Toxic Plants and Tortoises

Caesalpinia gilliesii seed pods are toxic to tortoises

Different species of animal vary widely in their susceptibilities to individual plant toxins. Most lists of toxic plants presented to reptile keepers are derived from studies (or experiences) based upon humans and commercial livestock. Although such lists are useful, they are by no means entirely reliable when applied to reptiles.

These different responses are explained by fundamental differences in diet, by differences in the anatomy and chemistry of digestive tracts, and by differences in feeding behaviors. Certainly, one major factor that influences the effect of ingestion of potentially toxic plant materials is the herbivorous reptiles reliance upon microbial digestion. This is paralleled in mammals by ruminants such as cattle, goats and sheep. Species that utilise a primarily microbial mode of digestion, whether mammalian or reptilian, possess a large fore stomach, or hindgut in the case of reptiles, toxins become well diluted and are absorbed relatively slowly. The rate of absorption in such species is far slower than in non-ruminants such as pigs, dogs, cats, or human beings. Microbial action in the rumen (or hindgut) can also destroy certain toxic plant compounds, although, conversely, it can also have a negative effect by promoting the release of toxic constituents as protective plant cellular material is degraded.

The most important factors in species differences in plant toxin susceptibility are probably the detoxification mechanisms specific to each species. These mechanisms can even vary within animals of the same species, or between different geographical populations of the same species, e.g., some (but not all) rabbits possess an enzyme known as atropinesterase that counters the effects of deadly nightshade; goats also possess an atropinesterase that makes them less susceptible to poisoning by the same plant. Deer are said to be able to feed on yew and rhododendron, and grey squirrels and American box turtles upon the deadly Amanita mushroom. Galapagos tortoises also consume highly toxic fruits in quantities sufficient to kill a horse. Among lizards, the ground iguanas Cyclura cornuta and C. carinata are known to feed extensively upon the fruits and leaves of the manchineel (Hippomane mancinella) and the fruit of the poisonwood tree (Metopium toxiferum), both of which contain powerful alkaloid toxins (Wiewandt, 1977 and Iverson, 1979).

Although it is commonly assumed that goats can feed with impunity upon almost anything, this is not true. Under natural conditions, the feeding pattern of the flock is determined by its leader, so that identical plants are eaten by all members at the same sites. Flock members learn to avoid the plants actively discriminated against by their leader – goats not kept as a flock, or kids separated from their mothers at an early age show little discrimination and are far more likely to succumb to plant poisoning. It is not known how widely learned toxin avoidance behaviour is in reptiles.

Poison plant lists usually do not discriminate between differing toxicity properties of separate plant parts in sufficient detail; it is also not often realised that many plants vary in toxicity seasonally, with toxin levels in some plants rising dramatically during episodes of drought. Even altitude can affect toxin levels in some plant species. In nature, the animal’s feeding behavior often varies to accommodate these changes or differences, for example, Dipsosaurus dorsalis consumes only the flowers of the creosote bush (Larea divaricata) and ignores the toxic leaves (Mares and Hulse, 1978).

In addition, there can be other, less obvious factors involved. For example, we do know that some wild mediterranean tortoises may sometime eat parts of wild lupin plants (Lupinous albus) without apparent harm, but there is no reliable data on precise frequency, quantity or precise parts consumed. Also, we know that sometimes wild lupins can be affected by a fungus called Diaporthe toxica which, when present, greatly increases the toxicity, and there are also different regional variations in alkaloid content. Hence, forming a view that such a plant is invariably 'safe' would be very risky. It is preferable to err on the side of caurion, as we do know that there have been fatalities associated with lupin species.

In captivity it is clear that reptiles are not necessarily able to discern dangerous plant toxins if offered unknown varieties – for example, tortoises have died as a result of eating daffodils and green iguanas have died after consuming azalea and oleander - therefore, although 'poison plant lists' are technically somewhat inadequate, in practice their use is highly advisable.

TOXIC PLANT LIST

The following list is derived from 'The Tortoise and Turtle Feeding Manual' (A. C. Highfield). Unfortunately, the exact degree of toxicity is not known in many cases. Some plants on this list may be safely consumed in small or moderate quantities, but may be fatally toxic under certain circumstances or in large doses. Use this list as a general guide.

