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Influence of
environmental humidity and dietary protein on
pyramidal growth of carapaces in African Spurred
tortoises (Geochelone sulcata) Weisner &
Iben 2003 : A criticism of the research
methodology
A. C. Highfield
This paper by Wiesner and Iben has been
uncritically cited numerous times and used frequently as
key source material in discussions and reference texts
related to the problem of so-called “pyramid growth” in
tortoises. One of the most striking recent examples
is this statement, by world famous chelonian expert Peter
C. H. Pritchard in his chapter “Evolution and Structure of
the Turtle Shell” which features prominently in “Biology
of Turtles” (CRC Press, 2008) pages 71-72:
“It has long been believed that the condition
resulted primarily from a diet too rich in protein, and an
alternative cause was believed to be an unnatural
ratio of calcium to phosphate (sic) in the diet ...
secondary causes were attributed to overfeeding,
dietary fiber, temperature, UV light and so on... however current
belief is that the condition derives from excessively
dry conditions and when hatchlings are raised on a substrate
of wet sphagnum, shells will develop normally (Weisner &
Iben, 2003).
My italics. Note how other factors are dismissed by use of
the past tense, and “current belief” identifies “excessively
dry conditions” as the sole cause and “wet sphagnum” as the
sole solution. The inconvenient fact that “wet sphagnum” (or
anything like it) is not typically present at all in
semi-arid tortoise habitats is entirely overlooked... and no
physiological mechanism of any kind is suggested that could
account for such an effect.
Furthermore, the above quotation is not directed merely at
African Spurred tortoises. It is directed at all
tortoises, without exception or limitation.
With such uncritical acceptance, and such widespread and
influential citation it is extremely important that the
claims made by that paper, and the methodology employed to
arrive at them, are indeed examined most carefully and with
a critical eye. It is also important to highlight and
identify cases where the paper is misinterpreted and
misquoted as this creates yet another layer of obfuscation
and confusion.
Humidity measurement and control
The most striking criticism of the data cited in their paper
Influence of environmental humidity and dietary protein on
pyramidal growth of carapaces in African Spurred tortoises
(Geochelone sulcata) is that the bands of experimental
humidity employed are so profoundly overlapping and
are of such wide individual range. This glaring and
fundamental defect that runs through the whole paper. The
terminology used is also contradictory in the extreme.
For example, they define what they term “dry environmental
conditions” as consisting of the ranges “24.3-57.8 %
and 30.6-74.8% relative humidity”. They then go on to define
“humid conditions” as comprising “45-99% relative humidity”.
It is difficult to know what to make of this, when a
relative humidity of (for example) 45% is described as
“humid” in one sentence and then classed as “dry” in
another.
It is also the case that very few ecologists or geographers
would normally accept relative humidity of 74.8% as in any
way representing dry conditions, or 45% as
representing humid conditions (such conditions are normally
defined as those containing a high percentage of water
vapor; noticeably moist). The use of such inconsistent
and erratic definitions applied to the measured relative
humidity levels must therefore give rise to serious concern
and further confuses the earlier claims made in respect of
“dryness” or “humid” conditions allegedly experienced in
natural habitats.
The criticism of the methodology employed goes far beyond
mere terminology, however. The actual bands used in the
experiment are cited as follows:
Group A: 24.3-57.8%
Group B: 24.7-55.5%
Group C: 30.6-75.8%
Group D: 47.9-99%
Group E: 45-99%
The authors state that these measurements represent
the “mean of eight weekly measured values of the maximum and
minimum relative humidity” in the experimental enclosures
employed which consisted of a series of 100 X 80 X 80
cm glass terraria fitted with two sliding glass doors.
Hiding places or “caves” made of brick measuring 30 X 65 X
10 cm were provided in each enclosure, and the substrate
used was the same in all units, comprising 4-5 cm of “bark
humus”. The humidity levels cited in the above table were
“measured directly under the top pane”, which would mean at
the very top of the inside of the unit close to the heat
lamps. They took a second set of humidity measurements in
the caves of the “three humid terraria” (no mention at all
is made of any such measurements in the “dry” terraria)
which it is claimed were “10-15% higher” (than what?). It is
unfortunately entirely unclear what these actual
measurements were as no actual figures are given. If indeed
the measurements were “10-15% higher” however, and were
calculated on the same basis as Table 1, that would mean
that humidity levels of up to 114% were achieved. This is a
scientific impossibility (the maximum theoretical level of
RH is 100%), therefore it is likely that they really meant
“10-15% higher than the average ambient” - but they do
not say this, and they provide no figures for what those
averages might have been, so again, we are left without a
clue as to what conditions these tortoises were really
subjected to.
Also, no temperature measurements of any kind are provided
anywhere in the paper. It is unclear why the authors
considered this critical information unimportant.
Information on temperature is absolutely essential when
considering relative humidity and the dew point, for
example. Temperature information is also extremely important
when evaluating any method of tortoise maintenance.
