Are animals that supposedly only eat one plant, really that restricted?

Factual Questions
1

Every so often you read about an animal that only eats one particular plant, or maybe animal. Like Pandas eat bamboo, koalas eat eucalyptus. Is that because the animal just can’t digest other items? Or is it simply that that item is what is readily available where the animal lives?

Because it seems to me that most life on earth is built out the same handful of compounds. Like plants are mostly cellulose and llignin and some other things I can’t recall right now. So assuming the animal can chew off and swallow the leaves and stems of whatever, shouldn’t it be able to digest it?

Like, if you dropped a few pandas and koalas into one of those ‘plants of the world’ type botanical gardens that didn’t have any bamboo or eucalyptus, would they just go on a hunger strike and starve themselves? Or might the caretakers find the pandas happily munching on sugar cane and the koalas doing major damage to the rhododendrons?

2

It’s been said that Monarch butterflies and their caterpillars must have milkweeds to feed upon. But it turns out that caterpillars can utilized other food sources to some extent, and adult butterflies consume nectar from a variety of flowers.

Koalas eating rhododendrons sounds like a really bad idea, as they’re pretty toxic.

3

This is a great question.

4

In fact, koalas can eat only specific eucalypts. There was an attempt to introduce them in Tasmania, but the correct eucalypts don’t grow there.

5

Pandas are actually, er, mal-adapted carnivores… they can’t process the bamboo all that well (Guardian article). Or from Wikipedia:

Wikipedia: Pandas and their transition from omnivory to bamboo-eating

Despite its taxonomic classification as a carnivoran, the giant panda’s diet is primarily herbivorous, with approximately 99% of its diet consisting of bamboo.[53] However, the giant panda still has the digestive system of a carnivore, as well as carnivore-specific genes,[54] and thus derives little energy and little protein from the consumption of bamboo. The ability to break down cellulose and lignin is very weak, and their main source of nutrients comes from starch and hemicelluloses.
[…]
The morphological characteristics of extinct relatives of the giant panda suggest that while the ancient giant panda was omnivorous 7 million years ago (mya), it only became herbivorous some 2–2.4 mya with the emergence of A. microta.[64][67] Genome sequencing of the giant panda suggests that the dietary switch could have initiated from the loss of the sole umami taste receptor, encoded by the genes TAS1R1 and TAS1R3 (also known as T1R1 and T1R3), resulting from two frameshift mutations within the T1R1 exons.[54] Umami taste corresponds to high levels of glutamate as found in meat and may have thus altered the food choice of the giant panda.[68] Although the pseudogenisation (conversion into a pseudogene) of the umami taste receptor in Ailuropoda coincides with the dietary switch to herbivory, it is likely a result of, and not the reason for, the dietary change.[64][54][68] The mutation time for the T1R1 gene in the giant panda is estimated to 4.2 mya[64] while fossil evidence indicates bamboo consumption in the giant panda species at least 7 mya,[67] signifying that although complete herbivory occurred around 2 mya, the dietary switch was initiated prior to T1R1 loss-of-function.[69]

They will eat meat if the opportunity presents itself: Panda filmed eating meat in China - BBC News. They just won’t really go out on their own to hunt or scavenge for it.

Well, specialist herbivores have different teeth and stomachs for digesting plant matter. The silica in leaves, for example, will do a number on your (human) teeth if you eat too many salads. Ruminants have specialized stomachs to better get nutrition out of plants, which is why some of them can be entirely grassfed.

And their adaptations aren’t just in the digestive tract alone, but also in other parts of their morphology… specialized claws, beaks, mouths, filters, sensory organs, gut biota, etc.

Many plants are also mildly (or occasionally greatly) toxic in some way, and that toxicity affects different animals differently… frogs can store plant poisons in their skins, birds can’t taste capsaicin in peppers, koalas have specialized livers for processing eucalyptus oils, etc. Eating unknown plants is a great way to get food poisoning or die in the wilderness.

Different animals also have different nutritional needs, depending on their genes and what they’re able to synthesize internally vs getting from the environment.

There’s also just their behavioral preferences, both instinctual and taught. Your average housecat technically could eat a variety of meaty things, but very few of them do. If you put in a panda in a Burger King, it probably won’t try to break open the coolers looking for food. If you force-fed it burgers, it might do OK for a while, but probably wouldn’t be too happy (would anyone…?).

Monophagy (specialists feeders who typically eat only one thing) usually got there through slow evolutionary processes in limited niches; it doesn’t necessarily mean that their entire system is solely capable of digesting just that one thing, but that on the whole, the organism is geared towards it (from detecting to acquiring to ingesting to processing, and ultimately, to preferring that thing).

Humans too… even as an “omnivore”, there’s a lot of plant matter we also can’t readily eat without substantial processing, whether for toxicity or hard seed coverings or simply taste. We just have technology on our side, while animals need to wait for evolution to do its thang.