Those plants which are especially dangerous to tortoises (credible cases where mortality has been reported following consumption) are clearly marked in red and with an asterisk.

Also note that unfortunately many reported cases fail to identify the specific scientific or latin name of the plant involved - and that 'common names' often vary by location. This is again another limitation of such lists. We can merely include the information available. We strongly advise caution if in any doubt at all.

NB: Some plants appear more than once under alternative names.

Acokanthera

Aconite/Aconitum (monk's hood)*

Amaryllis

Amsinckia (tarweed)

Anemone

Apple (seeds only)

Apricot (seeds only)

Autumn crocus (Colchicum autumnale)*

Avocado (leaves)

Azalea

Baneberry*

Beach pea

Betal nut palm*

Belladonna*

Bittersweet

Bird of paradise (Caesalpinia gilliesii)

Black locust

Bleeding heart

Bloodroot

Bluebonnet

Bottlebrush

Boxwood*

Buckeye horse chestnut

Buttercup

Caladium

Calla lily

Cardinal flower

Carolina jessamine

Casava

Castor bean

Chalice or trumpet vine

Cherry (seeds only)

Cherry laurel

China berry tree

Christmas berry

Christmas cactus (Euphorbia)

Christmas rose

Columbine

Common privet

Coral plant

Crocus*

Croton

Cyclamen

Daffodil*

Daphne

Death camus*

Deadly nightshade *

Delphinium

Destroying angel mushroom (death cap)*

Digitalis (Foxglove)*

Dogwood

Elderberry

Elephant ear (taro)

English ivy

Euphorbia

False hellebore

Fiddle neck (Senecio)

Fly agaric (amanita, deathcap)*

Four o'clock

Foxglove (digitalis)*

Gelsemium

Golden chain

Hemlock*

Henbane*

Holly

Horse chestnut

Horsetail reed (Equisetum)

Hyacinth

Hydrangea*

Impatiens

Iris

Ivy

Jack-in-the-pulpit

Jasmine

Jatropha

Jerusalem cherry

Jessamine

Jimson weed (thorn apple)*

Johnson grass, wilted

Lambkill (sheep laurel)*

Lantana camara

Larkspur

Laurel

Lily of the valley*

Lobelia

Locoweed*

Locust

Lupin*

Machineel

May apple

Mescal*

Milk weed

Mistletoe*

Moccasin flower

Monkshood*

Moonseed

Morning glory

Mountain laurel

Narcissus

Natal cherry

Nectarine (seed only)

Nicotine, tree or bush, (Nicotiana glauca) *

Nightshades*

Oak

Oleander*

Pear seeds

Pennyroyal

Peony*

Periwinkle

Philodendrons, some species

Pinks

Plum seeds

Poinsettia

Poison hemlock*

Poison ivy*

Poison oak*

Poison sumac*

Pokewood or pokeberry

Poppy (except California)

Potato (leaves)*

Privet

Redwood

Rhubarb (leaves)*

Rhododendron *

Rosemary

Russian thistle

Sage

Salmonberry

Scarlet pimpernel

Scotch broom

Senecio

Skunk cabbage

Spanish bayonet

Squirrel corn

Sudan grass

Star of Bethlehem

Sundew

Sweetpea

Tansy

Taro (elephant ears)

Tarweed

Tiger lily

Toad flax

Tomato (leaves & plant)

Toyon berry

Tree of heaven

Trillium

Trumpet vine

Venus flytrap

Verbena

Virginia creeper

Water hemlock*

Wild parsnip

Wisteria

Yellow star thistle

Yew*

Highfield, A. C. (2001) The Tortoise and Turtle Feeding Manual.

Iverson, J. B. (1979) Behavior and Ecology of the Rock Iguana, Cyclura carinata. Ph.D. dissertation, Univ. Florida, Gainsville.

Mares, M. A. and Hulse, A. C. (1978) Patterns of some vertebrate communities in creosote bush deserts. In: Creosote Bush: Biology and Chemistry of Larrea in New World Deserts. Dowden, Hutchinson and Ross, Stroudsburg, Pa.

Wiewandt, T.A. (1977) Ecology, behavior, and management of the Mona Island ground iguana Cyclura stejnegeri. Ph.D. dissertation, Cornell Univ. Ithaca, NY.

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