed water mass (Goldbogen et al. 2011). The larger the whale, the longer accelerations takes, the
longer filtration times are, and the fewer lunges per dive can be done. As body size increases, there would come
a point at which per-dive engulfment capacity levels off and then declines (Goldbogen et al. 2011), such that
intermediate body sizes may be able to filter more volume during a foraging bout than the largest whales
(Goldbogen et al. 2011). The allometric scaling of all these factors -- engulfment capacity, dive duration, and the
number of lunges per dive-- interact such that iforaging mechanics sets not only a upper limit on body size, but
also an optimum.
Dive Depth and Duration
The most direct consequence of lunge costs is their limitation of dive duration and depth, regardless of local
prey density. A typical dive has three phases (Goldbogen et al. 2011): (1) the descent, (2) a bottom phase of
foraging and filtering, and (3) an ascent. In each phase the cost of transit can undermine the energetic efficiency
of lunges.
Cetaceans minimize such costs with a variety of tactics, such as metabolic depression (e.g. bradycardia),
regional heterothermy (Berta et al. 2006), gliding during descent (Williams et al. 2000), steep dive angles (Sato
et al. 2004), passive acceleration by sinking between lunges (Croll et al. 2001), making use of the surface as a
plane for entrapment, and crepuscular feeding habits that exploit vertically migrating prey aggregations before
they disperse at the surface as plankters compete for foraging space (Goldbogen et al. 2011, Oleson et al.
2007).
Despite these adaptations, dives are still surprisingly short (Croll et al. 2001, Goldbogen et al. 2008). Usually,
diving capacity increases with size both within and across taxonomic groups (Butler & Jones 1982). This is not
because larger animals have higher mass-specific oxygen stores (rather, oxygen storage scales isometrically;
Lasiewski & Calder 1971; Hudson & Jones 1986; Glazier 2008), but because the metabolic rate at which those
stores are used is negatively allometric (Glazier 2008); larger animals have lower mass-specific metabolic rates
(Butler and Jones 1982; Halsey, Butler & Blackburn 2006, Goldbogen et al. 2011 scaling lunge-feeding). So in
theory, the largest marine mammals on earth should be among the best divers.
But the demands of lunge feeding cause fin and blue whales to dive shallower and deeper than their Theoretical
Aerobic Dive Limit (TADL) would allow (Schreer and Kovacs 1997, Acevedo-Guiterrez et al. 2002, Croll et al.
2001). Blue and fin TADL are 31.2 and 28.6 min, respectively, yet their foraging dive averages were as low as
10% and 30% of that duration, respectively (Croll et al. 2001). While their foraging dives are still longer and
deeper than non-foraging dives (Doniol-Volcroze et al. 2011), dive duration is still less than would be expected
(Croll et al. 2001). Moreover, dive times appear to be progressively truncated in larger whales (Goldbogen et al.
2011 scaling lunge-feeding). Blue whales and humpback whales (which are roughly half the blue whale’s size)
have been observed with the same general dive duration (7.8 min at the same depth of ~140m, Goldbogen et al.
2010).
This paradox was first noticed by Schreer and Kovacs (1997), who hypothesized that they did so because their
prey occur at shallow depths. However, this did not explain why dives times are shorter regardless of depth
(pointed out by Croll et al. 2001), nor does it account for the fact that the densest prey patches are often found
relatively deep (Sardou et al. 1996, Croll et al. 1998). It also does not explain the fact that large balaenids dive
for twice as long as balaenopterids at shallower depths, but with shorter recovery breaks in between
(Krutzikowsky & Mate 2000, Acevedo-Gutierrez et al. 2002). This puzzle instigated a proliferation of recent
studies in energetics (Croll et al. 2001, 2005, 2006), morphology (Lambertsen and Hintz 2001), kinematics (see
papers with Goldbogen, Potvin, and Pyenson), and optimal foraging (pioneered by Acevedo-Gutierrez et al.
2002).
