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Sleep-like behaviour in the Galapagos Tortoise (Geochelone elephantopus)

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Abstract

Sleep-like behaviour of Galapagos tortoises (Geochelone elephantopus) was studied at Volcan Alcedo, Isabela Island, Galapagos Islands, Ecuador. At midday, most tortoises found asleep were in the open with head and limbs extended; during cooler evening hours, sleeping tortoises usually occupied forms with head and limbs withdrawn, Variability in sleep-like postures during different periods of inactivity probably reflects alternative thermoregulatory strategies. Forms occupied by inactive tortoises (n = 53) comprised vegetation (51%), soil (30%) and other tortoises (19%), The absence of native predators on Galapagos implies a thermoregulatory rather than antipredator function for form use,
HERPETOLOGICAL
JOURNAL,
Vol,
2,
pp
51-53 (1992)
51
SLEEP-LIKE BEHAVIOUR
IN
THE GALAPAGOS TORTOISE
(GEOCHELONE
ELEPHANTOPUS)
FLOYD
E,
HAYES,
WILLIAM
KHAYES*.
KENT
R.
BEAMAN**, AND
LESTER
E,
HARRIS,
JR.***
Depanment
of
Biology,
Loma
Linda University,
Loma
Linda, CA 92350, USA
*Present address:
Depanment
of
Biology Southern College. Collegedale,
TN
37315,
USA
**
Present address: Section
of
Herpetology,
Los
Angeles County Natural History Museum, Los Angeles, CA 90007, USA.
***Present address:
Depanment
of
Biology. La Sierra University, Riverside, CA
92515,
USA.
(Accepted 2.10.90)
ABSTRACT
Sleep-like
behaviour
of
Galapagos
tortoises (Geochelone elephantopus) was studied
at
Volcan Alcedo, Isabela Island,
Galapagos
Islands, Ecuador. At midday, most tortoises found asleep were
in
the
open
with
head
and
limbs extended;
during
cooler evening hours, sleeping tortoises usually occupied forms with
head
and
limbs withdrawn, Variability
in
sleep-like postures
during
different periods
of
inactivity
probably
reflects alternative thermoregulatory strategies. Forms
occupied
by
inactive tortoises (n =53) comprised vegetation
(51%),
soil
(30%)
and
other
tortoises
(19%),
The
absence
of
native predators
on
Galapagos
implies athermoregulatory
rather
than
antipredator
function for form use,
INTRODUCTION
The
electrophysiological
and
behavioural
presence
of
sleep
has
been
documented
in
several species
of
terrestrial
chelonians
(e,g"
Vasilescu,
1970;
Flanigan,
1974;
Flanigan
et
al.,
1974;
Ayala-Guerrero, 1988).
However, Susic (1972)
and
Walker
and
Berger (1973)
failed to detect evidence
of
sleep
in
Caretta caretta
and
Geochelone denticulata, respectively. Consistent with most
studies
of
sleep
in
reptiles, the electrophysiological
characteristics
of
sleep
in
chelonians
have received
more attention
than
behavioural
or
ecological aspects,
Such
an
emphasis
has
emerged because
of
the interest
of
researchers
in
the evolution
of
sleep
phenomena,
in
particular
the electroencephalographic correlates
of
specific sleep states (see reviews
by
Karmanova,
1982;
Meddis,
1983;
Vasilescu, 1983). Consequently, all
investigation
of
sleep
in
chelonians
has
taken place
in
the laboratory without corroborative field studies,
The
studies above
confirm
that
sleep
in
chelonians
may
be
recognized
in
the
field
by
simple
behavioural
criteria. Behavioural sleep is defined as behavioural
quiescence associated with astereotypic posture,
an
elevated arousal threshold,
and
rapid-state reversibility
with relatively intense stimulation (Flanigan,
1974;
Flanigan
et
aI.,
1974), Several investigators reported
that
the
limbs
of
sleeping tortoises are extended, usually
posteriorly
and
parallel to the
body
axis; the
head
likewise is extended,
and
rests
upon
the
plastron
edge
or
substrate with
the
eyes closed (Flanigan,
1974;
Flanigan
et aI.,
1974;
Douglass
and
Layne, 1978),
Under
natural
conditions,
chelonians
might exhibit
much
greater variability
in
sleep
behaviour
than
that
observed
under
laboratory conditions.
