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Iguana protein issues
IGUANA EVOLUTIONIguanas have evolved to fill a certain niche - that is, herbivory in arboreal tropical environments. They are successful, because there are few competitors (especially reptilian ones). This pressure to efficiently digest plant matter has led to a longer, more sophisticated digestive system to enable fermentation and breakdown of otherwise quite hard to deal with matter. However, the pressure has been there because of the abundance of food, and perhaps the lower energy requirements necessary to locate and acquire food sources. Take a pinch of salt, add several hundred thousand years, and you have a digestive system nicely adapted for herbivory (I won't say perfectly adapted, because perfection is a very subjective quality!).
RESEARCH ON WILD IGUANAS
ADAPTATIONS TO HERBIVORY
EXCEPTIONS TO THE RULE?
CONSEQUENCES OF HIGH PROTEIN INTAKE
SPECIFIC EFFECTS OF ANIMAL PROTEIN ON IGUANAS
DO SAFE LEVELS EXIST?
BUT ISN'T ANIMAL PROTEIN REQUIRED FOR...?
FURTHER SPECULATION ON NATURAL VARIATION
GENERAL CONCLUSIONS
Those who think that green iguanas should for some reason be fed
animal protein are not considering their natural physiology or
behaviour. They know that animal protein (i.e. large amounts of
protein) will result in additional growth. They're right - just look
at beef cattle. However, much of this information has come from
captive farming operations where animals were intentionally grown
quickly for either reintroduction programs or for food purposes. In
the former, the longterm negative effects were not known, although if
it was enough to allow several breeding generations of wild animals
then its purpose would have been satisfied. In the latter, who cares
if an animal will die at 5 years if it's slaughtered for lots of meat
at 3 years?
Interestingly, many of those formulating diets for captive iguanas for
the pet industry still include animal protein, because there's a
belief that there must be some level at which animal protein
benefits growth without signficantly affecting longterm health. I
think this view is erroneous, particularly as they still have little
idea what kind of protein levels are harmful - taking a chance, if you
like, but if the food makes your ig grow, people are going to buy it,
right? Perhaps this is a factor - if someone buys product A that
includes animal protein, their iguana will appear to grow faster. But,
if they try product B because they hear it has no animal protein, but
their iguana's growth slows down a little, they might go back to
product A. Cynical, but probably not without merit! If someone does
decent longterm research and proves my view wrong, then fine, but
until they do my opinion stands. After all, a good artificial diet is
theoretically possible.
My overall conclusion to this is that no, we don't know it all. Far
from it. However, everything that we do know so far tells us to be
very cautious. I'm sure the odd bit of chicken, insect or whatever on
very rare occasions will do the iguana no harm. However, I really
don't see why this should be necessary, and I can't understand the
hangups about missing it out altogether. It's almost as though people
feel that iguanas must have a little animal protein every so often,
otherwise they might be missing out on something. I get the impression
others tout animal protein just because everyone else says don't
bother with it. When the idea of using animal protein is so entwined
within the literature of even fairly modern books, it's hard to avoid
this question - it keeps coming up time and time again. This is good,
because every time it comes up, we hopefully know a little more about
how the processes work. At the moment, however, we don't know enough,
and so my advice to people is always not to include it - not because a
tiny bit might not be beneficial, but because too much is definitely
bad, and we don't know how much is too much! People who use animal
protein are, in my opinion, playing dice with the health of their
iguana (longterm), and I don't think it's worth it - given that we
know a totally herbivorous diet is very healthy - but a good totally
herbivorous diet, that is! A bad diet, period, will kill your iguana.
But I think the best diet is totally herbivorous, until I see any
convincing evidence to the contrary, or firm data to show how much
animal protein can be safely utilised (if any).
