Postnatal ecdysis establishes the permeability barrier in snake skin: new insights into barrier lipid structures
1 Department of Zoology, University of Florida, Gainesville, FL 32611-8525,
USA
2 Department of Biology, William Paterson University of New Jersey, Wayne,
NJ 07470, USA
3 Department of Ornithology and Mammalogy, California Academy of Sciences,
Golden Gate Park, San Francisco, CA 94118, USA
* Present address: Department of Biology, National Taiwan Normal University,
Taipei, Taiwan 116, Republic of China
Author for correspondence (e-mail:
hbl{at}zoo.ufl.edu)
Accepted 8 July 2002
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Summary |
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Key words: snake, Lampropeltis getula, ecdysis, skin, evaporative water loss, skin resistance, hatchling, epidermal differentiation, lipid, permeability barrier, mesos layer, alpha keratin, endothermy
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Introduction |
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The water barriers of reptilian skin are of special interest, for several
reasons. First, reptiles live successfully in a wide variety of conditions,
ranging from aquatic to xeric terrestrial habitats. Second, lepidosaurian
species exhibit periodic losses of `epidermal generations' associated with
synchronized patterns of pan-body cellular proliferation and differentiation;
these losses are unique and quite distinct from the renewal of epidermis in
other vertebrates (Baden and Maderson,
1970; Maderson et al.,
1998
). Third, numerous studies have demonstrated that reptilian
skin is an important pathway for water loss and that rates of TEWL vary
inversely with habitat aridity (Dmi'el,
1998
; Gans et al.,
1968
; Lahav and Dmi'el,
1996
; Mautz,
1982a
,b
;
Roberts and Lillywhite, 1980
).
Barrier function in reptiles, as in mammals, appears to be genetically
determined. However, the barrier can be rapidly restored following trauma
(Maderson et al., 1978
), and
some species have been shown to exhibit plasticity for enhancing resistance to
TEWL under conditions of water stress
(Kattan and Lillywhite, 1989
;
Maderson, 1984
). Very little
is known, however, about the properties of integument with respect to the
important transition from the aqueous environment of the embryo to the
terrestrial environment of the neonate and adult.
The skin of full-term human and rodent newborns possesses a competent
permeability barrier at birth, and the timing of barrier formation is close to
the disaggregation of periderm and direct epidermal contact with amniotic
fluid (Hardman et al., 1999;
Kalia et al., 1998
;
Williams et al., 1998
).
Comparable data are not available for reptiles, despite their various
advantages as models for study (Dhouailly
and Maderson, 1984
). The purpose of the present investigation was,
firstly, to determine whether the first postnatal ecdysis affects TEWL and
skin resistance (Rs) in newborn snakes. Secondly, we
investigated the hypothesis that changes in TEWL or Rs are
related to changes in the mesos layer, which is the site of the permeability
barrier in snake epidermis. Finally, we quantified the influence of postnatal
ecdysis and associated changes in TEWL and Rs on reclusive
behaviors of the newborn snakes.
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Materials and methods |
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Measurements of evaporative water loss and skin resistance
We measured TEWL and skin temperature, and calculated skin resistance to
TEWL in 20 snakes obtained from six different clutches. Measurements were
repeated on four consecutive occasions, and each animal served as its own
control. The four consecutive trials were: (1) within the first 3 days
following hatching; (2) 2-4 days following the first ecdysis; (3) 14-16 days
following the first ecdysis; and (4) 4-10 days following the second
ecdysis.
For measurement of TEWL, each snake was lightly anesthetized by exposure to halothane vapor within a closed jar in order to induce immobility and apnea. The anesthetized snake was then fully extended and positioned loosely within a cylinder of 3 mm wire mesh. This tube, with snake extended, was then placed within a clear acrylic tube (41 cm x 2.5 cm i.d.) through which room air at ambient temperature was pumped at rates of 41.6-43.6 ml min-1 (Fig. 1). Airflow was maintained by an Applied Electrochemistry model R-1 flow control pump and calibrated with a volume meter. Excurrent air was directed through a Sable Instruments RH 100 RH/dewpoint meter before entering the flow pump. The RH/dewpoint meter was calibrated with dry nitrogen and water-saturated air prior to experimental measurements. Air temperatures and skin temperatures of snakes were measured with 30 g copper-constantan thermocouples.
