All plants contain harmful properties to some level, and it is that degree which determines the level of toxicity, if eaten.
For example, it is probably fine to feed a tortoise plants that contain oxalic acid, providing it is only a small portion of its daily intake. However, if you offer three or four different plants at the same feed, and they all contain high levels of oxalic acid and you follow this regime long term, then there is an increased risk of your tortoise developing dietary related disorders.
We've all heard of the exception to the rule, when people say such things as ‘My tortoise eats buttercups and nothing’s happened to him’. That may be true, but when there is a reference of just one tortoise dying after eating them, then you have to consider whether that particular plant is something you would like to chance offering your tortoise.
Some of the harmful properties found in plants are included on this page, together with brief information on the effects they could have on chelonia if ingested. The information provided is to encourage you to use your own initiative in knowing what some plants contain and which are probably safe to feed tortoises.
Alkaloids are organic compounds containing nitrogen found naturally in many plants. Some of the most well known harmful alkaloids are morphine, quinine, strychnine, nicotine and cocaine. There are some alkaloids that are harmless but others like those found within the Apocynaceae, Papaveraceae and Solanaceae families are very harmful. Alkaloids often have a bitter taste which helps to protect the plant from being eaten, but hungry animals will browse and may accidently eat plants containing high levels of alkaloids.
These are chemical alkaloids which are mostly found in three plant families: the Boraginaceae, the Compositae and the Leguminosae. Pyrrolizidine alkaloids are not toxic until they are eaten in quantity when they will cause damage to the liver and eventual liver failure. They may also cause malformation in offspring. The early signs do not become apparent immediately. Although the evidence is forthcoming regarding cattle and horses, we can only assume that it would also affect tortoises until we hear otherwise.
Coumarin is found in many plants, such as Sweet Woodruff, Mullein, Sweet Clover, Mayweed and many others. It is a sweet smelling chemical compound that is used as a flavour enhancer in pipe tobacco and some alcoholic drinks. Although Coumarin in its natural state in plants has no anticoagulant properties, when it comes into contact with certain fungi it is broken down into dicoumarol, an anticoagulant that inhibits synthesis of vitamin K in the liver, and whose derivatives are used in the production of warfarin. The presence of Coumarin in plants also encourages them to soak up nitrogen in the form of nitrates in the soil. While small amounts of Coumarin in a plant are unlikely to do any damage, the level of Coumarin present increases as the plant grows older, and so it is wise to feed only the young leaves of these plants, and only sparingly.
Cyanogenic substances in plants are those that are capable of producing cyanide. This substance can be a compound within the plant that is safe until attacked or eaten, at which time the sugar is partitioned off and cyanide is released. It can be found in plants of the Rosaceae family, the older and drying leaves and fruit of plants such as almond, apple, plum, apricot, peaches and cherries, and also the Euphorbiaceae family which includes spurge.
Hydrogen cyanide (historically called Prussic acid and also Hydrocyanic acid) is a product of chemical compounds (cyanogenic glycosides) that are naturally present in some plants. It is produced from these glycosides in combination with an enzyme and water. The glycosides and the enzyme are contained in the same plant parts, but they are in separate cells, and it is not until the cells are ruptured and the water is added that Hydrogen cyanide produced. So although cyanogenic glycosides are not toxic as such, if the plant is damaged or stressed in any way (for example, by eating, picking, cutting, drought or frost) then that releases both the glycosides and the enzyme and the toxic compound is produced.
Glucose is a product of plant photosynthesis and an excess is stored as glycogen to be used by the plant to give energy. Sugar that is ingested as fruit: apples, plums, berries, tomatoes (yes, tomatoes are fruit), ferments faster than that from a natural diet, causing high levels of endotoxins, compromising the normal gut flora which in turn may lead to liver abscesses and possibly prove fatal. Some tropical species of tortoise, such as Redfoots, have adapted to have fruit as part of their diet, but fruit should be avoided for all Mediterranean and grazing species such as Sulcata and Leopard tortoises.
A plant glycoside is a chemical compound that hydrolyses (is broken down by water) into sugar and one or more other substances.
Cardiac glycosides are steroid glycosides found in a number of plants including the Foxglove (Digitalis), from where we get today’s powerful heart drug, Digoxin, which is used to treat heart failure. Other plants containing cardiac glycosides include Oleander, Rhododendron, Azalea and Milkweeds. Ingestion can cause vomiting, irregular heart beat and eventual death; so remove plants containing cardiac glycosides from in or near a tortoise enclosure.
Goitrogens interfere with thyroid activity and could cause liver and kidney lesions. Goitrogenic glycoside can be found in plants of the Brassica and related families,including cabbage, kale, spinach and some hot peppery-flavoured plants like wild mustard, and can also contribute towards the formation of kidney and bladder stones.
Ranunculin is another glycoside which can be found within the Ranunculaceae family; for example, Buttercup, Delphinium, Wood Anemone and Clematis, and can cause mouth and throat irritation due to the acrid taste, colic and diarrhoea. While some say their own tortoises eat Buttercups and Clematis, it is not something that should be offered on a regular basis.
Saponins are a group of steroid glycosides which are capable of causing anti-nutritional effects. They are found in many plants including Alfalfa, Vetch, Spinach and trees such as Maples and Horse Chestnuts. High levels can speed up haemoglobin degeneration, irritate mucous membranes and act as an appetite suppressant. Although all saponins are not bad it would be best to feed with caution as only a small part of a tortoise's diet.
Oxalic Acid is a naturally occurring chemical substance present in many plants. It is harmful in high levels as it binds with calcium to prevent the absorption of this much-needed nutrient, and because of this over a short period of time it may result in calcium deficiency. Oxalic acid can be found in plants such as Spinach, Chard, Good King Henry, and Beetroot, which are all members of the Amaranthaceae (formerly Chenopodiaceae) family, as well as Parsley, Rhubarb and even Dandelions (which have a low level in young leaves but can have a higher concentration in older leaves). A low intake of oxalic acid doesn’t give rise to any problems, but it is best to avoid feeding large quantities of plants that are high in Oxalic Acid.
Raphides are bundles of needle-shaped crystals, usually of calcium oxalate, that develop as metabolic by-products in plant cells for defence against being eaten, for calcium storage and for structural strength. If an animal eats plant material containing large amounts of raphides, the soft tissues of the tongue and throat may be damaged. Although the effect of raphides on reptiles is unclear, it is known that mammals who eat a plant containing raphides can suffer painful oedema, and a sense of burning to the mouth and throat. For that reason we do not recommend feeding plants that have a high raphide content, but the exceptions to this are plants like Fuchsias where the raphides are so deeply embedded in the cells that they are not easily released when eaten and therefore unlikely to cause damage.
Tannins are complex astringent compounds that give plants vibrant colour and bitter taste. They are used for treating animal hides, in medicine, and also used as a stabiliser in pesticides. Tannins can inhibit the absorption of iron, leading to anaemia if too much is ingested. They are found in legumes, tree bark, Grapes, Strawberries, Pomegranates, Acorns, Nuts, and many other plants.
© The Tortoise Table, 2012. All rights reserved.