None of the published scientific field research on green iguana diets in
the wild suggests that they take animal protein, but it does show that they derive all their protein (and other)
requirements from plant materials in the diet. Anecdotal reports
which seem to have made it into the literature suggest that juveniles
have an increased propensity to take insects. There is the odd report
that iguanas also take bird eggs and carrion - there's even less
evidence for either of these. The nutritional research which has been
done (examining wild iguana gut contents) shows no evidence of animal
protein, although if it were present it would be in such low amounts
so as not to be detected by such a study. So, the natural diet of
green iguanas does not include any animal protein. This doesn't
discount the notion that they might accidentally ingest the odd insect
together with a flower head, or whatever, but I hardly consider this
to be anything significant... in the same way that cows don't eat
animal protein, but they're not going to wrinkle their nose and lightly brush
stray bugs from their juicy grass stems. My conclusion here is that they are
strict herbivores under normal circumstances.
Take a look at iguana morphology and physiology. Iguanas are not
equipped to hunt down and digest animal prey. Their digestive system
employs a hindgut fermentation process, which is to break down plant
matter, cell walls and carbohydrate. This isn't to say they couldn't
digest animal protein, but just that they're optimised for plant
matter. They also spend their entire time in the wild living on their
food plants. Why spend time and energy hunting animals when plant food
is all around them? So, my conclusion that they are strict herbivores
still stands.
Feral green iguanas (e.g. Puerto Rico) have been reported to take
insects more frequently that "incidental" would imply. Captive iguanas
will also readily take insects, mice and other sources of animal
protein if offered them (mice are often taken by iguanas that would
happily remove your finger at the same time - almost giving the
impression that they would eat anything presented to them). Some
people seem to have almost converted their iguanas into carnivores.
However, iguanas are nothing if not opportunistic. It seems that under
conditions of stress and poor food availability (and that includes
feral populations, which are mal-adapted to the local flora), they
will resort to using additional food sources until the situation improves.
This suggests that even in their native habitat they might resort to
limited omnivory if that becomes necessary for survival. Iguanas in
captivity are nothing if not stressed, and offered poor quality diets
on the whole. Therefore, eating insects, mice or whatever doesn't seem
unreasonable. Iguanas will happily eat their substrate if nothing else
presents itself. Herbivorous animals have been known to eat carrion,
including their own kind, when presented with no other food sources
(e.g. cows and sheep have been reported to do this). Given this
evidence, though, it seems that eating animal protein is a "last
resort" - i.e. it's a poorer choice of diet. Also, in the wild, if a normally
herbivorous iguana is forced to resort to eating some animal protein, the change will be
temporary until better food options present themselves.
Iguanas, and any animals for that matter, grow very quickly with
increased amounts of protein. If you're interested in fast growth
rates, usually when the animals are harvested for food, then increase
their protein intake. As animal protein is better than plant protein
for this, then feeding herbivorous animals animal protein will
increase growth rate. This is why there is so much controversy over BSE in
cows in the UK - beef cattle are pumped full of animal protein
(including spinal tissue, in this case) so they grow quickly. However,
the farmer is not interested in long-term health - the animals are
culled at an early age, with the exception of breeding animals which
recieve a different diet anyway. Dairy cattle are fed lower amounts of
animal protein. Long term health is affected by excess protein,
however. Veterinary evidence shows that excess animal
protein in iguanas leads to gout, dehydration, metabolic bone disease, renal
failure, kidney stones etc... and is one of the biggest causes of
relatively young iguana mortality. The reason these poor diets (i.e. high percentages of animal protein) exist in the older literature is that they
were formulated from farm-raising practises of green iguanas.
So, iguanas show good growth rates, but the longterm effects severely
shorten longevity.
Animal protein contains components which iguana digestive
physiology can't deal with in large amounts (e.g. fats, different
purine/pyramidine ratios, different amino acid ratios). This also very
likely unbalances the natural gut fauna the iguana possesses in the
hindgut (as has been shown in studies on other herbivores), reducing
digestion efficiency of plant matter which should make up the majority
of the diet anyway. Iguanas, being herbivores, are
physiologically adapted to process specific kinds of dietary protein -
i.e. those found in plant materials. While animal protein can be
metabolised, it literally overwhelms the system in excess (and it
doesn't take much to cause an excess - although we don't know exactly
how much). Nucleic acids in
the foods are degraded to nucleotides and ultimately pyrimidine and
purine bases - more are synthesised from amino acids in the liver. Any
of these which are not used by the body are broken down into uric acid
(or allantoin in some mammals, reptiles, fish). With animal proteins,
green iguanas end up with an excess of uric acid in the body fluids,
and over time this crystallises out - so uric acid crystals form in
tissues throughout the body, impairing their function and leading to
serious problems.