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Croll et al. (2001) proposed that some intrinsic constraint, namely the high cost of lunges, was limiting dive
duration and depth, and most energetics studies since have confirmed this (Acevedo-Gutierrez et al. 2002,
Goldbogen et al. 2007 big gulps hig drag, Goldbogen et al. 2008, Goldbogen et al. 2011 krill density, Goldbogen
et al. 2011 scaling lunge-feeding). As explained above, while energetic gain depends entirely upon prey
densities, foraging dive duration is limited regardless of the amount of prey engulfed by a physiological ceiling.
The power needed for each lunge accrues inescapable and significant oxygen deficits (Potvin et al. 2012). Even
if engulfed food contains more energy than that expended during a dive, the power required to execute a series
of lunges severely depletes metabolic stores. As a result, dive time is limited allometrically too.
Confinement to short dives limits prey access in both space and time (Goldbogen et al. 2011), with major
ecological and evolutionary consequences, particularly for larger species which require more time to (1)
accelerate to maximum lunge speed, (2) perform the engulfment, and (3) filter disproportionately large volumes.
Their vertical confinement suggests that large species may have evolved compensatory strategies, from
purposefully shallow but repeated dives to vast oceanic mobility. Despite being disadvantageous on the scale of
a single dive, a short and shallow dive profile might actually optimize energetic gains over the course of a meal
(i.e, repeated foraging dives). Minimizing surface recovery time between dives could maximize the overall time
spent foraging at depth. Short and shallow dives would allow the whale to spend less time in transit, less energy
on the overall dive, and less time recovering from dives. While the feeding rate per dive may suffer, overall
energy acquisition rates would theoretically increase (Doniol-Volcroze et al. 2011).
These compensations are all innovative, but surely not ideal. Shorter dives have a high opportunity cost and by
all accounts seem disadvantageous – unless remarkably dense prey patches can somehow be sought out and
exploited.
Prey Assessment
Most of the prey patches whales encounter will not be large or nutritious enough to be worth lunging on. For
lunge feeding to be cost-feasible, it must be invoked with caution. Rorquals must therefore assess the suitability
of the patches first, register their distribution in the water column, isolate the densest patches, decide whether to
pursue, and then exploit them as efficiently as possible if and when they do attack.
At the very near-scale, blue whales probably use a medley of sensory modalities to locate, assess and capture
prey. Prey patches can be tracked visually as well as mechanically, using the tactile vibrissae lining the rostrum
(Goldbogen et al. 2011). But how whales survey and assess the suitability of a field of super-organisms is much
less understood. This reflects one of the substantial gaps in our knowledge of blue whale natural history.
Whether a forager uses prior recognition (e.g., cues) or directly samples (e.g., a “taste test”), resolving
ambiguities in the quality of a prey field are time and energy costs (Stephens & Krebs 1986). One-lunge dives
have been observed in tagged blue whales, suggesting they conduct exploratory excursions into prey fields to
assess quality (Croll et al. 2001). Because the depth, patchiness, and overall nutritious content of a patch are
functions of euphausiid demographics (Decima et al. 2010), a small sample may be enough to compel the whale
to stay or leave. However, the exact cues blue whales are sampling for remain obscure. Patches may be
delineated by downwelling light when viewed from below (Calambokidis et al. 2007), but in productive waters
visual aids are probably uninformative for broad inspections of a prey field. Olfactory pathways in mysticetes are
reduced but still present (Berta et al. 2006), and it has been suggested that prey patches may be detectable and
tracked to some degree using auditory cues (Payne & Webb 1971).
The fact that the largest whales sample and assess their prey field implies that they forage according to
thresholds (Croll et al. 1998, Simrad and Laroie 1999). That is, there is a minimum prey density above which it
will suffice for the whale to linger and forage rather than continue a search (Stephens & Krebs 1986, Brodie et
al. 1978, Kenney et al. 1986). Blue whales have been observed foraging upon aggregations of prey that are two
orders of magnitude more dense than the local mean (Croll et al. 2001, Croll et al. 2005); these prey patches
also contained significantly larger adult krill individuals than the mean adult length in the local population (Croll et
-3
al. 2005). Goldbogen et al. (2011) calculated a “critical” krill density of 100 g m , below which a fin whale will be
9
unable to meet energetic demands even during non-stop foraging effort. The stakes are even higher for blue
whales, which require days of eating two tons of krill in order to survive a foraging season (Rice 1978).
