Predicted Respiratory System and Respiration Patterns of the Ancient Whale, Dorudon atrox

Lorrin-Alyssa Fujita and Gabriela Aguirre, ZOOL 430, University of Hawaiʻi at Mānoa

Abstract

Dorudon atrox was a large and active predatory whale extant during the Eocene epoch,

while the earth was in its “greenhouse” state. It was estimated that D. atrox’s resting metabolic

rate was 4,262 kJ/hour and its active metabolic rate was 6393 kJ/hour (Fujita and Aguirre, 2019).

On average, cetaceans use up to 80% of their lung volume during respiration and unlike most

mammals, respiration is not a reflex in cetaceans; instead it is a conscious effort (Mead, 2018).

By analyzing the respiratory dynamics of D. atrox, the first fully aquatic cetacean, we can

investigate the novel evolutionary adaptations through the evolution of whales and the

mechanisms underlying their breath holds which maximize oxygen uptake. Thus, D. atrox may

have made modifications to its breath rate in order to supply the necessary amounts of oxygen

depending on its activity, enabling it to fine tune its gas exchange when diving. In order to

supply oxygen efficiently, D. atrox possessed a total lung volume of 188.5 L and an alveolar

ventilation volume of 12.2 L where gas exchange occured. In this study, we propose the overall

respiratory dynamics of D. atrox, model its lung volumes and determine how D. atrox was able

to support itself when resting, active and while diving. D. atrox was able to support its RMR and

AMR with breath rates of 3 and 5 breaths per minute, respectively. It was also found that when

diving, 53 L of O2 was needed for the span of a 10 minute dive and could be supplied with

approximately 3 breaths before descending into the water.

Introduction

Dorudon atrox lived approximately 37 million years ago during the Eocene epoch in the

warm waters of the Tethys sea (Uhen, 2004). It is estimated that D. atrox was 5 meters in length

and approximately 1 meter in height (Figure 1; Uhen, 2004). In a body mass scaling analysis

done by Uhen (2004), D. atrox was estimated to weigh 2,240 kg - a mass and body size

comparable to the modern beluga whale. Despite its large size, D. atrox was an active predatory

whale with a high daily metabolic rate of 151,302 kJ/day (Fujita and Aguirre, 2019) accounting

for swimming, chasing prey, escaping predators and resting. More specifically, its resting

metabolic rate (RMR) was 4,262 kJ/hour and its active metabolic rate (AMR) was 6393 kJ/hour

(Fujita and Aguirre, 2019). D. atrox’s activities and lifestyle required specialized mechanisms to

allow for efficient oxygen delivery to meet its demands.

Figure 1. Sketch of Dorudon atrox by Gabriela Aguirre

Existing towards the end of Basilosaurid evolution, there were many novel characteristics

D. atrox possessed and lacked, serving as a basis for modern whale evolution. Species prior to D.

atrox were semi-aquatic mammals, not requiring any adaptations to maximize respiration in the

water as they were able to haul themselves on land if needed (Gingerich, 2012). However, D.

atrox was the first fully aquatic cetacean (Gingerich, 2012). According to paleontological

evidence, the posterior positioning of the external nares allowed for respiration at the surface of

the ocean with minimal head movement upon ascent (Uhen, 2004; Heyning and Mead, 1990).

This morphological feature is now a characteristic retained by all modern cetaceans (Uhen,

2004). Unlike air breathing terrestrial mammals, the respiratory tract of cetaceans is seperate

from the mouth and pharynx (Davenport et al., 2013). Instead, the upper respiratory tract starting

at their blowhole is connected to their larynx by the vestibular and nasopalatine cavity. Directly

following this is the lungs (Davenport et al., 2013; Figure 2). D. atrox also had a compressed

region of cervical vertebrae, increasing its thoracic length (Uhen, 2004). As a result, it increased

the length of the rib cage as well. The rib cage of D. atrox served as a protective bony cage

which enclosed around the lungs and thoracic region of the body and was comprised of 17 rib

bones (Uhen, 2004). Evidence suggests that various ribs along the thoracic vertebrae were not

attached directly via costal cartilage (Uhen, 2004). Instead, there was separation of the ribs

which could have allowed the chest to expand or collapse. This anatomical feature may support

the idea that D. atrox was able to compress its lungs for dives.