In
this
paper
we
report
and
analyse
our
observations
of
sleep postures
and
sleep sites selected
by
Galapagos
tortoises
(Geochelone elephantopus)
in
their
natural
ecological
setting,
We
further discuss
how
the
patterns
that
we
observed
may
reflect alternative thermoregulatory
strategies,
METHODS
During
22-24
June
1984,
we
studied free-ranging
tortoises
(G.
e.
vandenburghi) from alarge resident
population
(3,000-5,000 individuals;
MacFarland
et al.,
1974)
on
Volcan Alcedo, Isabela Island,
Galapagos
Islands, Ecuador. Tortoises were considered to be asleep
when
completely immobile with the
head
resting
on
the
plastron
or
substrate
and
the eyes closed, However,
without electrophysiological
data
we prefer to call this
behaviour
"sleep-like",
For
tortoises found
in
sleep-like
repose, we recorded (1) whether
the
subject occupied a
"form" (facing
into
asoil
bank,
vegetation
or
another
tortoise;
d.
Auffenberg
and
Weaver,
1969;
Judd
and
Rose,
1977;
Strass et al., 1982); (2) whether
the
head
and
neck were extended, partially extended
or
withdrawn; (3)
the
number
of
legs extended; (4) the composition
of
forms (soil, vegetation
or
another
tortoise)
when
used;
and
(5) the substrate (soil
or
grass)
upon
which non-
form users rested,
Observations were
made
at
midday
(1200-1400
hr)
and
near
dusk
(1650-1730 hr).
Maximum
and
minimum
daily temperatures during the study were 18,5°C
and
15.6°C, respectively.
Humidity
ranged from
93,0%
to
97,0%;
all three days were mostly
overcast
In
several
cases «
10%)
we
could
not
record all
data
for
an
individual without the risk
of
disturbing
other
nearby
tortoises, Conventional two-sample chi-square tests
(P
statistic; Siegel, 1956) were used to test for associations
between variables associated with sleep-like behaviour.
RESULTS
Depending
on
the time
of
day, inactive tortoises
assumed
two distinctive postures: (1)
at
midday, most
tortoises found asleep were
in
the
open
with
head
and
limbs extended, whereas (2)
during
cooler evening
hours, sleeping tortoises usually occupied forms with
head
and
limbs retracted (Table
1),
The
substrate (soil
or
grass) selected by inactive tortoises
in
the
open
did
not
vary
during
these periods,
nor
did
the
composition
of
forms (Table
1).
52
FLOYD
E. HAYES,
WILLIAM
K.
HAYES,
KENT
R.
BEAMAN,
AND
LESTER
E.
HARRIS, JR.
Number ofTortoises
Variable Midday Dusk df
X2
P
Exposure
Open
59
20
47.52
<0.001
Form 6
46
Substrate
Soil
22
14
3.20
0.07
Grass
26
5
Form Composition
Soil
3
13
Vegetation 3
24
2
2.16
0.34
Other tortoise 0
10
Head
Extended
48
3
Partially Extended
11
17
2
71.76
<0.001
Withdrawn 6
46
Number of
Legs
Extended
Zero
14
58
One
12
7
Two
23
24
64.84
<0.001
Three
15
0
Four 4 0
TABLE
1.
Conditional
ceIl
totals
for
variables associated
with
tortoises in sleep-like
repose
at
midday and
dusk.
Substrate
comparisons
are
for
tortoises
in
the open (non-form
users).
Regardless
of
the
time
of
day,
the
head
and
limbs
of
sleeping tortoises were more likely to
be
extended when
in
the
open
than
when
in
forms,
and
there was a
positive association between leg
and
head
extension
(Table 2). Forms occupied by tortoises during
both
periods
combined
(n =53) comprised vegetation (51%),
dirt (30%)
and
other
tortoises (19%).
Variables Number ofTortoises df
X2
P
Exposure XHead Exposure
Open Form
Head Extended
50
I
Head PartiaIly
22
62
81.90
<0.001
Extended
Head Withdrawn 7
45
Exposure X
Legs
Legs
Extended
54
5
41.37
<0.001
Legs
Withdrawn
25
47
Head X
Legs
Legs
Extended Withdrawn
Head Extended
49
2
Head PartiaIly 9
19
2
94.60
<0.001
Extended
Head Withdrawn
51
TABLE
2.
Conditional cell totals
for
variables associated
with
tortoises
in
sleep-like
repose,
independent of
time
of
day.
DISCUSSION
Voigt (1975) described thermoregulatory postures
in
the
desert tortoise (Gopherus agassizii)
that
were similar
to
our
observations
of
G.
eiephantopus. At midday,
when
shade
was often sought,
heat
was offloaded by extension
of
limbs
and
head; this posture was also
maintained
during
periods
of
basking,
when
body
temperatures
rapidly
increased. At night,
heat
was retained
by
a
decrease
in
surface to volume ratio
when
the
head
and
limbs were withdrawn.