There's no doubt that excess animal protein kills iguanas in the
longterm. However, that's not the question. The question is, is there
a certain amount of animal protein (very low percentage of daily
intake) which will either not harm the animal, or perhaps even be
beneficial because of its high protein content? Hindgut fermentation
works best in larger animals (more efficient per unit volume of food),
but what about hatchling iguanas which are much smaller? Would perhaps
the odd insect lead to increased growth with no obvious health
problems until the hindgut area grew large enough to be able to reach
a greater operating efficiency? (i.e. they're not utilising all the
purines/pyramidines in their diet, so animal protein doesn't cause
uric acid concentrations in the blood to go up significantly because
not all the purines/pyramidines are being utilised)
It is quite commonly believed that animal protein is the only way an iguana
can get any or enough calcium in it's diet (because animal bones contain
calcium). This is incorrect - calcium is present in varying quantities in most
plants, but still many iguanas suffer from calcium deficiency which leads to
metabolic bone disease. In order to prevent these symptoms, a balanced diet needs
to be created which contains sufficient calcium and sufficient vitamin D3 (which may
be present in the diet, supplements, or more usefully through synthesis in the skin
by the action of UVB wavelengths in sunlight). At the same time, the inhibiting effects
on calcium metabolism of phytates, oxalates, acid and fat must be minimised. Powdered
calcium powder helps to supplement a diet which might otherwise be low in this important element.
It is also sometimes believed that animal protein is the only way an iguana can get any protein,
period! This simply isn't true - plants also contain proteins, but with different ratios of
amino acids to animal tissue (the former of which the iguana digestive system has evolved to cope with).
Ranging from Mexico to Brazil as they do, iguanas come from a
diverse array of habitat types and it's very likely, given the
morphological and behavioural varitations that exist, that similar variations
in dietary preferences also exist. Perhaps some populations do select
limited amounts of animal protein, and perhaps these animals do have
lower life expectancies. We simply don't know this yet. There are
clues: some iguanas appear to reach much smaller adult sizes than
their counterparts. It's one of my speculations that these animals may
be adapted to a poorer diet (caused by adaptation to the different
foods available in another habitat), and even if presented with a
richer diet don't grow as quickly as those from other populations.
Although Iguana iguana is one species, there are many different races
which can often become available in the pet trade. Remember, though,
that captive farmed iguanas (which make up by far the majority of
animals available) are coming from a limited number of breeding
operations, which are harvesting eggs/raising animals from the same
area, and therefore diversity in captive populations may be more
limited. I'm a little wary, though, of suggesting a perfect diet for
all iguanas (from Mexico to Brazil) - it doesn't make any sense that
they would all benefit equally from exactly the same diet given that
they don't live in the same environments, but until we learn more (and
we haven't yet) the best we can do it provide diets which we know are
beneficial to the widest number of animals.
Iguanas are herbivores. However, like adding leaded fuel to an unleaded car, you can still
throw other things through the digestive system, and it will still try
and do what digestive systems are good at - digest it! A small amount
of leaded in an unleaded car might make it hiccup at little, but no
lasting harm. In captivity, though, people put too much leaded fuel in
and destroy the engine of their iguana over time. The whole digestive
system can cope with much of the contents of animal protein, but
because the rest of the body isn't used to dealing with this kind of
food source (inc. high protein, fat etc...), things get thrown out of
balance if there's too much of it (this doesn't normally happen under
natural conditions, otherwise selection pressure would tend to lead to
more omnivorous animals).