Figure 2. Sketch of D. atrox respiratory system by Gabriela Aguirre

The ability to compress its lungs was important as marine mammals experience an

extreme amount of pressure changes while foraging for food and needed specialized mechanisms

allowing for the lungs to collapse (Fahlman et al., 2019). Collapsing of the lungs is facilitated in

diving whales by having small lung volumes; lung volume in diving whales is not used to

increase oxygen stores, instead they possess modified oxygen affinities in the blood to mitigate

for the reduction of alveolar space (Snyder, 1983). According to Snyder (1983), the degree of

blood oxygen stores in diving mammals can be up to three times that of terrestrial mammals and

in addition, their muscle oxygen stores can be up to ten times that of terrestrial mammals. The

enhancement of oxygen transport is due to the metabolic adaptation of mitochondrial oxidative

metabolism within their skeletal muscles (Kjeld et al., 2018). The degree of storage is

proportional to the metabolic demands of the mammals oxygen utilization (Snyder, 1983). In

order to estimate the duration and depth of D. atrox’s dives, the modern beluga whale was used

as a guideline, since it is a fellow Odontocete comparable in activity level, metabolic activity,

size, and mass (Quakenbush, 2019; Kaplan, 2008; Fujita and Aguirre, 2019). Therefore, we

estimated that on average, similar to the beluga whale, D. atrox dove to approximately 20m for a

durations of 10 minutes (Heide-Jorgensen et al. 1998).

Traditionally, mammalian lung air is inspired through the trachea and into the lungs

(Withers, 1992). The lungs contain the bronchus which then subdivides into smaller and smaller

bronchioles, ending with small sack-like alveoli. Alveoli are the primary location in which gas

exchange occurs whereas the trachea, bronchus and bronchioles are sites of dead space where no

gas exchange occurs (Withers, 1992). On average, cetaceans use up to 80% of their lung volume

during respiration (Mead, 2018). Unlike most mammals, respiration is not a reflex in Cetaceans;

instead it is a conscious effort (Mead, 2018). Therefore D. atrox may have made modifications to

its breath rate in order to supply the necessary amounts of oxygen depending on its activity,

enabling it to better fine tune its gas exchange. Here we explore the overall respiratory dynamics

of D. atrox, modeling the physiology and capacity of its lungs using mass-specific allometric

scaling equations present in Withers (1992). We also determine how D. atrox might have been

able to support its resting metabolic rate (RMR) and active metabolic rate (AMR) based on its

lifestyle. Furthermore, we examine how much oxygen D. atrox would need to supply its body

when actively diving to a maximum depth of 20 meters below sea level for 10 minutes.

Methods

Lung Volume Model

The mass-specific allometric scaling values in Withers (1992) for respiratory variables

were used to calculate the total lung volume (VL ) and tidal volume (Vt ). Dead space volume

(VD) was determined using the scaling variables from Stahl (1996). The same generic equation

Variable [ml] = a (Mass of D. atrox [in kg or g]) b [1]

was used, where a and b are the given scaling values for mammals and the variables are VL and

Vt in Withers (1992) and VD in Stahl (1996) to estimate each variable. Mass was used in either

kilograms or grams, depending on the equation. Alveolar ventilation volume (VA) was then

found by subtracting the tidal (Vt ) and dead space volumes (VD).

During the Eocene, atmospheric oxygen levels were estimated to reach a high of 23%

(Falkowski et al., 2007). The partial pressure of oxygen in the air (pO2 insp) was estimated and

assumed to be 22 kPa and the partial pressure of alveolar oxygen (pAO2) was estimated to be 13

kPa (Sharma et al., 2019). These values were then used in the equation

pO2 exp = ) pO2 fresh air + ) pAO2 fresh air( V tV d ( V t

V a [2]

to determine the pressure of expired air out of the lungs.