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During measurements, ambient air temperature and humidity varied within the
range 24.5-25.6°C and 46.2-56.8%, respectively, in different trials, but
remained virtually stable during individual trials. Ambient humidity was
lower, however, during the fourth measurement (following second ecdysis) when
it varied from 38.1-49.8% in different trials. All measurements used in
calculations of TEWL were made after the flow-through system had equilibrated
to constant excurrent humidity with a snake inside the chamber. Evaporative
water loss was calculated from the relationship
TEWL=[e
i]x
a,
where
is water vapor density of incurrent or excurrent air and
a is rate of airflow. In separate
experiments we examined mass changes of physical models to incorporate a
correction factor for absorption of water vapor by the acrylic tube, using the
equation: corrected TEWL=0.81xmeasured TEWL.
The skin resistance to evaporative water flux was calculated as
Rs=RtRb,
where Rt is the total resistance and
Rb is the boundary layer resistance. The total resistance
was calculated from the relationship
Rt=[s(RHx
a)]xTEWL-1,
where
s is the saturated water vapor density at skin
temperature,
a is the saturated water vapor density at ambient
chamber temperature, and RH is the relative humidity of ambient air
(Spotila and Berman, 1976
).
The boundary layer resistance was determined as above, utilizing measurements
of TEWL from `wet' snakes that evaporated as a free water surface. This
separate set of experiments included seven different measurements from either
agar models of snakes used in the study, or anesthetized snakes that were
wrapped with a single layer of fine, water-saturated tissue paper. Values of
Rb were less than 5% of Rt.
To measure skin surface areas of snakes, we carefully wrapped each
individual with a layer of thin Parafilm® fitted carefully to head and
body contours, while the snake was still anesthetized at the conclusion of
TEWL measurement (Lillywhite and
SanMartino, 1993). This Parafilm was then removed from the snake
and laid out on a piece of paper. This paper was cut to match the area of
Parafilm and weighed on a balance to convert mass to area, using mass:area
calibrations derived from the same paper.
Ultrastructure and histochemistry
Pre- and post-shed (first ecdysis) skin samples were frozen on dry ice and
stored at -70°C until sectioning. These samples were embedded in OCT
compound, and 10-12 mm sections were cut on a cryostat maintained at
-20°C. Sections were transferred onto slides and stained with Fat
Red7B for neutral lipids, washed in 70% alcohol followed by water,
mounted in glycerine jelly, observed and photographed. For routine
ultrastructure, freshly obtained skin samples were fixed in Karnovski's
fixative for 24 h, washed in sodium cacodylate buffer, osmicated in 1% osmium
tetroxide, dehydrated through a graded series of alcohol, and routinely
embedded in Epon 12. To demonstrate the barrier lipid structures, skin samples
were post-fixed with 0.5% ruthenium tetroxide (RuO4) instead of
osmium tetroxide (OsO4) for 1 h and then processed as above. With
respect to routine histology, semi-thick sections (0.5-1 mm) of
OsO4-fixed samples were stained with Toluidine Blue for light
microscopy, while silver gray sections were double-stained with uranyl acetate
and lead citrate, then visualized using a Zeiss EM 12 microscope. Silver gray
sections from RuO4-fixed samples were observed with and without
double staining to evaluate the lipid structural organization.
Behaviour
We observed the behaviours of snakes used for measurements of TEWL and
noted a tendency for individuals to be less reclusive in damp moss following
the initial ecdysis. Therefore, we devised an experiment to test whether
snakes altered humidity selection following ecdysis. We used a total of 26
newborn snakes in this experiment, different from the ones used for
measurement of TEWL.
Each snake was kept individually in a plastic shoebox, as above, provided with wet and dry microenvironments. To create a dry microhabitat, we placed about 4 g of dry sphagnum moss in a plastic cup 11 cm in diameter by 4.5 cm in height. The wet microhabitat was prepared the same way except that 10 ml of water was added to the moss inside a second cup. We allowed ample time for the moisture to distribute and wet the moss evenly, resulting in moss that was moist to the touch. Each of the two cups was positioned at opposite ends of the box, and their relative position was determined randomly in each trial. The humidity and temperature within the box and both cups were checked regularly throughout the experiment. The humidity inside the box was 69-88% (mean=78.6%), and the temperature was 24.4-26.1°C (mean=25.3°C). The humidity (66-85%; mean=76.1%) and temperature (24.3-26.0°C; mean=25.3°C) of dry cups were similar to those within the box, while the wet cups had constantly higher humidity (95.0-98.0%; mean=98%) and a slight tendency to lower temperature (24.0-25.5°C; mean=24.8°C) than either the dry cups or the greater box environment.