6

This was a really interesting read: Koala Genome Sequence Reveals Secrets about Their Strange Biology | National Geographic

It’s not just any one single thing that lets the koala eat eucalyptus, but a combination of their specialized senses (they can determine which leaves are less toxic), genetic detoxification genes (livers and stomachs adapted for processing it better), the rearing of their young (mother makes the young koalas eat a special poop she formulates to inoculate their gut biomes), and other behaviors (sleeping a lot to conserve energy).

There’s been some success in expanding their diets by feeding them poop from less picky koalas: Poo transplants can alter koalas' gut microbiome so they can eat different types of leaves - ABC News

Maybe if you fed one enough poop from your local fast food restrooms, they’d start to get a hankering for Big Macs…? For science… anyone?

7

@Reply gave some great posts there. They touched on something that probably warrants expansion.

A koala in the wild could probably survive on at least some plants that aren’t eucalyptus. If it thought to try eating any of those things. A fussy human eater who lives on chicken nuggets could probably subsist equally well nutritionally speaking on hamburgers. But not if they’re unwilling to put it in their mouth.

Fussy young humans have the advantage of parents with wider experiences and of language. Lotta animals get rather little training from parents. And in the case of monophages like e.g. koalas, the parental lesson is “Eucalyptus is food. All else is not.”

Bottom line:
I could easily see many monophages starving while surrounded by stuff they could safely and nutritiously eat. If only they thought to try it.

8

Koalas sometimes eat brush box, paperbark, and bloodwood trees- the leaves only of course.

9

Don’t forget that humans cannot survive on a strictly vegan diet without vitamin supplementation. I believe it is B12 for which there is no vegan source. A few ants a day would likely suffice. I think that’s where cows get theirs.

10

If you’re just gonna feed some critter fast food poop, you should just cut to the chase and give 'em the fast food.

11

Nice! I mean, I’m willing to try, but I’m coming up short on koalas here in the Pacific Northwest. Looks like the San Diego Zoo might be my closest.

12

A third of the butterfly species in Germany absolutely need nettles during their larval phase. They won’t lay their eggs anywhere else, and the larvae won’t eat anything else.
For the silkworm the same applies, but they need mulberry leaves, preferably white mulberry. When I had some as a kid I tried to feed them salad, because mulberry tree leaves were difficult to get. It did not end well. The larvae were stunted and grew a sick colour. Then they died.

ETA: The stinging nettle, Urtica dioica, is called Brennnessel in German, with a triple n and a double s.

13

I believe B12 actually mostly comes from soil bacteria and is present on unwashed plants. The reason we get it from animals is because they bioaccumulate it from their feed and supplements. If we didn’t triple wash our salads, we’d get more of it too.

Vegan B12 supplements come from fermentation, not animal sources.

Nonetheless, your overall point still stands that the typical vegan diet requires careful analysis (and often supplementation) to maintain nutritional adequacy; it is very far from the diet that we evolved to eat.

14

It gets worse. Koalas will only eat leaves off this particular type of plant if they pick and choose them by themselves. You can present them with a freshly picked bowl of eucalyptis leaves and they will just look at them, not recognising them as even being food.

15

Coindentally, “nnn… ss%&*!!” is also the sound I tend to make whenever I come into contact with one of these. Isn’t language fascinating?

16

Indeed it is.
But the reason it is written like that is grammatically logical, at least within the parameters of the German rules for grammar.

17

Which strongly suggests their preference is far more a mental matter than actual digestive nutritional necessity.

Their feeding process is so ritualized that they can’t even recognize the same food as food without the associated ritual.

18

It is not so simple: they would die if they ate the older leaves that have accumulated more toxins. They want to choose the young, tender leaves that are not as poisonous yet.
Yes, and then the ritual took over. But there is a reason for the ritual.

19

What about corn?
It is ‘well known’ that archaeological teeth samples show wear, indicating use of sandstone for grinding corn. Do you get similar results just from eating too much of the wrong kind of corn?

20

Do you mean the silica left over from grinding, not from the plant itself?

Not sure… never heard of this. I would hypothesize (read: blindly guess) that since what we eat is typically the fruit of the corn plant (its kernels) and not its leaves, maybe there’s less silica there to begin with? And maybe since the kernels are smaller relative to something like a spinach leaf, maybe each one gets individually chewed less before swallowing? I don’t know; I’m just speculating.

I also thought I once read that cooked vegetables (as opposed to raw salad leaves) were less damaging to enamel, but I can’t find a ready cite for that anymore… it’s possible I misremembered?

And I also don’t know the strength of any of these impacts relative to the rest of your diet and your oral hygiene habits.

Sorry! It was just an interesting aside I learned once upon a time, not something I studied in depth.

(Oh, and to link it back to the OP… some herbivore teeth will keep growing throughout much of their lives, to counteract this sort of wear)