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First published online December 16, 2008
Journal of Experimental Biology 212, 50-55 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.019778
Swimming for your life: locomotor effort and oxygen consumption during the green turtle (Chelonia mydas) hatchling frenzy
The University of Queensland, Physiological Ecology Group, School of Integrative Biology, Qld 4072, Australia
e-mail: d.booth{at}uq.edu.au
Accepted 24 October 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: aerobic metabolism, swimming, sea turtle, performance, oxygen consumption
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Gyuris, 2000;
Pilcher et al., 2000), but on
mainland rookeries predation rates may be much lower averaging about 5% (for a
review, see Whelan and Wyneken,
2007). Immediately after entering the water, hatchling sea turtles
begin a phase of hyperactivity (frenzy) that is characterised by almost
continuous swimming for up to 24 h
(Wyneken and Salmon, 1992;
Wyneken, 1997). Swimming is
achieved by `power stroking' bouts lasting 2–10 s in which the front
flippers are moved in an up and down flying motion to generate
lift-drag-lift-based forward thrust (Carr
and Ogren, 1960; Davenport et
al., 1984; Salmon and Wyneken,
1987; Wyneken,
1997; Burgess et al.,
2006). These power stroking bouts are typically separated by brief
1–5 s periods of `dog paddling'. This occurs when the head is raised to
take a breath and the gait switches from front flippers only movement to one
in which diagonally opposite flippers move together
(Salmon and Wyneken, 1987;
Wyneken, 1997;
Burgess et al., 2006). As the
time since entering the water lengthens, power stroke rates slow, dog paddling
bouts become longer, and periods of `rest' may occur, when there are no
flipper movements (Burgess et al.,
2006). While swimming out to sea, hatchling green turtles
(Chelonia mydas Linnaeus) take no evasive action when approached by
aquatic predators; their sole anti-predator tactic appears to be swimming as
fast as possible across the predator-rich shallow near-shore waters and thus
minimising the time they are exposed to heavy fish predation
(Gyuris, 1994).
Because the swimming effort of hatchlings is prolonged, it is assumed to be
supported almost exclusively by aerobic metabolism
(Wyneken, 1997). However,
measurement of blood lactate levels suggests that both aerobic and anaerobic
metabolism power swimming within the first 10–15 min of hatchlings
entering the water (Baldwin et al.,
1989). In a study that measured oxygen consumption
(
O2) of
hatchling sea turtles during and after the frenzy,
O2 during the
frenzy period was found to be considerably greater than in the post-frenzy
period (Wyneken, 1991;
Wyneken, 1997). However, given
the fact that swimming effort during the frenzy period declines considerably
as time proceeds (Wyneken,
1997; Booth et al.,
2004; Burgess et al.,
2006), it is unlikely that
O2 would remain
constant during the frenzy period. In this study I measured both the swimming
effort and the
O2 of newly
emerged green turtle hatchlings simultaneously and continuously during their
first 18 h of swimming in order to test the hypothesis that rates of oxygen
consumption should correlate with swimming effort during this vital
period.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). To test whether the direction and angle of the pull on the
tether affected the force recorded by the force transducer, a 98 mN spring
balance was attached to a tether and stretched sequentially to 20, 29, 59, 78
and 98 mN in all directions and at various angles. In all cases the direction
and angle of the tether did not influence the force recorded by the force
transducer. Water temperature was monitored continuously by a thermocouple
connected via a thermocouple-to-analogue converter (SMCJ-T
Omega.com, Omega
Engineering Inc., Stamford, CT, USA) to the data acquisition system
(Fig. 1). Each hatchling swam
continuously for 18 h, after which it was released into the ocean.
Oxygen consumption was measured using open flow respirometry. The lid of
the respiratory chamber was sealed with vacuum grease, and air was drawn by a
pump at 
80 ml min–1 through Tygon® tubing connected
to the lid, and sequentially through an indicating soda-lime carbon dioxide
absorber, an indicating Dierite® water absorber, a mass flowmeter (GFM17
Aalborg, Orangeburg, NY, USA) and an oxygen analyser (PA-1B Sable Systems, Las
Vegas, NV, USA). Room air entered the chamber through the small hole in the
lid of the chamber through which the tether connecting the force transducer to
the hatchling passed. Outputs of the flowmeter and oxygen analyser were
connected to the data acquisition system
(Fig. 1). The oxygen analyser
was calibrated with high purity nitrogen and dry carbon dioxide-free room air
immediately before and after swimming trials.
O2 was
calculated using equation 4(a) of Withers
(Withers, 1977) after a
washout correction was applied using the method of Bartholomew and colleagues
(Bartholomew et al., 1981).