Behavioral Innovation
Despite their common use of the lunge feed to catch super-organisms, balaenopterid species demonstrate
unique behavioral plasticity in feeding (Nemoto 1959, Berta et al. 2006, Sharpe and Dille 1997). Relative to her
congenics the blue whale exhibits the most stereotyped feeding modes, perhaps because fewer behavioral
adjustments are needed to engulf krill than schooling fish. Yet some degree of plasticity still exists. Regional
variations in diurnal feeding patterns (Croll et al. 1998, 2001; Calambokidis et al. 2007, Doniol-Volcroze et al.
2011) probably reflect differences in prey type and behavior. Goldbogen et al. (2012) described 360 degree
rolling maneuvers used by blue whales in the California Current to orient themselves for a lunge and visually
survey the prey field in three dimensions. Daytime foraging dives have been seen to involve steep upward pitch
angles, during which the krill patch is backlit by downwelling sunlight and the whale’s form blends with the
abyssal shadow (Calambokidis et al. 2007, Goldbogen et al. 2011).
Ecological Entrapment
Again, these compensations are innovative, but they push the largest whales even further down a path of
ecological canalization. As the species evolve to target and exploit the very densest patches with greater
success, they grow larger, specialize even further and are progressively confined to an ever stricter rubric for
foraging success. Fin and blue whales cannot subsist on average concentrations of their prey. Only extreme
density anomalies and super-aggregations of prey (Nowacek et al. 2011) will sustain them. Thus larger whales
are increasingly confined to a smaller subset of suitable habitats in which they can subsist (Potvin et al. 2012).
Imperative of Expanse
The costs of lunge feeding and the size required to do it efficiently limit both the depth and the prey conditions in
which whales can feed. The percent of usable ocean is reduced. Therefore, in order to meet nutritional needs,
the amount of ocean the largest whales can access must increase. This access is conferred by mobility, the
ability to cover large areas at low energetic costs.
Mobility allows predators to accumulate nutrition from patchy resources across their foraging range (Burness et
al. 2001, Carbone and Gittleman 2002, Croll et al. 2005, Williams 2006, Croll et al. 2008). Blue whales are the
ultimate interoceanic integrators, accruing biomass during peregrinations that span entire basins and are among
the largest ranges for individuals of any mammal species (Clapham et al. 1999 for review; Reeves et al. 2002).
One tagged blue whale travelled 2,959 km in 30.5 days (Mate et al. 1999); another travelled 20,000 km in 1.5
years (Bailey et al. 2009). Blue whales are also known to forage year-round (Bailey et al. 2009), even in lowlatitudes (Reilly & Thayer 1990; Branch et al. 2007, Bailey et al. 2009). Although the blue whale’s annual travel
pattern broadly adheres to a north-south migration (Burtenshaw et al. 2004), compared to other species blue
whale mark-recapture rates are generally low (Calambokidis et al. 1990, Christian et al. 2006) and their breeding
grounds, if they have any, are largely unknown.
In contrast, smaller but still enormous species like the humpback rely on their mobility to migrate between
seasonal habitats and in one of them (usually a summertime temperate foraging ground) meet their total annual
food requirements in just 4 to 6 months of dedicated foraging. Even fin whales, the second largest animals on
earth (but two-thirds to a half the weight of a blue whale), can afford to establish philopatric foraging grounds
and reduce their foraging effort in winter months (Falcone et al. 2011, Pilkington et al. 2012).
In both life strategies, surviving their journey requires 1) the mobility to succeed, 2) sufficient filtering equipment
to eat enough when food is found, and 3) a sufficient body frame to store those accrued lipids for prolonged
periods of prey-poor oligotrophic conditions during which the whale cannot eat (Croll et al. 2001, Berta et al.
2006).
10
Scale
As a long-lived, well-traveled predator, the spatial vastness of the blue whale’s ecological stage poses a major
challenge to any attempt at studying it (Stommel 1963, Levin 1992, Paine 2006). At various points along that
spatial continuum, the blue whale could be best modeled as an intermittent ram-feeder, central place forager
(returns to the surface during a foraging bout), area-restricted search specialist, or interoceanic integrator.