Only eight weekly measurements per group with just the mean
of the maximum and minimum values were cited. The range is
so huge, and the method of measurement so unreliable and
imprecise we have no idea at all what the true average
relative humidity was for each group. None whatsoever. How
can you possibly reach any valid conclusion on the effects
of a particular level of relative humidity, when you are
subjecting the tortoises to anything from 45% to 99% for
unspecified periods as they did with ‘Group E’ for example,
or 30.6% to 74.8% in ‘Group C’? Most people would regard 30%
as quite dry and 75% as quite humid......the range is so
enormous for each group that the results are completely
meaningless. You cannot possibly infer anything from those
results.
We simply have no idea at all of how long these animals were
subjected to these various levels of humidity. Take ‘Group
C’ again. In any 24 hour period, were they subjected to
30.6% for 6 hours and 74.8% for 18 hours? Or the reverse? Or
some other period? We can only guess.
The method of taking a once weekly measurement from the very
top of the tank is also incredibly misleading and
inaccurate. Of far more relevance would be a set of
continuously levels at just above the substrate taken at
various points (near the animals) - but they did not even
measure these and we have no idea what they were. They would
certainly not be the same as what was recorded at the top of
the tanks where the heat sources were situated. Higher?
Lower? We simply have no idea (again). It is also usual in
trials like this to specify what equipment was used to
obtain the measurements. This is important, as there is
likely to be a big difference between the results obtained
from say, a greenhouse hygrometer and a piece of properly
calibrated precision laboratory equipment. The authors were
aware of this, because they do cite the make and model of
the atomic absorption spectrophotometer used. They fail to
say what they used for the most important measurements of
all, however... the hygrometers. This is a very disturbing
omission. Did they use the same instrument for all the
measurements? Was it calibrated? We just don’t know.
The method of humidity control employed is said to have
consisted of 40 X 40 X 15 cm plastic bowls filled with
demineralized water and “atomisers to produce fog”. The use
of a “fogger” type atomizer to produce humidity is also
problematic. These do not directly produce humidity. They
produce water droplets that are then vaporized by
temperature. We have no idea what these temperatures were,
as already pointed out. They are very prone to producing a
‘cold fog’ on the bottom of the tank while creating very
high humidity at the top... amphibian keepers like them for
this very reason. They also produce a profound evaporative,
localized chilling effect toward the bottom of the terraria.
Anyone who has ever used one will recognize this
characteristic. We again have no idea what the effects of
this really were in this trial, because insufficient data is
provided. We can be absolutely certain, however, that what
was happening at “ground level” next to the animals would
profoundly differ from what was recorded on those mere eight
occasions at the very top of the tanks.
Unfortunately, from an experimental point of view, other
factors that would substantially influence the allegedly
“controlled” humidity within these enclosures were also
present: in each, a 13 cm diameter dish with drinking water,
and the provision of soaked pellet food and “endive salad”.
These would both evaporate adding to humidity within the
units in an uncontrolled manner under the heat lamps. As so
little environmental data is provided, it is impossible to
guess what impact this may have had. It is disconcerting
that this possibility does not even seem to have been
considered by those conducting the experiment. It would have
been preferable if feeding and drinking could have been
conducted in a separate, isolated area within the
“controlled” environment to eliminate this variable.
General maintenance
The authors state that while the animals were provided with
“light for activity” for eleven hours a day, the UVB tube
and heating light was only turned on for three hours in the
morning (9-12 am) and 90 minutes each afternoon (3-4.30 pm).
This is a quite unusual regime and is certainly not typical
of what most captive tortoises experience. It would
certainly have had some quite profound effects upon feeding,
activity and upon digestive efficiency. It is yet another
strange and unquantifiable variable present in this study.
We cannot know what effects (if any) it had on the results
or the behaviour of the tortoises.
One effect we can predict with confidence is that if the
heat lamps were turned on for three hours in the morning,
and again for 90 minutes in the afternoon, this would have
had an immediate and massive impact on the RH experienced
within the units, producing either a considerable rise, or a
severe drying effect depending upon the amount of water
evaporated within the unit. It is quite impossible it had no
effect at all. This very considerable variable
is not even mentioned by the authors and absolutely no data
as to the levels that resulted is presented. The net result
is that these tortoises experienced wildly fluctuating,
unspecified levels of RH on a daily basis. It is hard
to reconcile this with the claims subsequently made as to
the effects of any one particular level or levels of
relative humidity.
Diet
There are a number of other serious methodological
criticisms of this study that can be made, including the
failure to adequate monitor and regulate actual calcium and
phosphorus intake for the duration of the experiment, which
would appear to have been highly variable and subject to
individual preference (loose cuttlefish bone was provided in
each enclosure and a tortoise could consume as much – or as
little – of this as it preferred). Given that calcium intake
and overal calcium to phosphorus balance is such a critical
factor in bone development, the lack of control here is very
worrying from the point of overal experimental validity. The
authors conclude, however, that even on what they call a
“higher protein” diet, the tortoises raised in what they
call the “high humidity” (45-99% RH!) environment of
Group E had less “pyramiding” than those raised on a lower
protein diet in Groups A and B.