Control runs of the system without a turtle in it indicated that oxygen
consumption of organisms in the seawater was so small that it was
undetectable, so any detected oxygen consumption could be confidently assigned
to the hatchling turtles.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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O2 data were
averaged into 2 min intervals and plotted against time in order to track
swimming performance and
O2 over time
(Fig. 2). Swimming performance
(quantified by swim thrust) decreased with time and could be divided into
three phases based on the pattern of decline in thrust
(Fig. 2B): (1) rapid fatigue
(0–2 h), (2) slow fatigue (2–12 h), and (3) sustainable swimming
effort (12–18 h).
O2 was highly
correlated with swim thrust during the rapid and slow fatigue phases, but
poorly correlated during the sustainable swimming phase
(Table 1;
Fig. 3) and, as a consequence,
also decreased with swim time (Fig.
2A). Hatchlings having the greatest
O2 also produced
the greatest swim thrust (Table
1). Thrust production efficiency [swim thrust per watt, calculated
assuming lipid was the substrate being metabolised during swimming and that
every litre of oxygen consumed corresponded to the expenditure of 19.7 kJ of
energy (Schmidt-Nielsen,
1997)] had a tendency to increase as swimming time passed
(Fig. 2C), being least
efficient during the rapid fatigue phase, of intermediate efficiency during
the slow fatigue phase, and of greatest efficiency during the sustainable
swimming phase (Fig. 2C;
Table 1). To statistically test
whether thrust production efficiency changed significantly between the three
phases (0–2, 2–12 and 12–18 h), the 2 min values for thrust
production efficiency for each of the five hatchlings were averaged across
each of the three phases to give a single mean value of thrust production
efficiency (Table 2). A
repeated measures ANOVA in which mean thrust production efficiency was the
dependent variable and swimming phase was the (repeated) independent variable
was applied to these data and indicated that thrust production efficiency
changed with swimming phase (F2,8=6.765,
P=0.019).
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Stroke rate during a power stroking bout, mean maximum thrust per power stroking bout, and the proportion of time spent power stroking were averaged over 10 min periods (Fig. 4). Stroke rate during a power stroking bout decreased very rapidly within the first 30 min, decreased at a slower rate over the first 4 h and continued to decrease at a slow rate until 12 h, and then did not change significantly between 12 h and 18 h (Fig. 4A). Mean maximum thrust produced during power stroking declined rapidly in the first 2 h, did not consistently increase or decrease from 2 to 6 h, declined steadily from 6 to 12 h and did not change significantly between 12 h and 18 h (Fig. 4B). The proportion of time spent power stroking appeared to remain constant for the first 6 h and then decreased steadily until 12 h, after which it did not consistently increase or decrease (Fig. 4C).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Gyuris, 2000;
Pilcher et al., 2000). Once
turtles have crossed the fringing reef and entered deeper water it is
generally assumed that the density of predators in the surface waters
decreases dramatically (otherwise all hatchlings would be predated within the
first couple of weeks of entering the sea) so turtles can slow their swimming
effort without increasing the chances of predation greatly and join the
currents that will ultimately take them into the open ocean for their oceanic
life history stage.
Direct measurement of hatchling swimming speed has been made in the field
by tethering small floats to hatchlings and following these floats (e.g.
Salmon and Wyneken, 1987;
Gyruis, 1994; Gyruis, 2000; Pilcher et
al., 2000) but this method is logistically intense, requiring
individuals to be tracked by surface craft and such resources were not
available for this study. Another study
(Pilcher and Enderby, 2001)
used a swimming flume to assess hatchling swimming performance. However, the
drawbacks of this method are that swimming speed is chosen by the
experimenter, and that only one hatchling can swim at a time. Using hatchlings
tethered in tanks to assess swimming performance as in this study is a good
compromise because tethered hatchling swimming behaviour is similar to that of
free-swimming hatchlings (Wyneken and
Salmon, 1992; Wyneken,
1997), and the only assumption that needs to be made is that the
thrust measured from tethered hatchlings is directly related to swimming
speed. The added advantage of this method is that swimming effort can be
quantified into the different components of power stroke rate during a power
stroking bout, the maximum thrust produced per power stroke and the proportion
of time spent power stroking, and other energetic measurements such as oxygen
consumption can also be made.