Our
observations (see also
Douglass
and
Layne, 1978; Auffenberg
and
Iverson,
1979) suggest
that
tortoises sleep
much
of
the time
when
in
these various postures. Hence, variation
in
sleep-like
postures during inactivity likely reflects alternative
thermoregulatory strategies.
In
the
absence
of
body
temperature measurements, it
cannot
be determined
whether
the
midday
postures
of
G.
eiephantopus represented
basking
or
shading
behaviour. Considering
the
cool, overcast conditions
at
the time
of
our
study, the tortoises were more likely
trying to absorb
rather
than
offload heat.
In
apparent
contrast, the
Aldabran
tortoise (Geocheione gigantea),
which must seek
shade
at
midday
during
warm
weather,
remains active
throughout
cool, cloudy days (Swingland
and
Frazier, 1980).
The
use
of
forms may reduce overnight
heat
loss from
convection,
conduction
and
radiation,
and
may
reduce
water loss as well (Auffenburg
and
Weaver, 1969;
Judd
and
Rose, 1977). Forms
may
also provide
concealment
from predators
during
periods
of
inactivity (Strass et ai.,
1982). Because native predators are
absent
from
Galapagos
(MacFarland
et al. 1974), athermoregulatory
function for form use seems most plausible for
Galapagos
tortoises; however,
antipredator
strategies are
still retained
by
G.
elephantopus (Hayes et ai., 1988).
Our
observation
that
tortoises frequently aggregate
at
night
suggests apossible thermoregulatory function similar to
form use. Boersma (1982) found
that
aggregates
of
marine
iguanas (Ambiyrhynchus cristatus), also endemic
to Galapagos,
maintained
higher
overnight
temperatures
than
solitary iguanas, as observed likewise
in
other
reptile groups.
Maximization
of
heat
retention during
nocturnal
inactivity may facilitate increased
morning
foraging
time.
Marlow
(in Huey, 1982)
found
that
burrow
use
by
G.
agassizii
at
night
appeared
to decrease
morning
basking
time
and
increase foraging time relative to more
exposed form users (see also
McGinnis
and
Voigt, 1971).
Heat
conservation
at
night
may
also facilitate digestion
and
reduce vulnerability to
predation
(Boersma, 1982).
With the exception
of
studies
by
Susic (1972)
and
Walker
and
Berger (1973), there is
ample
evidence for
behavioural sleep
in
chelonians. Walker
and
Berger
(1973, 1980) argue
that
sleep
in
reptiles is merely
an
ectothermic manifestation
of
thermoregulatory
behaviour,
and
not
actual sleep. However, sleep
and
thermoregulation certainly are
not
exclusive activities
in
birds
(Amlaner
and
Ball, 1983)
or
mammals
(Walker
and
Berger, 1980),
and
recent electrophysiological
evidence also refutes
the
position
of
Walker
and
Berger
(Hartse
and
Rechtshaffen, 1982). Accordingly, we
suspect
that
thermoregulation is a
dynamic
and
integral
aspect
of
reptilian sleep,
both
processes serving
unique
but
complementary
functions.
The
relationship between
reptilian sleep
and
thermoregulation merits further
investigation.
ACKNOWLEDGEMENTS
We
thank
the Charles
Darwin
Research Station,
Galapagos
National
Park
and
the
DeRoy
family for
assistance
and
logistical support.
Loma
Linda
SLEEPING
IN
GIANT
TORTOISES
53
University
and
L.
E.
Harris, Jr., provided funds
in
support
of
this study.
Our
research was conducted
under
apermit granted to LEH, Jr.,
and
KRB.
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Some giant tortoises (Geochelone gigantea} migrate to the relatively shadeless coast during the rainy season to obtain the seasonal flush of food which results in migrant females laying larger clutches than those tortoises that stay inland. The migrants face the conflict of maximizing food intake and minimizing the risk of death from overheating. Internal body temperature was measured using radio pills: other observations were made of ambient temperature, critical thermal maximum,sward height, tortoise density and body size distribution of grazing tortoises in relation to distance from the nearest shade center. The results are: (i) on cool days tortoises are active all day but on hot days they shade at midday, (ii) CTMax is 36-38°C, (iii) larger tortoises spend longer in the shade, (iv) the need to shade imposes limitations on feeding range, (V) different size tortoises do not have different size feeding ranges. the third, which rainy season (Swi than their conspecifics who have remained inland. However there are hottest time of yi Indeed, death from overexposure is the major apparent cause of 611 often single trees, to which the tortoises must retire during the heat of the day Risk of overheating not only limits the maximum distance from each center at which on feeding range, (v) different size tortoises do not have different size feeding ranges
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