Oxygen Needed to Support its RMR and AMR with Respective Breath Rates

When resting, D. atrox is assumed to be breathing at the surface of the ocean with an

RMR of 4,262 kJ per hour. Assuming that D. atrox receives 20 kJ per Liter of O2, the equation

VO2 = RMR in [kJ/hr] ÷ 20 kJ/L O2 [3]

was used and converted to find the amount of oxygen needed in [L of O2/min]. The volume of air

breathed per minute (VE ) was then found using the equation

VE = (VO2 in [L O2/min])(Pbarometric) ÷ (pO2 insp in [kPa] - pO2 exp in [kPa]) [4]

where it is assumed that Pbarometric is 101 kPa and the partial pressure of oxygen in the air (pO2 insp)

was estimated and assumed to be 22 kPa (Sharma et al., 2019). This value was then used to

calculate the breath rate (BR) by dividing the minute volume (VE ) by the tidal volume (Vt ).

This same process was then repeated for the AMR value of 6393 kJ/hr where D. atrox is

assumed to be swimming.

Pulmonary diffusion conductance, DLO2

The pulmonary diffusion of O2 breathed in from the capillaries was determined using the

mass-specific regression equation from Weibel, Taylor and Hoppeler (1991)

DLO2 = 1.55 Mb1.084 [ml O2/min·mmHg] [5]

to determine the rate of diffusion of oxygen per kg of mass. The resulting rate for DLO2 was then

converted to ml/min/kPa/kg to match the units of pressure above. Using the calculated DLO2

value from [5] and assuming that pAO2 - pcO2 is 2.7 for vertebrates (Withers, 1992), the

equation

VO2 = DLO2 in [ml O2/min/kPa/kg] ༝ (pAO 2 - pcO2 ) [6]

was used to estimate the predicted value of VO2 and compared to the calculated VO2 from [3].

Simulated Diving Model

The active metabolic rate (AMR) of D. atrox was used to determine the amount of

oxygen needed during a simulated dive. It was assumed that D. atrox dove to about 20 m below

sea level for 10 minutes at a time (Heide-Jorgensen et al. 1998). and that the hydrostatic pressure

of water increases by one atmosphere, or 101 kPa every additional 10 meters (Withers, 1992).

The VO2 [eq. 3] for D. atrox’s AMR calculated above was used to estimate the minute

ventilation volume (VE ) and the amount of breaths that D. atrox needed to take when

resurfacing.

Results

The overall lung composition of D. atrox was large, allowing it to take in large amounts

of oxygen (Table 1). The dead space was very minimal in comparison to the alveolar ventilation

volume (Table 1), accounting for 27% of the tidal volume whereas the alveolar ventilation

volume accounted for 73% of the normal tidal volume.

When comparing the need to support RMR and AMR it was found that the amount of

oxygen needed by D. atrox increased by 1.8 L, the volume of air inspired per minute increase by

26.8 L and breath rate increased by 1.7 breaths per minute (Table 2). When diving, D. atrox

required 53.3 L of O2 in 10 minutes and this could have been supplied by 3 breaths before

descending (Table 2).

Table 1. The resulting lung volumes of D. atrox.

Total Lung Volume, VL Tidal Volume, Vt Dead Space, VD Alveolar Volume, VA

Values 188.5 L 16.8 L 4.54 L 12.3 L

Table 2. Values of required VO2 , VE , and breath rates in order to support RMR vs AMR and the

simulated dive.