Likewise, the blue whale’s perspective of its prey changes along this continuum from individual crustaceous
zooplankter, to “super-organism”, to prey field, to a “prey-scape”, to a poleward-propagating wave of food. And,
at every step of this expanding scale, in both space and time, the blue whale must display a foraging strategy
that is efficient, reliable, and resilient (Goldbogen et al. 2012b; Fig. 2). Understanding how this can be is critical
to our reconciliation of the species’ natural history.
Figure 2. Varieties of the blue whale experience, and the relationship between blue whale foraging strategy and events in
prey and the environment. The z axis here is “biological determinism”, that is, the importance of an event in governing the
lebenplan of the animal. Figure inspired by Stommel (1963) and Haury et al. (1978).
Voracious Vagrancy
To feed year-round or nearly so, blue whales rely on their mobility to peregrinate between oceanographic
hotspots, wherever and whenever these may occur (Burtenshaw et al. 2004, Croll et al. 2005, Bailey et al.
2009). Blue whales concentrate foraging effort predictably upon the dense krill swarms that are formed and
maintained downstream from an upwelling center in close proximity to steep bathymetric features (Croll et al.
1998, 2008). The phenology of their prey in a region is a consequence of the interplay between local
oceanography and geography (Croll et al. 1998, Berta et al. 2006). Generally, however, krill outbreaks occur
sequentially, cascading poleward as spring conditions transgress into higher latitudes. Blue whales must exploit
such ephemeral prey anomalies right at their peaks, riding the crests of annual “prey waves” as they propagate
(Burtenshaw et al. 2004, Croll et al. 2005). To do so, they perform high-speed, directed transits from one
predictably productive area to the next (Croll et al. 1998).
11
Upon arrival in a promising foraging area blue whales slow down and adopt a circuitous course, thought to be
indicative of “area-restricted search” (ARS) for suitable prey conditions (Bailey et al. 2009). From tag data, the
average time spent in ARS mode within a feeding area is approximately 21 days (Bailey et al. 2009). The cues
compelling a whale to enter and exit ARS mode remain unknown, but three weeks may represent the average
time it takes for a blue whale group to reduce the density of anomalous swarms in prey field to the initial ambient
concentration, at which point the opportunity cost of lingering would outweigh the advantages (Charnov 1987).
These patterns suggest that individuals do not return reliably to any known area. Blue whales are not sitefaithful, but food-faithful. Blue whales “vote with their flukes” (John Calambokidis, pers. comm.).
Dynamic Immunity
A corollary to the home range advantage is that predators are immune to local dynamics of their prey. They look
down upon ecosystem dynamics from the top of a food chain and they have the mobility to avoid unsuitable
conditions as needed. Prey supply in an area will only impact whale presence, not population. If krill are absent,
blue whales will go elsewhere (sensu large pelagic seabirds; Weimerskirch 2002). If conditions compel them to
expand or restrict their annual range, the can easily do so (Bailey et al. 2009, Calambokidis et al. 2009).
Conversely, if a single blue whale is present, tons of krill may be removed from the ecosystem within hours
(Rice 1978). Though K-selected as a species (Berta et al. 2006), in terms of local ecology the blue whale
functions in more of an r-selected fashion, abruptly colonizing newly available prey fields, exploiting resources
thoroughly, then dispersing. This is only possible due to their extreme size and associated mobility.
Blue whales are long lived (90 years or longer, Sears & Perrin 2009), whereas their euphausiid prey live only
~21 months (E. pacifica, Iguchi et al. 1993) to 5 years (E. superba, Pakhomov 2011). The population dynamics
of long-lived, far-ranging, K-selected predators are typically smoothed and low-amplitude (barring catastrophes
like whaling), and may serve to dampen and stabilize the highly variable dynamics of their short-lived prey
(sensu Hassel & May 1974, Steele 1974, Murdoch & Oaten 1975, Verity & Smetacek 1996).
