The cost of locomotion in North Atlantic right whales (<italic>Eubalaena glacialis</italic>)

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Nowacek, Douglas P.

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Nousek McGregor, Anna Elizabeth

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2011-01-06T16:01:16Z

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2011-01-06T16:01:16Z

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2010

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Marine Science and Conservation

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Locomotion in any environment requires the use of energy to overcome the physical

forces inherent in the environment. Most large marine vertebrates have evolved

streamlined fusiform body shapes to minimize the resistive force of drag when in

a neutral position, but nearly all behaviors result in some increase in that force.

Too much energy devoted to locomotion may reduce the available surplus necessary

for population-level factors such as reproduction. The population of North Atlantic

right whales has not recovered following legal protection due to decreased fecundity,

including an increase in the intercalf interval, an increase in the years to first calf and

an increase in the number of nulliparous females in the population. This reproductive

impairment appears to be related to deficiencies in storing enough energy to meet the

costs of reproduction. The goal of this study was to determine whether increases in

moving between prey patches at the cost of decreased foraging opportunities could

shift these whales into a situation of negative energy gain. The first step is to

understand the locomotor costs for this species for the key behaviors of traveling and

foraging.

This study investigated the cost of locomotion in right whales by recording the

submerged diving behaviors of free-ranging individuals in both their foraging habitat

in the Bay of Fundy and their calving grounds in the South Atlantic Bight with a

suction-cupped archival tag. The data from the tags were used to quantify the oc-

currence of different behaviors and their associated swimming behaviors and explore

three behavioral strategies that reduce locomotor costs. First, the influence that

changes in blubber thickness has on the buoyancy of these whales was investigated

by comparing the descent and ascent glide durations of individual whales with differ-

ent blubber thicknesses. Next, the depth of surface dives made by animals of different

sizes was related to the depth where additional wave drag is generated. Finally, the

use of intermittent locomotion during foraging was investigated to understand how

much energy is saved by using this gait. The final piece in this study was to deter-

mine the drag related to traveling and foraging behaviors from glides recorded by

the tags and from two different numerical simulations of flow around whales. One, a

custom developed algorithm for multiphase flow, was used to determine the relative

drag, while a second commercial package was used to determine the absolute mag-

nitude of the drag force on the simplest model, the traveling animal. The resulting

drag estimates were then used in a series of theoretical models that estimated the

energetic profit remaining after shifts in the occurrence of traveling and searching

behaviors.

The diving behavior of right whales can be classified into three stereotyped be-

haviors that are characterized by differences in the time spent in different parts of the

water column. The time budgets and swimming movements during these behaviors

matched those in other species, enabling the dive shapes to be classified as foraging,

searching and traveling behaviors. Right whales with thicker blubber layers were

found to perform longer ascent glides and shorter descent glides than those with

thinner blubber layers, consistent with the hypothesis that positive buoyancy does

influence their vertical diving behavior. During horizontal traveling, whales made

shallow dives to depths that were slightly deeper than those that would cause ad-

ditional costs due to wave drag. These dives appear to allow whales to both avoid

the costs of diving as well as the costs of swimming near the surface. Next, whales

were found to glide for 12% of the bottom phases of their foraging dives, and the

use of `stroke-glide' swimming did not prolong foraging duration from that used by

continuous swimmers. Drag coefficients estimated from these glides had an average

of 0.014 during foraging dives and 0.0052 during traveling, values which fall in the

range of those reported for other marine mammals. One numerical simulation deter-

mined drag forces to be comparable, while the other drastically underestimated the

drag of all behaviors. Finally, alterations to the behavioral budgets of these animals

demonstrated their cost of locomotion constitutes a small portion (8-12%) of the

total energy consumed and only extreme increases in traveling time could result in a

negative energy balance. In summary, these results show that locomotor costs are no

more expensive in this species than those of other cetaceans and that when removed

from all the other stressors on this population, these whales are not on an energetic

`knife edge'.

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https://hdl.handle.net/10161/3088

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Conservation Biology

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Ecology

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Biomechanics

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behavior

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Eubalaena glacialis

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hydrodynamics

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locomotor costs

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marine mammal

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North Atlantic right whale

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The cost of locomotion in North Atlantic right whales (Eubalaena glacialis)

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Dissertation

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