RMR Model AMR Model Diving Model

VO2 3.5 L O2/min 5.3 L O2/min 53.3 L O2 /10 min

VE 53.5 L O2/min 80.3 L O2/min 1606 L O2 /minute

Breath Rate 3 breaths/minute 4.7 breaths/minute 95.6 breaths/minute

Discussion

D. atrox’s tidal volume at rest was reasonable for its size as tidal volumes in cetaceans

vary with mass (Table 1; Snyder, 1983). According to our model, the dead space volume of the

lungs proposes limitations by utilizing approximately a quarter portion of the total tidal volume.

As a result this would have reduced the volume of air that could potentially reach the alveoli.

However, the amount of dead space in cetaceans is typically minimized, this is achieved by

possessing a short cervical region which the trachea follows (Uhen, 2004; Figure 2). This means

that in contrast to our model, the dead space length in which oxygen travels is most likely

reduced, and therefore reaches the alveoli sooner for gas exchange.

At rest, D. atrox was assumed to remain at the surface of the water with its nares exposed

in order to facilitate breathing. In contrast when active, D. atrox was assumed to be swimming,

migrating through the Tethys sea, occasionally ascending to the surface to breathe as needed

(Uhen, 2004). It was found that D. atrox’s normal tidal volume at RMR could only house

one-third of the volume breathed per minute, requiring a total of three breaths per minute (Table

2). Conversely at AMR, D. atrox could house one-fourth the volume, requiring a total of 5

breaths per minute (Table 2). As a result of periodic submergence of the external nares during

activity, breathing is inhibited. In order to mitigate inability to intake more oxygen while

swimming D. atrox’s tidal volume would need to increase allowing it to obtain more oxygen

within a single breath to meet the higher O2 demands.

Additionally, whales require adaptations for diving that allow them to alleviate and

reduce the effects caused by increased hydrostatic pressure. Based on paleontological evidence,

it was assumed that D. atrox remained in relatively shallow waters spending its time swimming

and diving at a depth of 20 m (Uhen, 2004; Heide-Jorgensen et al. 1998). According to the

diving model, it was found that in order to sustain a 20m dive for approximately ten minutes, D.

atrox would have needed 53 L of O2 (Table 2). When diving, cetaceans experience apnea -

periods of time in which there is no influx of new oxygen (Falhman et al., 2019). Generally, as

these periods of apnea increase, the partial pressure of oxygen decreases and the partial pressure

of carbon dioxide increases in the lungs (Noren et al., 2012). Shallow dives encourage the

depletion of oxygen stores and the build-up of carbon dioxide whereas deep dives encourage

cessation of oxygen delivery in the lungs (Ponganis, 2011).Therefore, as a result of shallow

diving we can assume D. atrox experienced O2 dept and CO2 build up. To combat this, it can be

proposed D. atrox possessed similar adaptations for gas exchange in the lungs and peripheral

tissues like its fellow modern cetaceans. However, further analyses of how oxygen storages

within it’s skeletal muscles and diffusion capacity could help mitigate the O2 dept and CO2 build

up would be needed (Kjeld et al., 2018; Pongáis, 2011).

Respective Contributions Primary Author - Lorrin-Alyssa Fujita Secondary Author - Gabriela Aguirre All sections were written and edited by both authors. Appendix Metabolic Rates

- From previous design projects (Aguirre and Fujita, 2019). BMR = RMR = 63.6(2,240,000 g)0.76 = 4,262,315 J/hr → 4262 kJ/hr AMR = 1.5(4,262,315 J/hr) = 6,393,472 J/hr → 6393 kJ/hr

Lung Volume Model

- Using mammal values from Table 13-8 in Withers (1992) and Table 1 in Stahl (1966) - General Equation: Variable [ml] = a Mass of D. atrox [in kg or g] b

- Mass of D. atrox is 2,240 kg (Uhen, 2004) - pO2 insp was estimated and assumed to be 22 kPa (Sharma et al., 2019) - pAO2 was estimated to be 13 kPa (Sharma et al., 2019)

Total Lung Volume (VL )

a = 0.035 and b = 1.06 (Withers,1992) VL = 0.035(2,240,000 g)1.06 = 188,508 mL → 188.5 L