Selection for Size
A precondition to mobility is large body size (Berta et al. 2006), which brings about low costs of transport (see
below), greater overall speeds and range, the ability to capture large quantities of prey at a time (Williams 2006),
and an extensive frame for accommodating energy reserves in the form of blubber (which is also insulating and
requires less metabolic maintenance; Brodie 1975, Berta et al. 2006). The mobility needed for the survival of the
largest whales would not be possible without being large. Size and life strategy thus go hand in hand. Together
they may have been a mechanism of ecological isolation between fin and blue whales, and have probably
preserved the size discrepancy between the two species.
The benefits of high speeds and efficient long-distance migration (i.e. mobility) provide selective pressures for
large bodies (Goldbogen et al. 2007, Croll et al. 2008). Mobility allows animals to search vast areas efficiently to
find, capture, and assimilate enough food to support large size (Schmidt-Nielsen 1984). The general relationship
between body size and the energetic cost of transport (COT; Tucker 1975) – the power required to move a given
body mass at some velocity – suggests that large cetaceans have very low COT values for their clade, and the
largest of them – the blue whale – may have the lowest of all (Williams 2006, Berta et al. 2006). In addition, the
distance travelled per unit body length decreases. Speed is also a critical component of locomotor performance;
since it directly impacts both drag and daily travel distances. There is a certain optimum speed at which a body
-1
form travels most efficiently. Blue whales typically migrate at ~4 km hr (Bailey et al. 2009), but can sustain
-1
speeds of up to an incredible 8.3 m/s (29.88 km hr , Sears 2002). Locomotor performance is also determined by
the characteristics of appendages. Blue whales have highly streamlined bodies with small, high-aspect ratio
flippers and flukes, suggesting that mobility is a key selective pressure for the blue whale (Woodward et al.
2006).
Body size can be an evolutionary cause or effect of all aspects of a foraging strategy: modes of capturing and
assimilating prey, defending niche space against competitors, and behavioral nuances that maximize energy
gain. In fact, size influences all chapters of natural history at all levels of biological organization (Dial, et al. 2008,
Schmidt-Nielsen 1984, Goldbogen et al. 2011). Because of this, any selective pressure that affects size will
12
ultimately inform the ecology and evolution of a species (Blanckenhorn 2000, Goldbogen et al. 2011). The
implication is that an ecology of physiological constraints and selective pressures interacted to direct the
3
evolution of extreme size in rorquals and no single factor can be said to explain the phenomenon fully .
For all its advantages, enormity comes with two major costs relevant to foraiging: 1) greater nutritional needs
(return to the lunge feed energetics) and 2) reduced maneuverability (see below). Both restrict the largest
whales to even more restrictive prey conditions, confining them even more resolutely into an ecological corner.
Sacrifice of Maneuverability
Because the surface area of appendages increases isometrically while body mass increases allometrically,
larger whales are less adroit at sharp turns, rapid accelerations, and other agile maneuvers (Goldbogen et al.
2010; Webb and de Buffrenil 1990; Domenici 2001; Goldbogen et al. 2011). Body proportions differ among
mysticete species according to where they fall in the continuum of adaptive space delineated by the conflicting
pressures of mobility and maneuverability (Woodward et al. 2006). In both size and proportions, the fin and blue
whales are molded for expansive, efficient travel.
At the core of every foraging strategy are two directives: (1) find food, and (2) eat it (Williams 2006). Generally,
mobility achieves the former while maneuverability achieves the latter. But these two mandates are often in
conflict (Woodward et al. 2006). Each mysticete species strikes its own balance according to its specializations
in habitat, prey, and feeding modality. This balance is embodied in each species’ shape and size. The tension
between mobility and maneuverability escalates in the largest bodies; and their exclusivity scales allometrically,
and as it does the possibility of moderation or compromise diminishes. The largest species must choose a side.
Prey Specialization
The forfeiture of maneuverability must be compensated by adjustments in diet (Goldbogen et al. 2012a).