The current study indicates that swimming effort as quantified by the mean
swim thrust generated has three distinctive phases during the frenzy swimming
period: (1) a rapid fatigue phase (0–2 h), (2) a slow fatigue phase
(2–12 h), and (3) a sustained effort phase (12–18 h). Changes in
the power stroke rate during a power stroking bout, the thrust produced per
power stroke and the proportion of time spent power stroking all contributed
to this pattern as has been found previously for green turtle hatchlings
(Burgess et al., 2006). To
generalise, the very rapid decrease in swimming effort observed during the
rapid fatigue phase of swimming (0–2 h) was caused by a combination of a
dramatic decrease in power stroke rate in the first 30 min which was followed
by a less dramatic decrease in power stroke rate between 30 and 120 min, and a
continuous decrease in power stroke thrust between 0 and 120 min
(Fig. 4A,B). The proportion of
time spent power stroking did not change significantly during this period
(Fig. 4C). During the slow
fatigue phase (2–12 h) decreased swimming thrust was caused by several
factors. Firstly, stroke rate during a power stroking bout declined steadily
throughout this phase (Fig.
4A). Secondly, towards the end of this phase decreases in both the
thrust produced during power stroking and the proportion of time spent power
stroking occurred (Fig. 4B,C).
During the sustained swimming effort phase (12–18 h) there were no
significant changes in power stroke rate, power stroking thrust or proportion
of time power stroking.
The rapid decrease in effort during the first 2 h of swimming is likely to
be caused by the depletion of muscle cell glycogen stores
(Hill et al., 2004) and a
decrease in the contribution made by anaerobic metabolism. Anaerobic
metabolism is significant during the first 10–15 min of swimming as
indicated by an increase in blood lactate concentration at a rate of
approximately 1 µmol ml–1 of blood per minute in
free-swimming green turtle hatchlings
(Baldwin et al., 1989). Clearly
this accumulation of lactate cannot continue indefinitely and the rapid
decrease in stroke rate per power stroking bout during this time
(Fig. 4A) probably reflects a
shift to predominantly aerobic metabolism. The patterns of change in swim
thrust, stroke rate during a power stroking bout and the proportion of time
spent power stroking are very similar to those obtained from Heron Island
green turtle hatchlings emerging from eggs artificially incubated at 30°C
(Burgess et al., 2006) and
naturally incubated eggs during the 2006–2007 nesting season (T. Ischer,
K. Ireland and D.T.B., unpublished data) indicating that these general
patterns are somewhat stereotypic for green turtle hatchlings hatched from the
Heron Island rookery.
Energetics of swimming
The method used in the current study allowed measurement of swimming effort
and O2 within 2
min of hatchlings being placed in the water. I found that the greatest
swimming effort and oxygen consumption occurred within the first 10 min of
hatchlings entering the water, and that swimming effort and oxygen consumption
decreased rapidly during the first 2 h of swimming
(Fig. 2).
O2 directly
followed the decline in swimming effort during the first 12 h of swimming, an
observation consistent with the assumption that hatchling sea turtle swimming
is powered predominantly by aerobic metabolism
(Butler et al., 1984;
Wyneken, 1997). As a
consequence, as hypothesised, there was a strong correlation between swimming
effort and O2
(Table 1;
Fig. 3). Only a few studies
have measured O2
in hatchling sea turtles while swimming [leatherback Dermochelys
coriacea (Lutcavage and Lutz,
1986; Wyneken,
1991; Wyneken,
1997; Jones et al.,
2002; Jones et al.,
2007); olive ridley Lepidochelys olivacea
(Jones et al., 2007);
loggerhead Caretta caretta
(Wyneken, 1991;
Wyneken, 1997); green
(Wyneken, 1991;
Wyneken, 1997). Of these
studies only Wyneken (Wyneken,
1991; Wyneken,
1997) made measurements in the crucial first few hours of
swimming, but even this, the most comprehensive study, confined
O2 measurement
to just the 0.5–2 h interval after the hatchlings first entered the
water, a time when the hatchlings in the current experiment were rapidly
decreasing their swimming effort (Fig.
1A). Hatchlings in the present study had generally lower rates of
O2 compared with
those in Wyneken's study (Wyneken,
1991; Wyneken,
1997) which might be explained by a difference in the swimming
effort of different populations of green turtles. Wyneken
(Wyneken, 1991;
Wyneken, 1997) found
O2 for a 26 g
green turtle hatchling (the mean mass of hatchlings in the current study) to
decrease from 34 to 31 ml h–1 (average 33 ml
h–1) during the 0.5–2 h interval of the frenzy swimming
phase, and I found a maximum value of 34 ml h–1 during the
first few minutes of swimming but
O2 was generally
much lower than this for most of the swimming frenzy and decreased from 30 to
18 ml h–1 (average 24 ml h–1) for the
0.5–2 h interval. Hatchling
O2 in the
current study averaged 10 ml h–1 from 12 to 18 h of swimming,
much lower than the post-frenzy value (20 ml h–1) and only a
little above the resting value (8 ml h–1) reported by Wyneken
(Wyneken, 1991;
Wyneken, 1997), although
Prange and Ackerman (Prange and Ackerman,
1974) reported the resting metabolism of newly hatched green
turtles to be just 2.6 ml h–1. This suggests that the green
turtle hatchlings in the current study were considerably less active swimmers
than those studied by Wyneken (Wyneken,
1991; Wyneken,
1997).