Tidal Volume (Vt ) a = 0.0075 and b = 1.0 (Withers,1992) VL = 0.0075(2,240,000 g)1.0 = 16,800 mL → 16.8 L

Dead Space (VD ) a = 2.76 and b = 0.96 (Stahl, 1966) VL = 0.0075(2,240 kg)1.0 = 4,540 mL → 4.54 L

Alveolar Volume (VA ) VA = Vt - VD = 16,800 mL - 4,540 mL = 12,260 mL → 12.26 L

Partial Pressure of Expired O2 pO2 exp = ) pO2 fresh air + ) pAO2 fresh air( V t

V d ( V tV a

pO2 exp = ( ) 22 kPa + ( ) 13 kPa = 15.3 kPa4,450 mL16,800 mL 16,800 mL

12,260 mL Oxygen to Support RMR

- RMR is 4,262 kJ/hr (Aguirre and Fujita, 2019) - Assume air gives 20 kJ/L O2

VO2 = RMR in [kJ/hr] ÷ 20 kJ/L O2

= 4,262 kJ/hr ÷ 20 kJ/L O2 = 15.3 L O2/hr ÷ 60 min = 3.5 L O2 per minute VE = (VO2)(Pbarometric) ÷ (pO2 insp- pO2 exp)

= (3.5 L O2 per minute)(101 kPa) ÷ (22 kPa - 15.3 kPa) = 53.5 L O2 per minute Breath Rate (BR) = VE ÷ Vt = 53.5 L O2 per minute ÷ 16.8 L = 3 breaths per minute Oxygen to Support AMR

- AMR is 6393 kJ/hr (Aguirre and Fujita, 2019) - Assume air gives 20 kJ/L O2

VO2 = AMR in [kJ/hr] ÷ 20 kJ/L O2 = 6393 kJ/hr ÷ 20 kJ/L O2 = 319.65 L O2/hr ÷ 60 min = 5.33 L O2 per minute

VE = (VO2)(Pbarometric) ÷ (pO2 insp- pO2 exp) = (5.33L O2 per minute)(101 kPa) ÷ (22 kPa - 15.3 kPa) = 80.31 L O2 per minute

Breath Rate (BR) = VE ÷ Vt = 80.31 L O2 per minute ÷ 16.8 L = 4.7 breaths per minute Pulmonary Diffusion of O2 and estimated VO2

- Using the mass-specific scaling equation from Weibel et al., 1991

DLO2 = 1.55 Mb1.084 [ml O2/min/mmHg] = 1.55(2,240 kg)1.084 = 6,637 mL O2/min/mmHg → 670,337 mL O2/min/kPa

- Assuming pAO2 - pcO2 for vertebrates is 2.7 (Withers, 1992) VO2 = DLO2 in [ml O2/min/kPa/kg] ༝ (pAO 2 - pcO2)

= 299 mL O2/min/kPa/kg ༝ 2.7 = 807 mL O2/min/kPa O2 Dept During Simulated 10 min Dive

- AMR is 6393 kJ/hr (Aguirre and Fujita, 2019) - Assuming air gives 20 kJ/L O2

- Assuming kPa increases by one atmosphere every additional 10 m (Withers, 1992) and that D. atrox dove on average to about 20 m depth for approximately 10 min ((Heide-Jorgensen et al. 1998).

VO2 = AMR in [kJ/hr] ÷ 20 kJ/L O2 - = 6393 kJ/hr ÷ 20 kJ/L O2 = 319.65 L O2/hr ÷ 6 = 53.27 L O2/10 min

VE = (VO2 )(Pbarometric) ÷ (pO2 insp- pO2 exp ) = (53.27 L O2 per minute)(202 kPa) ÷ (22 kPa - 15.3 kPa) = 1606 L O2 per minute

Breath Rate (BR) = VE ÷ Vt = 1606 L O2 per minute ÷ 16.8 L = 95.6 breaths per minute

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