Nutrition

‘What and how do I feed my
tortoise?’ is one of the top questions people have. It does not take
much research to become completely confused about this. Part of the
problem is that there is no such thing as a ‘perfect’ tortoise diet- for
that matter, we really don’t have a ‘perfect’ diet for dogs, babies, or
humans in general!

Our starting point are the guidelines that the Association of  Zoos and Aquariums’ Nutrition Advisory Group came up with. They suggest offering a diet that …
  • Is nutritionally balanced. This basically means it meets the actual needs for calories, proteins, carbs, vitamins, etc.
  • Reasonably duplicates natural feeding behaviors.
    The animal should eat it as much like they would in the wild as
    possible. If the animal slowly grazes all day, it is inappropriate to
    make it gorge in a few minutes.
  • The animal consumes consistently. We should offer things the tortoise will usually eat without forcing it, etc.
  • Is practical and economical to feed. It should be cost- and time-effective to offer the diet.
 
Natural tortoise diets

Most
tortoise species follow a few basic rules in the wild. They generally
live in places that are amazingly low in nutritious foods- even peak
growth rain forests that look so lush have amazingly little usable foods
most of the year. 
 
Tortoises
are designed to live in places like this- they take food most other
animals ignore or cannot live on, eat it slowly, digest it slowly, and
spend most of their time sleeping so they can live on the small amount
of nutrition they took in. They may roam an area the size of several
football fields to find enough nutrition for the day. When the do
find a pile of something tasty- fallen fruit, yummy flowers, carrion-
they will gorge, then sleep (sometimes for weeks), then gorge again
until it is all gone.
 
Seasons
play a big role as well. During the dry season, they live on tough
grasses, leaves, stems, and other drier foods that tend to be low in
calories but high in calcium and fiber. During these lean times, they
add very thin growth rings to the shell. When the rainy or wet seasons
roll in, they splurge on flowers, fruits, and fungi- bursting with
carbohydrates, nutrients, and flavor. These periods spur growth and this
is reflected in wide growth rings. This pattern of feast and famine
probably plays a major role in overall health and shell development,
preventing pyramiding, etc.
 
Compare
this to captivity where we generally offer meals that are already high
in calories and fats, and low in calcium and fiber; offer the same
nutritional profile all year along with a similar climate all year; and
greatly restrict their ability to exercise. It is no wonder that so many
captive tortoises do so badly.
 
Most tortoises eat about the same things overall, but there are two general categories:
 
These
eat almost entirely plants, and not even all plants equally since most
eat few if any fruits. Most of these tortoises live in grassland or arid
habitats, and many of them live in places where they often need to
brumate in the cold season. They get about 75% of their calories from
carbohydrates (sugars found in the plants), 20% from proteins, and the
remaining 5% from fats (note- too much fat in their diet causes stomach
disorders!) Most of the calories will come from plant matter although
almost all tortoises will relish the occasional piece of meat. (Mader)
 
These
tortoises get much of their needed metabolic water from the plants and
the digestive processes- but they should still have access to drinking
water. 
 
A
typical diet profile would be roughly 95% vegetation and vegetables-
grasses, hays, leaves, stems, flowers, ‘weeds’, fungi, etc.; less than
5% fruits or vegetables with seeds, and very little meat. (Mader)
 
Forest dwelling omnivores (red- and yellow-footed tortoises, Kinixys, Indotestudo and Manouria species, American box turtles, and most terrestrial turtles)
tend to eat a wider variety of foods, usually including fruits and
meats. Much of this is probably due to the lower availability of
calories and nutrients in the wild foods. (The frequent heavy rains tend
to wash the nutrients out of the soil and in response the plants tend
to concentrate what little they can get in the fruits.) Most of these
tortoises do not brumate, although they may aestivate in hotter or drier
weather. They should get about 50% of their calories from
carbohydrates, and 25% each from fats and proteins. (Mader)
 
Most
omnivores drink more freely, or at least eat foods with higher water
contents. Again, clean water should always be provided.
 