Curiously, the three lateral views of example tortoises from
Groups B, C and D (no images whatever from groups A and E
were published) all show growth abnormalities to some
degree. There is an apparent slightly reduced level of
deformity on the example tortoise from Group D compared to
that of Group B. However, due to the lack of control
over calcium intake and Ca:P balance (cuttlefish bone is
rich in phosphorus) we cannot be sure how much of this
effect might be due to any difference in humidity, or how
much might be the result of different amounts of cuttlefish
bone consumption or of food intake. It appears no attempt
whatever was made to measure or record this variable. A very
wide range of blood calcium, phosphorus values were recorded
in each group throughout the study, but interpreting it is
difficult when such unquantified variables are present. We
also have no idea how truly representative the very few
animals depicted really are.
Finally - but certainly not least - the values of
dietary protein provided in the diet are disturbing. By just
about any normally accepted chelonian nutritional standards,
all of these tortoises without exception were fed a high
protein, highly digestible (low fiber, 10.9 to 13.4%) diet.
Not one group was tested using a diet that could be defined
as “low protein” or “high fiber”. The lowest protein level
provided was 13.7% and the highest, a rather staggering
30.7% (both DM basis). It would have been far more realistic
(and useful) if a genuine “low” protein, high fiber option
had been included. This could be defined as something
offering circa 7.5% protein (DM) and at least 30% fiber
content, which more closely represents a typical ‘wild’
dietary profile. There is no question whatever that
the dietary regime used would have promoted artificially
high rates of growth in all of the groups. This fact is
admitted by the authors.
Misquotes and distortions
In stark contrast to the claims of what I might term “vocal
supporters” of this particular paper (who consistently
misinterpret it and misquote it), at no time do the authors
themselves actually seek to deny the role of protein intake
in the development of ‘Pyramidal Growth Syndrome’ (PGS),
indeed, in their summary they clearly state “it is
understandable that the abnormal combination of a dry
environment and a persistent high level of protein may lead
to health disorders, such as PGS. Such management conditions
are very common both in private and institutional
situations, which probably explains the high prevalence of
PGS in tortoises in captivity” and go on to state “some
influence of dietary protein is probable”.
They also openly admit that “the combination of dry
environmental conditions and comparatively high growth rates
induced by a nutritionally dense diet led to pyramidal
growth in the African spurred tortoises of this study”,
thereby also acknowledging the key role of high growth rates
and diet - another link which “supporters” of this paper
have consistently misrepresented. Do not forget the fact
that all of the tortoises in this study - without exception
- developed “pyramiding” to some degree or other. Not a
single really smooth, deformity-free animal was produced out
of the entire fifty.
While I decry the methodology of this particular study, and
many of the conclusions reached, I do actually agree with
them on those particular points. High growth rates, promoted
by nutritionally “dense” diets are also a factor and must be
considered along with environmental influences. Each
of these factors is involved and we need to consider them
all. We should not concentrate on just one aspect to the
exclusion of the others.
There is one other widespread distortion and massive
misunderstanding of what this paper proposes. These authors
absolutely did not advocate keeping tortoises at constant
levels of very high humidity. Nowhere do they even suggest
that. What they actually say is very different. They simply
say that “areas with a relative humidity of nearly 100% for
hiding should be provided to the tortoises at all times”.
This is not at all the same thing as forcing animals to
endure constant levels of high humidity. They are
specifically referring to the provision of a “humid hide”
where a tortoise can retreat - voluntarily - on a temporary
basis - not recommending an enforced 24/7 “sauna” type
environment with no means of escape. This latter is,
unfortunately, an apt description of the conditions adopted
by some keepers who refer to this paper as their
inspiration.... where constant round-the-clock 80-90%
humidity at high temperatures (26C+) is the only environment
ever available to them. Such conditions deprive those
animals of thermoregulation opportunities, subject them to
constant levels of humidity that would only be experienced
on an infrequent basis in nature, and which could have
unknown metabolic and physical consequences over the long
term.
It is a strange and somewhat ironic fact that the authors of
this paper were really very close to identifying the true
causes of “Pyramidal Growth Syndrome”. Their experimental
methodology may have been defective, but the link between
humidity and this type of growth is real. Had they persisted
further, and inquired in far more detail into the one
material that is extremely affected by humidity and that
almost completely surrounds a chelonian - keratin - they
might have arrived at a genuinely viable answer. Instead,
they overlooked keratin entirely (the word is not mentioned
once in the entire paper), and they sought refuge in vague,
unsubstantiated and unviable theories involving
“intracellular and intercellular” pressure of osseous (bone)
tissues caused by dehydration. Had they instead looked at
what effect external hydration and dehydration has on
keratin, and also at the two key modes of cell
proliferation in chelonian keratin, a great deal of
confusion would have been avoided.
(c) 2014 A. C. Highfield / Tortoise Trust
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