Compared to schooling fish, euphausiids are non-evasive. By targeting prey as agile (or not) as itself, the blue
whale is freed to dedicate its body form towards maximally efficient travel from one prey patch to the next
(Woodward et al. 2006). That said, euphausiid escape responses can still undermine a whale’s foraging effort.
3 Two questions can be asked: 1) Why are they so big? And 2) why aren’t they bigger? That is, have the largest whales to a maximum size
or an optimal size? Some constraints and pressures may be more determining than others.
Constraints on size can be internal (i.e., physiological limitations) or external (i.e., limited resources of food or space). The influence of these
constraints tends to be context- or state-dependent. That is, a feature that is constrained in one environment (e.g., body weight on land) may
not be in another (e.g., weight in aquatic environments); and a trait that favors selection of larger body sizes in smaller animals can do the
opposite in larger animals. Many ubiquitous survival concerns -- the quality of diet (Case 1979), digestive physiology (Clauss et al. 2003),
energetic efficiency (Alexander 1998), locomotor performance (Alexander 2005, Domenici 2001, Vogel 2008, Potvin et al. 2012), etc. – can
select positively, negatively, or negligibly for larger size depending on the combined states of other variables (Alexander 1998). Constraints
can also be ancestry-dependent: the phylogenetic history of a species bestows an ancestral bauplan that is evolutionarily plastic in some
respects but nonnegotiable in others (Gould & Lewontin 1979). This predestines the adaptive potential of traits.
There are innumerable selective pressures that influence body size, and their traction depends on the interference of other pressures and
the presence or absence of constraints. Every advantage conferred by size can be toppled by a disadvantage under certain conditions.
Allometric relationships in the rorqual body form can lead to advantages, such as lower mass-specific metabolic rates (Kleiber 1947, Boyd
2002) and larger gastrointestinal tract capacity (Williams et al. 2001), as well as disadvantages such as the greater mass-specific power cost
of feeding and overall caloric demand (Williams et al. 2001). Larger body sizes may be more mobile, but are usually less maneuverable (see
below). Larger body sizes confer lower surface-to-volume ratios, which can assist heat retention (Irving 1973, Berta et al. 2006) but also
pose problems for heat dissipation in the largest animals (Alexander 1998). Larger heads would enable directional audition of far-ranging low
frequency vocalizations among conspecifics (Payne & Webb 1971), but there are obvious energetic costs associated with this. Larger body
size may reduce the risk of predation in some cases (Williams 2006), but the loss of maneuverability may render a large organism more
vulnerable to its enemies (Blanckenhorn 2000) and more detectable to both predators and prey (Costa & Williams 2000, Cooper &
Stankowich 2010). In matters of sexual selection, larger body size can be an advantage in female choice (Eric Archer, pers. comm.) and
male competition (Blanckenhorn 2000), but devoting time and energy to growth can delay reproductive maturity or disadvantage a
contending suitor (Blackenhorn 2000). Larger species tend to occur in smaller populations, which are more vulnerable to local extinction
(McLain 1993, Alexander 1998). These and the few that follow comprise only a small subset of all the trade-offs associated with any body
size.
13
To subsist as obligate euphausivores (Goldbogen et al. 2010), the species can counteract krill evasion by
evolving larger mouths (Goldbogen et al 2012b) or by targeting especially dense aggregations (Goldbogen et al.
2011).
Tautological selection
Both the problems (energetic costs and maneuverability) and the solutions listed above (mobility, size and prey
specialization, but also threshold foraging, behavioral tactics and mobility) serve to confine the largest whales
even more resolutely into an ecological corner (Berta et al. 2006, Goldbogen et al. 2011). By conferring mobility,
size both creates and overcomes the problems of maneuverability and feeding at low trophic levels. Ironically,
the only way to support large size is to scour entire ocean basins for the densest available prey fields, and the
only way to achieve that mobility and endure the long periods of subpar conditions is to be large. In this way,
size and mobility are necessarily coupled; one cannot exist without the other, and moreover, neither would exist
without the other. They are inseparable elements of the same life tactic, two sides of the same coin, a textbook
example of what might be called “tautological selection”: the blue whale needs its large size to forage in a way
that supports its large size. The species is in an evolutionary arms race with itself. Its Besslerian predicament
would be a recipe for runaway selection if the blue whale were not already at the ceiling of biological feasibility,
but this selective cycle may have contributed to the initial divergence of the blue whale lineage. The
inescapability of this foraging strategy entrenches the blue whale’s position as the world’s largest predator
Conclusion
The extremes of the lunge feed require that the species evolve extremes in other respects. As enormous, longlived, mobile animals that feed obligately on ephemeral swarms of micronekton, the largest whales are facing
selective pressures for size and mobility that compound to create a life strategy of higher and higher stakes.