At the beginning of the experiment, each snake was placed in the center of the box. The location of each snake was recorded once during the late afternoon, at night, and the following morning. Then, before repeating the test, each box was wiped clean, and two new cups were placed in the box. We tested each hatchling snake for seven days before shedding. After shedding we waited for 1-2 days and then retested the same individuals for another week.
Because the location of each snake in the afternoon, evening and morning was generally the same, we used only the location of each morning observation in the analysis of behaviour. Each snake had seven trials before and after shedding. The probability of a snake staying in the wet or dry cup was computed for each individual.
Stastistics
Data are reported as means ± S.E.M. To evaluate changes in measured
variables related to TEWL, we performed a repeated-measures analysis of
variance (ANOVA), followed by post hoc tests to examine differences
between specified trials. In other circumstances, we employed paired
t-tests, as described elsewhere in the text. Behavioural data for the
percentage occurrence of snakes in wet versus dry containers were
analyzed first using non-parametric Wilcoxon signed-rank tests. All analyses
were performed using SAS Stat View© 5.0.1 for Windows.
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Results |
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Evaporative water loss, skin resistance and skin temperature
TEWL of newborn snakes was nearly twice as great at hatching as in the same
individuals following the first ecdysis (81.2±5.2 versus
45.7±1.5 µg cm-2 h-1, respectively),
reflecting a doubling of the skin resistance (Rs) after
skin shedding (441.7±24.9 versus 865.7±30.7 s
cm-1) (Fig. 3).
Subsequent measurements showed a downward trend of TEWL, but the changes were
not statistically significant, partly because of a decrease in the humidity of
room air that increased the tendency for TEWL during the fourth measurement
period. However, Rs, which is independent of ambient
conditions, decreased significantly following the second ecdysis
(Fig. 3). The variance of
measurements within and between clutches was similar
(Fig. 4), and clutch effects
for TEWL and Rs before and after postnatal ecdysis were
not significant (ANOVA, all P>0.05). Inspection of
Fig. 5 illustrates how
subsequent measurements of Rs of the same individuals
before and after ecdysis tend to covary. This pattern suggests that much of
the variation in TEWL and Rs among individuals represents
true biological variation rather than errors in experimental measurements.
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The guts of hatchling snakes were visibly distended with yolk that gradually diminished during the first 2 weeks following hatching. Skin temperatures were elevated 0.60±0.06°C above ambient air temperature during the initial measurements of TEWL, then converged toward ambient in subsequent measurement trials as visible evidence of yolk disappeared (Fig. 6).
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Ultrastructure and histochemistry of the skin permeability
barrier
Histology of epidermis
Histologically, the pre-shed epidermis showed about five layers of
nucleated cells and compact and ß layers. The mesos layer was not
discernible at a histological level (Fig.
7, insets). The epidermis showed a perceptible staining for
neutral lipids when stained with Fat Red-7B
(Fig. 8B, inset). Post-shed
epidermis showed fewer nucleated layers, more pronounced
and ß
layers, and perceptible lipid staining with Fat Red-7B
(Fig. 9B, inset).
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Ultrastructure of neonatal, pre-shed epidermis
The epidermal organization in snake skin is complex, and further
complicated by the stage of the skin-shedding cycle at the time of biopsy. In
this study we restricted our observations to the general morphology of the
layers, and focussed especially on the organization of the barrier lipid
structures, as revealed by the RuO4 post-fixation. Due to its
highly reactive nature, RuO4 is destructive to the cytosolic
elements, especially keratin, and hence evaluation of RuO4-stained
tissues has to be complemented with routine OsO4-fixed samples
(Menon and Ghadially,
1997).
In low-magnification, survey electron micrographs (Fig. 7A), the outer ß layer appears to be artificially separated from the underlying mesos layer. Within the ß layer, individual cell boundaries are apparent. The mesos layer is composed of about three layers of extremely flattened, thin cells, which periodically show a slight `ballooning' of electron-lucent cores. Desmosomal connections between overlapping mesos cells are apparently absent or very rare.
Below the mesos layer is the layer. As seen in
Fig. 7A, there are about 2-3
cell layers of mature
cells, with characteristically dense,
keratinized cytosol, subjacent to which are 1 or 2 cell-thick immature
cells. Below the immature
cells lie two layers of nucleated cells. As
the basal cell is rather large and not flattened, this may be indicative of a
very early renewal phase.
