(Note: This original post was made on Facebook on October 8, 2018. I am adding it to my blog a year later, since some of the commentary and discussion stands alone as useful in a general-interest way when it comes to explaining toxicity versus venom.)
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Today a guy came to my nursery to buy plants. He was full of questions, which I welcome and expect since that is how people learn how to grow things successfully. At one point he wanted to know whether there was any truth to something that a neighbor lady of his had told him: The reason why cacti don’t come back from the roots when rabbits eat them was because rabbits have venomous saliva that kills the plants.
Now I have a moral dilemma. I cannot decide whether this should replace my current favorite misconception of, “I’d like to buy a 12-inch tall saguaro with two arms, please” or not. I’ve heard the Mini-Me saguaro thing numerous times over the years, but venomous bunnies is a totally new one to me. Monty Python would be proud.
Okay Facebook, advise me.
Leif: It’s a prehistoric function from when Leporidae were carnivores and they must kick their back legs in order for the venom to be released.
I just spent some time looking into Komodo dragon ecology and their venom and there are some good articles about it, including National Geographic and Wikipedia. Without rehashing the stuff in the articles I read, basically the bacterial “dirty mouth” hypothesis was generated by the observations of a guy who observed, but didn’t scientifically test, Komodo dragons in 1969. From the article linked below: “In 1969, an American biologist named Walter Auffenberg moved to the Indonesia island of Komodo to study its most famous resident—the Komodo dragon. This huge lizard—the largest in the world—grows to lengths of 3 metres, and can take down large prey like deer and water buffalo. Auffenberg watched the dragons for a year and eventually published a book on their behaviour in 1981. It won him an award. It also enshrined a myth that took almost three decades to refute, and is still prevalent today.”
There are good explanations about why the theory has persisted, including that it’s such a weird one that it grabs attention and appeals to our sense of the absurd and esoteric. It was also based upon watching the behavior of non-native water buffaloes that were attacked by dragons who took much longer to die because they were far too large for the dragons to kill outright. Native prey for the dragons include pigs and deer, which they generally try to kill outright via a deadly combination of their venom and massive blood loss and traumatic wounding.
But water buffaloes escape the initial dragon attacks due to their size, and then try to nurse their wounds by standing in feces-contaminated water holes, which leads to a slow death by a combination of infection and being weakened by a dangerous, but not entirely lethal, dose of dragon venom. Once dead, the dragons will then feed upon the water buffalo carcasses. That helped enshrine the myth that dragons purposely use bacteria in their mouths to kill prey, but it was entirely due to an unnatural interaction between dragons and a not-normal-sized prey animal they were not evolutionarily adapted to successfully kill. Smaller animals they attack die within minutes usually, due to both the venom, which inhibits blood clotting and also reduces blood pressure, and the physical gaping wounds. This one-two punch is an effective strategy for Komodo dragons to procure food.
Here’s the entire NGM article, very interesting to read along with the one on Wikipedia.
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Me: I have never heard the Venomous Rabbit Saliva Theory before yesterday. This leads me to wonder how Science Neighbor Lady came up with it. Here’s an unscientific theory of my own, not having talked to the research subject at all myself, which I guess makes me the Pot calling the Kettle black….
She observed that rabbits, and numerous other gnawing rodent-type pests, will eat cacti in her yard, sometimes even very spiny ones, under desert summer conditions. This probably surprised her when she saw it (it surprised me too when I first moved to Arizona and started suffering attacks within a few days of setting my newly-relocated-from-Colorado plant collection outside!) and she was likely under the impression that cacti were nearly impossible to kill. I hear that assertion often and it’s simply not true. In fact, cacti and other succulents which are lumped into the general group are not that hard to kill. Here’s why….
Anatomically, most cacti have only one growth point located at the apex of each stem, and they have relatively few stems when compared to the tens of thousands of apical stems a tree has at the tip of each little branch, so damaging a cactus’ growth beyond repair is really quite simple. Removing the apical cells where the plant grows immediately stunts the cactus’ future growth, and once is all it takes to set the plant back for years on end. Plus, unlike many herbaceous or woody plants which have subterranean buds at a ground-level crown or along the roots that can resprout, cacti lack those. Therefore removal of all top growth automatically kills them without any hope for recovery. It really doesn’t take much to permanently kill a cactus as long as you attack it physically.
Physiologically, cacti and succulents are generally slow-growing because they have internal mechanisms to save them water in a dry climate. This makes them extremely efficient at saving water, but it comes at the cost of very slow growth compared to more water-expending plants. This water storage capacity also makes them magnets to rabbits, and other critters, in dry periods because often there is no free water around for thirsty animals to drink, so they consume cactus flesh instead to meet their survival quotas. A hungry and thirsty rabbit stressed by summer heat will return to a cactus they find time and time again, chewing more off of it daily, until the slow-growing plant with one apex per stem is fatally damaged. It is rabbit behavior that kills cacti and succulents in conjunction with inherent plant physiology that disables quick recovery from such damage. Not venomous saliva.