Metabolic, morphological and physiological traits are strained to the absolute limits of the rorqual Bauplan, and
only the most exceptional foraging conditions will suffice. This is the essential tension of an enormous
locomoting forager.
While the blue and fin whale are both enormous, their size differences translate to substantially unique life
strategies. Fin whales exhibit site fidelity, seasonal fasting and relatively predictable migration patterns, a life
tactic that carves out ecological space for the species between blue whales and the smaller rorquals. Blue
whales, in contrast, are size-limited not by ecological pressures but by physiological ceilings. They still exhibit
seasonal movements, but their unpredictability, their expanse, and their near-constant foraging effort pushes the
species into a foraging class of its own.
To be tenable, the lunge-feeding mode must be nested within a foraging strategy that straddles numerous
scales in space and time. The result is a bewildering medley of foraging tactics that can be described only with
piecemeal analogies. At the scale of a prey patch, a lunging whale has been compared to a pelican (Field et al.
2011). A rorqual’s size provides enormous capture rates and gulp volumes, enabling her (but also requiring her)
to specialize in anomalously dense, relatively immobile prey. However, krill escape responses can still degrade
capture rates and maneuverability still matters. It is at this scale that the blue whale’s size seems almost
maladaptive, like a limousine trying to navigate a maze of alleyways. These downsides are countered in part by
both higher lunge speeds and the efficiency of big bodies with even bigger heads (Goldbogen et al. 2012). Both
are bestowed by a larger, sleeker, fusiform body. In turn, large size exacerbates the maneuverability problem,
reinforces the imperative of efficiency, and entrenches itself as the solution.
At the scale of a foraging dive, the whale resembles a central place forager, like a breeding seabird confined to
a certain radius from its colony (sensu Ashmole 1963). At the scale of her megascale foraging migration, the
blue whale is most like the largest oceanic seabirds, constantly on the move and tracking environmental
changes to narrow in on suitable prey-scapes. As with the albatrosses (Pennychuik 1987), the key to this
versatility is the blue whale’s body shape and size.
14
Yet size can be compensatory only to a point. It is limited to an optimum set by 1) the biomechanical limitations
of the rorqual Bauplan, and 2) by the caloric value of the regional prey field. Even by targeting highly nutritious
prey, the largest whales are unable to meet their caloric requirements with the average conditions of their prey
field. Instead, they rely on anomalously dense aggregations, which only occur as punctuated and disparate
events in space and time. Blue whales must take full advantage of these “flash swarms”, wherever they are and
as soon as they occur. To do so, the blue whale lineage has evolved into extraordinarily mobile, far-ranging
predators. Their vast mobility allows the whale to integrate prey biomass across immense spatial scales and
brief time scales, surveying oceanographic conditions in search of sufficient prey swarms, at once freed to roam
the ocean yet confined to a strict subset of conditions within it. With the seasons, oceanographic cascades are
triggered throughout the blue whale’s home range, directing her grandest movements and structuring the critical
dimension of her foraging strategy.
The key to this mobility is the blue whale’s great size. While social, courtship, defense, thermoregulation, and
reproductive pressures can influence morphology (Woodward et al. 2006) and demographic tactics (Stearns
1976), the blue whale does not appear especially regulated by any of these. Body size and mobility certainly
accommodate these needs and may be more selectively favorable as a result, but the blue whale is enormous
first and foremost because of the problems of scale in its foraging strategy. The species devotes all aspects of
its natural history to accommodating a body size large enough to eat enough to be large. Blue whales are,
above all else, enormous locomoting foragers.
15
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