As mentioned earlier, and noted elsewhere in the literature,
RuO4 staining causes considerable disruption to cytosolic
structures and keratin, while superbly staining the extracellular barrier
lipid structures (mortar) and cellular lipid inclusions, including
lipid-enriched organelles. In the mesos layer of pre-shed skin, the
extracellular lipids stained by RuO4 showed some lamellar
organization, but lacked the tight bilayer organization that characterizes
barrier efficacy. The somewhat chaotic organization seen here
(Fig. 8A) is reminiscent of
what has been reported in fetal mammalian skin before the attainment of
barrier competence (Azsterbaum et al.,
1992). Within the
layers, the outer, mature
cells
showed large inclusions of lamellar lipid structures intermixed with
electron-lucent lipid material. Unlike the mesos layer, the
layer
showed prominent desmosomal connections between adjacent and subjacent cells.
In immature
cells, the cytosol contained many vesicular and
membrane-bound structures, most notably large lipid inclusions of varying
morphologies. Multi-lamellar bodies of these snake
cells (mlb;
Fig. 10) closely resemble
avian multi-lamellar bodies (Fig.
10, inset). There were also large elongated lipid structures with
a lamellar substructure (ll) and electron-lucent cores (l), and large
electron-lucent lipid inclusions with lamellar lipids at their periphery
(arrow).
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Ultrastructure of post-shed epidermis
In low-magnification, survey electron micrographs
(Fig. 7B), the outer ß
layer showed a clearer syncytial organization, as compared to the pre-shed
stage. The artifactual separation (resulting from tissue processing) between
the ß and mesos layers was also seen in the post-shed samples. However,
the outermost mesos cell layer remained partly attached to the ß layer
(Fig. 7B). The mesos layer is
approximately 6-7 cells thick, which reflects a doubling of cell numbers
following the first ecdysis. Individual cells of the mesos layer were quite
similar to those of their counterparts in the pre-shed stage in all features,
including the paucity of desmosomal connections.
The layer was also thicker than in the pre-shed stage, and
consisted of about 4-5 mature
cell layers. Below this were two cell
layers of viable, nucleated cells, including the basal layer that rests on the
basement lamina.
Intercellular domains of the mesos layer showed lipid structures with well-defined, continuous bilayer organization (Fig. 9A, arrows) as opposed to the pre-shed samples. The bilayers were not anchored to any desmosomal structures (in contrast to lipid bilayers in mammalian stratum corneum).
Within the mature cells, lamellar inclusions were retained
(Fig. 10), similar to what is
seen in pre-shed epidermis. However, as immature
cells were not seen
in the samples we examined, mlbs of the kind seen in the pre-shed stages were
not observed.
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Discussion |
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Perhaps the most profound change in hydric environment during the ontogeny
of any squamate reptile is the natal transition from the `aqueous' environment
of the embryo to the aerial (terrestrial) environment of the neonate or
hatchling. It is well known (anecdotally) that newborn snakes generally shed
their skin within 24-36h after birth or hatching, although in some species the
timing is longer (Ernst and Zug,
1996). There are two factors that are likely to stimulate early
postnatal shedding. First, neonatal snakes continue to assimilate yolk while
growth processes contribute to body elongation, even before feeding
(Fig. 2). Ecdysis is presumably
necessary to accommodate these increases in body length. Second, it seems
likely that the natal transition from embryonic fluids to air stimulates
ecdysis as an important means of adjusting the permeability barrier
(Maderson, 1984
). In king
snakes we have demonstrated a twofold increase in Rs as a
result of the first postnatal ecdysis, correlated with a structural doubling
of the permeability barrier (Figs
3,
7).
Previous studies of TEWL in snakes have reported measurements for adult
animals, but there is little information about neonates or juveniles. Dmi'el
(1985) reported measurements of
TEWL and Rs for a range of body mass that included
hatchlings of the desert snake Spalerosophis diadema, and he found
that Rs was independent of body mass. However, shedding
histories of the snakes were not mentioned, so it is not known whether the
hatchlings in Dmi'el's study were measured before or after postnatal ecdysis.
While the Rs of newborn king snakes is less than half that
of older snakes (Fig. 3), it is
nonetheless several-fold greater than Rs that has been
measured in an aquatic species of snake
(Lillywhite and SanMartino,
1993
). It appears the periderm and embryonic epidermis slough
within the egg (Alibardi,
2002
), and a permeability barrier (beta and mesos layers) of
partial competence is formed within the epidermis prior to hatching
(Maderson, 1985
), similar to
barrier formation in mammals (Williams et
al., 1998
). The further increase of Rs at the
second postnatal ecdysis (Fig.