Now you know. Someone might want to inform Neighbor Lady too.
Me: The very notion that herbivores would develop a venomous saliva to attack plants makes absolutely zero ecological sense. Plants cannot run away or attack, so venom is thoroughly unnecessary as an evolutionary tool to catch plant prey or indulge in self-defense. Venoms evolved in numerous predatory animals (at least in the arthropod, reptilian, and piscine orders) because with those, there are valid survival applications to catching your prey with it. And since venoms can also be used for self-defense, other non-predatory animals developed it, especially those with resources to defend such as bees and ants with honey or concentrated nurseries of larvae and brood that could represent a useful food supply to others.
But if your prey is immobile like plants are, then you don’t need venom to procure it. This is why I know of no venomous vertebrate herbivores – not rodents, birds, deer, horses, etc. There are a few mammalian insectivores such as tenrecs, shrews, and slow lorises, who have venomous glands in their mouths that secrete compounds to mix with saliva, and inject this into the site of a bite. This handful of venomous mammals use their bites to both disable their animal prey and for self-defense. Plus the platypus is a marsupial (not a mammal) that has venomous spurs on the hind legs that are used for fighting and self-defense as well. But aside from these rare exceptions, most mammals and marsupials are nonvenomous.
BTW there is a difference between venom and poison or toxicity. These words are commonly used interchangeably, but they are different concepts, and while allied they are substantially different. Venom is a type of protein-based substance that is delivered via an injection, usually a bite or a sting, that deters or incapacitates the recipient. Venom is delivered via an active process, purposely used for prey, self-defense, or both.
Toxicity is passive and is not delivered via bites or stings. Toxins are present in certain tissues of a given organism, or in all tissues of the organism, and they represent a health hazard when consumed or in some cases even touched. But toxins are not delivered via fangs, teeth, sharp spines, or stingers. Again, it’s passive. Now that doesn’t mean that toxins aren’t potent and dangerous because clearly many are, but they are usually used as self-defense against consumption only and not as aggression, like predation is.
There are clearly many toxic plants, well-defended on a chemical level, and this group includes succulents such as many euphorbias, many milkweeds, and some aloes for example. There might be a few cases where certain plants such as stinging nettles could be considered semi-venomous since those have specialized hairs on the leaf surfaces that are designed to break off and inject irritant compounds into the bodies of attackers. This makes nettle leaf hairs similar to the actions of a sting or bite, although obviously the plant cannot seek out contact on its own volition in the same way an animal can, and therefore it’s still inherently defensive in nature as opposed to aggressive. But basically stated, almost all chemically-defended plants are toxic, but not venomous.
Clearly some plants use toxins to defend themselves against herbivores including rabbits, and rabbits have evolved the ability to detox certain compounds, making the plants palatable to their kind. Monarch butterfly caterpillars are another well-known example of an animal that has evolved to handle the toxins in milkweeds they eat, and even use them to their advantage by becoming toxic to birds themselves thanks to their milkweed diet. But getting back to rabbits, note how while the rabbit might need to evolve methods of detoxifying plants in their diet, that they don’t need to develop venoms either in order to get their food in the first place. This is why rabbit saliva is not venomous. It’s biologically pointless.
Well that was quite the lecture. Hope I didn’t poison anyone’s brain. 😉
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(Postscript: I hope this discussion of various aspects of venomous saliva, herbivory, Komodo Dragons, and the definitions of venomous versus toxicity will be useful to future armchair gardening theorists. 🙂 )
Have to wonder how some of these myths persist.
Lack of education, lack of deeper thought, lack of researching into whether an assumption is correct or not. Like that silly myth that you shouldn’t water plants (tomatoes, cactus, African violets are all plants I have heard about) in the middle of the day because water droplets act like magnifying glasses that concentrate sunlight which burns holes into your plant’s fruits, stems, or leaves. Dumb myth. If things worked that way, why don’t YOU get 3rd degree burns from water droplets after exiting a swimming pool on a hot summer day? Why doesn’t your dog? Why don’t birds? Why doesn’t lawn grass fry when sprinklers are on? What about when a rain shower passes and the sun comes out afterwards? Morning dew at sunrise?
Go ahead and try watering ANYTHING on a sunny day. There will be absolutely zero harmful effects from magnifying glass sunrays via water beads. There might be other reasons not to water midday but the sunburn droplets is not one of them. Yet this stupid myth is still trotted out sometimes by thoughtless people.
Can you tell this one irritates me? The venomous rabbit one was at least new and is not widespread and might in fact be unique to that one lady. 😉