Meadow yellow

Meadow yellow
Bulbous Buttercup (Ranunculus bulbosus) in a Devon meadow

Thursday, 11 November 2010

Are Yew poisonous

Our bathroom has some intimacy with a large Yew tree (Taxus baccata), the fulsome evergreen branches hanging a metre off the window unnoticed for much of the year. But in the autumn the multitude of mini matt red barrel-shaped berries colours up this dark patch of garden. This week whilst attending to some toiletry I noticed two squirrels feasting on the yew’s fruit. Having always heeded the warning that almost every part of the yew tree is poisonous, I was curious at how these animals coped with the toxins, or wondered if the tree was as dangerous as we are led to believe. Indeed it opened up a bigger question; how do animals know what is and isn’t poisonous?

Numerous scientific studies have determined that most tissue of the yew tree, including the seeds and foliage are rich in taxin(e), a complex mixture of toxic alkaloids (Thomas & Polwart, 2003), and hence it’s genus name of Taxus. The toxins appear to impact primarily on the cardiac tissue of humans and other mammals, leading to heart failure and potentially death. Other effects reported in humans include dizziness, vomiting, abdominal pain and convulsions (Wilson, et al., 2001). This toxicity remains prevalent all year round and even when dried, but peak concentrations are found during the winter (Wilson, et al., 2001). Most incidents of poisoning appear accidental occurrences, with contaminated feed to livestock, but there are a few human reported cases.

The poisonous nature of the Yew has been known for ages. Primitive cultures made use of yew juice dipped arrows as an animal poison for hunting and the Celts committed ritual suicides by drinking extracts from yew foliage (Wilson, et al., 2001).

However, interestingly there is one part of the tree that does not contain taxin which may well explain the recent diet of our squirrels; the aril, or soft succulent scarlet tissue surrounding the seeds and forming the berry. So our ever-clever squirrels can enjoy the fruitful bounty of yew without toxic effect, but I wonder what they do with the seeds – do they spit them out or pass them out undigested in a parcel of ready-made fertiliser. This seems like a clever strategy of the yew to direct wildlife to only consume those parts of it that aids its seed dispersal. However for us humans, BEWARE -only the wiliest of wild foragers should attempt to eat the yew berries, ensuring none of the deadly seeds are consumed.

Thomas, P. A. and Polwart, A. (2003) Taxus baccata. Journal of Ecology, 91( 3): 489-524.
Wilson, C. R., Sauer, J. and Hooser, S.B. (2001) Taxines: A review of the mechanism and toxicity of yew (Taxus spp.) alkaloids. Toxicon, 39 (2-3): 175-185.

2 comments:

  1. I recall that a businessman in Surrey/Sussex committed suicide by eating over 400 Yew Seeds, and that was supposed to be many times higher than the fatal rate (not known).

    I'm aware that one Castor Seed can kill a human, but no idea on how many Yew Seeds would do the same.

    Interesting blog!

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  2. I ate 4 yew berries as a child and survived, i recall i had stomach cramps as a result !

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