3) demonstrates a continued capacity for improvement of barrier
function, as previously shown for lizards
(Kattan and Lillywhite, 1989
;
Kobayashi et al., 1983
) and
for birds (Menon et al.,
1996
). Our measurements demonstrate there is a threefold
improvement of barrier effectiveness (Rs) over the two
shedding cycles examined in the present investigation
(Fig. 3). However, the maximum
effectiveness of the barrier in this species, and its facultative mechanism,
remain to be determined.
The postnatal changes in barrier effectiveness that we describe here for
king snakes differ strikingly from those of altricial species of birds endemic
to xeric environments. Nestlings of zebra finches (Taenyopygia
guttata) have a remarkably tight water barrier that progressively
decreases in efficacy as they fledge, allowing evaporative cooling for
thermoregulation (Menon and Menon,
2000). However, under conditions of water deficit, adult zebra
finches are capable of rapid facultative waterproofing. We do not yet know
whether adult snakes are capable of facultative changes in permeability
barrier effectiveness as shown for some lizards and birds.
Newborn humans and rodents possess a competent permeability barrier at
birth, with rates of TEWL at least as low as in adults
(Williams et al., 1998).
Barrier formation begins during late gestation and involves a progressive
increase in the thickness of skin layers, formation of a multilayered stratum
corneum, secretion of lipid lamellar bodies in the interstices of stratum
corneum, and transformation of short lamellar disks into compact, continuous,
lamellar unit structures (Aszterbaum et al., 1992). The keratinization and
barrier formation in skin coincide with changes in the composition of amniotic
fluid and are thought to be essential for protection from amniotic fluid
during late gestation (Hardman et al.,
1999
; Parmley and Seeds,
1970
). Also, contact of rat fetal skin with air accelerates
barrier formation (Williams et al.,
1998
). Little is known about the processes underlying permeability
barrier ontogenesis in reptiles. However, present data for king snakes suggest
that emergence of the integument from embryonic fluids and its subsequent
contact with air are essential for completion of barrier competence in the
newly hatched animals, which might render them potentially useful models for
mechanistic investigations of barrier development.
Ultrastructure and histochemistry of hatchling integument
From the morphological data, there is a clear correlation between the
reduced cutaneous water loss in post-shed snakes and qualitative and
quantitative changes in the mesos layer, which is the acknowledged site of
permeability barrier in snakes (Lillywhite
and Maderson, 1982). The increased number of cell layers in the
mesos layer in post-shed skin, together with the continuous, organized bilayer
structures of the barrier lipids, would contribute to a tighter permeability
barrier, in contrast to the less organized lipids of the mesos layer in
pre-shed skin. A similar structure-function relationship in barrier competency
is seen when xerically stressed birds upregulate their permeability barrier
(Menon et al., 1996
) as well
as during the fetal mammalian barrier maturation in late gestation
(Azsterbaum et al., 1992
;
Hardman et al., 1999
). The
chaotic organization of bilayers in pre-shed ophidian skin is reminiscent of
the similarly disorganized lipids of mammalian fetal skin before attainment of
functional competency.
An interesting feature of the snake mesos layer is the apparent paucity of
desmosomal connections within this layer
(Fig. 7). In mammalian
permeability barrier formation, desmosomes play important roles in (i)
providing initial anchoring for the secreted lamellar body contents that
subsequently undergo enzyme-mediated processing into mature lamellar bilayer
structures, and (ii) providing cohesion to the corneocyte `scaffolding'
(bricks) structure that supports the organization of lipids (mortar) providing
the permeability barrier. Again, it is the gradual dissolution of the
desmosomes in upper stratum corneum, mediated by lamellar body-derived
proteases, that allows controlled desquamation in mammals. Such a pattern of
desquamation does not occur in snakes, due to the syncytial nature of the
outer beta layer and the unique pan-body shedding cycles that characterize
ophidian skin. However, it is quite possible that a sequence of desmosomal
degradation similar to that in mammalian stratum corneum might occur within
the mesos layers of snake epidermis during its early formation in the pre-shed
condition, when the second generation is formed beneath the one that is
destined to be shed. As the mesos layer is physically protected, by virtue of
being sandwiched between the ß and layers, no desquamation could
result from the desmosomal degradation within the mesos layer. It is
interesting to speculate on the functional benefit of a barrier layer that is
free of desmosomes and yet protected from desquamation and loss. From a
structural point of view, the stress propagation through desmosomes could
conceivably weaken the delicate mesos layer, while in its absence, this vital
barrier layer could be protected from the physical stresses of locomotion,
preventing shearing or other damage to the `waterproofing' lipid bilayers.
