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Aposematism, or how Nature tells us that carotenoids are POISONOUS

What is aposematism, you ask?  From the Amateur Entomologists Society website (entomology is the study of insects):

Aposematism is the term used to describe the use of bright colouration to advertise that an organism is dangerous or unpalatable.

Let's get one more bit of info on what aposematism does, from the Encyclopedia Britannica:

Aposematic mechanism, biological means by which a dangerous, or noxious, organism advertises its dangerous nature to potential predators. The predator, having recognized the dangerous organism as an unfavourable prey, thereupon desists from attacking it. Aposematic, or warning, mechanisms have evolved along with protective systems; it is advantageous for the protected organism not to risk the injury that is likely to occur in even a successfully repelled attack by a predator.

The most common aposematic mechanism is the possession of bright, contrasting colours, such as the black and yellow of many wasps and the red of ladybird beetles. Other organisms, such as the North American rattlesnakes, employ acoustic warning systems.

What are the main colors of carotenoids?  Yellow, orange, and red.  What colors do animals often use for aposematic coloration?

Contrasting coloration in terrestrial mammals

Animals that signal their unprofitability to potential predators are often bright red, orange, yellow or white in combination with black (Cott 1940)

So, from the above, we can see that certain insects, reptiles, and mammals (this is not a complete list, just those examples) use aposematic coloration, and that coloration is often in the yellow/orange/red spectrum, and that coincidentally, carotenoids just so happen to be a major source of yellow/orange/red coloration in plants.  It is possible that carotenoids are a WARNING to us not to eat those plants, because they are POISONOUS?  I would say that is VERY likely.  What's that?  Aposematic coloring is COMMON in plants?  You don't say...

Aposematic (Warning) Coloration in Plants (PDF attached below)

Aposematic (warning) coloration seems to be a common defense in plants, eventhough it was practically ignored by botanists and zoologists until recently. The first papers fully dedicated to visual aposematism in plants were published after the year 2000. Similar to the situation in animals, aposematic (warning) coloration (yellow, orange, red, brown, black and white, and combinations of these colors) seems to be a common defense in plants, although (except for anecdotal mentions) it has been practically ignored by botanists and zoologists until recently. The fact
that many aposematic animals use both plant-based pigments and sequestered poisonous molecules to become aposematic highlights the absurdity of neglectingthe aposematic nature of so many plants. Aposematic coloration was referred to here in the broadest sense, considering any visual warning phenomenon that may deter herbivores. Aposematic coloration is expressed by thorny, spiny and prickly plants, by poisonous ones, and by plants that are unpalatable for various reasons. Plants that mimic aposematic plants or aposematic animals are probably common, despite the small number described so far. Colorful mimicry of insect infestation or herbivore damage to tissues in order to repel herbivores is also found in plants. Many types of aposematic coloration simultaneously serve other functions, such as physiological, communicative and even other defensive functions. It is therefore difficult in many cases to evaluate the relative functional share of aposematism in various color patterns of plants and the specific selective agents involved in their evolution. Aposematic coloration is part of a broader phenomenon of defensive coloration in plants, which has also received insufficient attention. The fact that botanists have not usually considered the operation of aposematic coloration or other types of defensive coloration is evident from the lack of a regular and systematic description of these color patterns in the majority if not all of the thousands of published floras.

Warning displays may function as honest signals of toxicity

Many prey species use colourful ‘aposematic’ signalling to advertise the fact that they are toxic. Some recent studies have shown that the brightness of aposematic displays correlates positively with the strength of toxicity,
Thus, contrary to the prevailing theoretical orthodoxy, warning displays may in fact be honest signals of the level of (rather than simply the existence of) toxicity.
As with sexual signals, aposematic coloration is commonly imparted by pigments including carotenoids...

Let's go even deeper into this and make some interesting connections.

First, carotenoids and retinoids are fluorescent when exposed to UV light:

It seems that many toxins are fluorescent:

Certain animals (we're NOT one of them) have evolved to see UV fluorescence:

What Animal Can See Ultraviolet Color?

The vast majority of animals with ocular media, including fish, amphibians, insects and other invertebrates, reptiles, birds and some mammals are capable of seeing ultraviolet light. Goldfish, mantis shrimp, butterflies and eagles are a few examples, while hedgehogs are one of the most UV-sensitive mammals.

Prior to several studies published in 2014, the nature of UV-sensitive mechanisms in animals, particularly mammals, was unknown. Most animals that can see into the ultraviolet spectrum possess ocular lenses that let UV light into the eye where a pigment sensitive to UV absorbs it. However, even without these pigments, all examined photoreceptors in the animal kingdom exhibit some sensitivity to UV radiation if this light reaches them. Cats, dogs, red pandas, cattle, reindeer and okapi are a few of the mammals that a study proved could, at least partially, see UV light.

If there was ever a time for "guilt by association" by both colors (yellow, orange, red) and fluorescence (all Poison/"Vitamin A" is fluorescent), with aposematism and toxicity, I would say that time is now.

Still think you should "Eat the Rainbow"?  Don't say you didn't see the "warning" signs Nature provided you.

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