The
typical omnivore tortoise diet would be 75% vegetation and vegetables,
20% fruits or vegetables with seeds, and 5% meats or high-protein foods.
(Mader)
 
Prepared diets

You
can get chows and other prepared foods for tortoises just as you can
for dogs or yourself (think cereals and other processed meals). While
there are several options that work nicely, the three
most consistently recommended brands in the US are:
There
are a lot of varying opinions about using prepared diets, from those
who use them most of the time, to those who never do. The arguments for prepared chows include:
  • They are well-researched with years of clinical trials.
  • They
    are an easy and cost effective way to offer all the nutrition the
    tortoise needs (when the directions are properly followed.)
  • They are quick and convenient to use.
  • They fulfill the AZA’s guidelines listed above.
Arguments against chows include:
  • Most
    use cereal grains, that even when processed are not a natural food
    source for reptiles (or humans according to some people). 
  • The use of sweeteners. These are often called ‘addictive’, even when they are added to improve flavor and calories. 
  • Improper
    nutrition. Many people feel that, whether we are talking about
    tortoises or people, there is just no way to replace a natural diet with
    a processed diet and be completely healthy.
Interestingly enough, there are also arguments against fresh food diets! 
  • Store
    bought foods have lost much of their freshness and nutritional value
    because of the time spent in transit and storage, and may be
    contaminated with chemicals, fungi, etc. 
  • It
    is difficult for the typical keeper to offer a truly nutritious and
    balanced meal that really suits the tortoise’s needs. Because you only rarely really know what the nutritional values of any given food item are, it is far too easy to overdo some nutrient while mot offering any of another. 
Natural or prepared diets?
Most people fall into one of these categories:
  • ALL PREPARED FOODS. People
    in this group are confident that the makers of the chow have created a
    solid, balanced, and nutritious diet for the animals, and appreciate the
    cost and convenience of the prepared diet. 
  • MOSTLY PREPARED FOODS. This
    category feeds their animals mostly chows or kibbles, with some fresh
    foods on the side to add interest, variety, and to help offer
    micro-nutrients that may be missing from the chow. Many prepared food
    makers actually recommend this.
  • HALF AND HALF. Recommended by veterinarian Greg D. Harrison to prevent what he calls the ‘Improper Diet Cascade’.
    The idea is that the fresh and prepared foods will balance the
    weaknesses in the other half. He also points out some things to consider
    when using prepared diets, such as storing them as to prevent
    rancidity.
  • MOSTLY FRESH FOODS. The
    idea here is that a good diet built on variety, balance, and locally
    available produce is the best food for most animals, but that prepared
    chows can offer some benefit as well, so they are used as a supplement
    to help ensure good nutrition, or when fresh foods are less available. 
  • ALL FRESH FOODS.People
    in this category feel they can buy or grow foods that serve their
    animal’s needs well and/or that there are problems in commercial chows.
    (However, many experts feel that without more research and preparation
    than most people are willing to do, that it can be very difficult to
    ensure that the foods offered actually meet the needs of exotic
    animals.)
 
How much to feed?

This
is another tricky area. Some people feel that the tortoise can regulate
its own intake and cannot really be over-fed, others worry about
obesity and limit the intake. Either way, the key goal is to help ensure
healthy growth and natural behaviors.

One way to determine this
is to calculate the daily calorie need: the weight in kilograms, to the
0.75th power, times 32 gives us the ‘base metabolic rate’ for any
reptile. (WTkg^.75×32=BMR). The BMR is how many calories the animal
needs when healthy but not active- such as being kept indoors in a
smaller enclosure. You increase the daily calories offered for animals
that are more active- mating, nesting, reproducing, etc. and reduce it
for animals that are in any form of slow-down (approaching brumation or
aestivation), or not well. See ‘Healthy Weight‘ and ‘Dosages and Guidelines‘ for more. (Mader)

 
Another, more convenient, solution is to roughly mimic the natural diet-

  • Offer
    a nearly unlimited amount of plants that are low in moisture and
    calories but high in calcium and fiber. This includes most grasses,
    hays, leaves, lettuces, sprouts, greens, flowers, stems, and fungi. This
    can also include vegetables that are low in sugars like bell peppers,
    cauliflower, and broccoli. Note that not all species eat things like
    grasses or hays- especially the omnivorous species.
  • Limit
    things that are high in calories, fats, and proteins- fruits,
    vegetables with higher sugar content like pumpkin, and any type of meat,
    including invertebrates.

    • With herbivorous tortoises, this sort of food should be limited to occasional meals or snacks.
    • For
      omnivorous tortoises, this category can be offered in small amounts
      (about the size of the tortoise’s head) more often- one to three times a
      week, for example.

Overfed
tortoises will grow in ways that may not be healthy. One study found
that tortoises feed free amounts of Mazuri Tortoise Diet added height
and width to their shells without adding length (Lickel). High calorie
foods can cause obesity, limit the tortoise’s ability to retract or
breathe, and affect internal organs.

It
is almost always better to underfeed a captive tortoise because of the
limited amount of exercise they can get and because most of the foods we
offer are so much ‘richer’ in nutrients than the wild diets are.

 
Resources

 

Revised 5-29-2012. (C) Mark Adkins