Another interesting feature concerns the lipid inclusions within cells of
the layer. Within the mature
layers, these inclusions show
lamellar as well as electron-lucent morphologies, bearing close resemblance to
what has been described for avian stratum corneum
(Menon and Menon, 2000
). In
the immature
cells of pre-shed skin, multilamellar bodies
(Fig. 10) and different stages
of `dissolution' of lamellar inclusions into electron-lucent lipids are
dominant features, again very similar to what is seen in avian transitional
cell layers (Menon et al.,
1996
). These observations point to an intriguing possibility that
cells themselves might be involved in the barrier homeostasis, which
has not previously been suggested for ophidian epidermis. The facultative
waterproofing ability of avian epidermis
(Menon et al., 1996
) resides
in its capacity to modulate the type of lipids secreted, i.e. non-bilayer,
electron-lucent lipids under basal conditions, but lamellar lipid structures
under xeric stress, leading to significantly decreased evaporative water loss.
The retained bilayer lipids in the avian stratum corneum under basal
conditions, as well as that seen in the ophidian
layer (previously
named cholesterol clefts by Jackson and
Sharawy, 1978
), might represent a reserve barrier mechanism.
Whether snakes can modulate TEWL by secreting lamellar lipids from the
layer has not yet been evaluated. We speculate that this might be possible,
and such a mechanism could perhaps underlie the large variation of TEWL that
is observed among neonate snakes. Experimental tape stripping of scales
results in
layer hyperplasia, and in this type of barrier repair, no
mesos layers are formed until the next skin shedding cycle
(Maderson et al., 1978
).
Careful ultrastructural investigations on the
layers during the repair
response that follows tape stripping might reveal whether newly formed
cells are secreting lamellar lipids to reseal the barrier-defective areas,
without necessitating a pan-body epidermal renewal needed to form the mesos
layer.
Endothermy and behaviour of hatchlings
The transient elevation of skin temperatures, averaging 0.6°C above
ambient air temperature, are sufficient to designate hatchling king snakes as
endothermic (Fig. 6). The guts
of the hatchlings we studied were visibly distended with yolk, and skin
temperatures gradually converged toward ambient temperature, as visible
evidence of yolk disappeared and the snakes increased in length (Figs
1,
6). These results strongly
suggest that the endothermic conditions of snakes reflect an elevated
metabolic rate related to the digestion and assimilation of yolk
(Bakker and Andrews, 1984;
Beaupre and Zaidan III, 2001
).
The condition appears to be analogous to postprandial calorigenesis (`specific
dynamic action'), which is capable of producing remarkable elevations in
metabolism in snakes (Secor and Diamond,
2000
). The presence of internal yolk in newborns has been reported
for a number of squamate reptiles and appears to be an important energy
supplement used for synthesis in growing neonates
(Beaupre and Zaidan III, 2001
;
Stewart and Castillo, 1984
;
Troyer, 1983
). Newborn water
snakes (Nerodia rhombifera) contain both fat bodies and yolk remnant,
which account for 71% of the total lipid present at birth and 43% of the
original yolk lipid (Stewart and Castillo,
1984
).
The skin permeability barrier is considered important for water balance and
potentially influences the behaviours of newborns and hatchlings. Our
observations of newborn king snakes demonstrate that they are reclusive and
seek humid microenvironments prior to their first ecdysis. While the majority
of hatchlings continued to seek shelter in humid environments following
ecdysis, a significant fraction abandoned humid containers in favor of drier
ones (Table 1). Thus, it
appears quite probable that the degree of water movement across the integument
has an important influence on the dispersal of newborn snakes away from birth
sites. In this context, it is of interest that neonatal pit vipers of numerous
species remain with their mother until the first ecdysis, after which maternal
care is abandoned and the young disperse
(Greene et al., 2002). The
degree of water movement across the integument has perhaps been important in
influencing the evolution of parental care in viviparous species of squamates.
The low resistance of neonatal skin to water loss, coupled to a transiently
high metabolic rate, would promote high rates of evaporative loss in the
newborn. Insofar as yolk assimilation precludes the necessity for immediate
prey capture, aggregation of snakes in protected places would reduce surface
area/volume ratios and thereby mitigate a tendency to dehydration.
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Acknowledgments |
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