The relatively broad range of swimming effort and
O2 recorded
during the rapid and slow fatigue swimming phases resulted in relatively high
correlation coefficients between swimming effort and
O2
(Table 1), a result similar to
that reported for hatchling leatherback and olive ridley turtles within the
first 4 weeks of hatching (Jones et al.,
2007). In contrast, the relatively narrow range of swimming effort
and O2
experienced during the sustained effort phase resulted in low correlation
coefficients (Table 1) and this
may also explain why only a weak correlation between swimming effort and
O2 was found in
hatchling sea turtles previously (Wyneken,
1991).
The tendency for swimming efficiency to increase with time as swim thrust
decreased is probably related to the fact that the frequency and speed of
muscle contraction decreased with swim time and thus the energy needed to
counter the inertial forces of moving limbs through the water decreased
(Vogel, 1989), and the fact
that faster limb movements create large wakes, and larger wakes dissipate more
energy (Prange and Schmidt-Nielsen,
1970). The rate of energy expenditure as indicated by the rate of
oxygen consumption clearly tracks the swimming effort of hatchling green
turtles (Figs 2 and
3). The rate of energy
expenditure decreased precipitously within the first 2 h of entering the
water, and declined more slowly between 2 and 12 h before decreasing to a
sustainable level after 12 h of swimming.
The dry mass of residual yolk of hatchling green turtles incubated at 28
and 30°C averages 1.5 g (Booth et al.,
2004) and has an energy density of 32.5 kJ g–1
(D.T.B., unpublished data) so the residual yolk would contain about 49 kJ of
energy. Only about 4.8 kJ of energy was expended during the first 18 h of
swimming (corresponding to 6.4 kJ day–1, but during the
sustained effort phase energy expenditure was 4.7 kJ day–1),
so it would appear that a hatchling could survive at least 10 days of
continuous swimming without the need to feed. Taking into account the fact
that green turtle hatchlings do not swim continuously throughout the day after
their first day at sea (Wyneken and
Salmon, 1992) this non-feeding period may stretch to 2 weeks.
Similar theoretical calculations also indicate that leatherback and olive
ridley hatchlings could survive up to 3 weeks without feeding
(Jones et al., 2007). Pilcher
and Enderby (Pilcher and Enderby,
2001) reported that the swimming ability of green turtle
hatchlings was compromised if hatchlings were contained in beach enclosures
for 6 h after emergence from the nest. They suggested that this decrease in
swimming performance may be due to depletion of limited energy stores in
hatchling turtles during the time that they were trapped within enclosures.
However, the calculations above clearly indicate that energy depletion from
body stores would not limit the swimming ability of green turtle hatchlings
within the first 10 days of hatching. It is far more likely that muscle
fatigue is the cause of decreased swimming effort during this time
(Burgess et al., 2006).
Ecological implications
Because the chances of a green turtle hatchling surviving the reef flat
crossing depends on, among other factors, swimming speed
(Gyuris, 1994), it is not
surprising to find that hatchlings put their maximum swimming effort into the
first few minutes of swimming. Given that fringing reefs surrounding coral
cays are typically 100–600 m wide and that swimming speeds of green
turtle hatchlings during the frenzy phase are typically 1.0–1.6 km
h–1 (Pilcher et al.,
2000; Wyneken,
2000) it should take a green turtle hatchling between 10 and 40
min to cross the fringing reef. The rapid fatigue phase in which the swimming
effort is greatest lasts approximately 2 h, so hatchlings should be well
beyond the fringing reef by the time their swimming effort begins to plateau.
Once the deeper waters outside the fringe reef are reached, the swimming
effort can be eased and the residual yolk can supply enough energy to support
continuous swimming for at least 10 days without feeding, and given that green
turtle hatchlings do not swim continuously beyond the first 24 h
(Wyneken and Salmon, 1992)
this theoretical non-feeding period could be as long as 2 weeks.
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Footnotes |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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