Is it possible that technologically-capable, spacefaring species would depend on a single source of food?

Consider something like the leaf cutter ant, which eat nothing but fungus, they evolved agriculture long before humans existed. They harvest plant material to farm the fungus and have been doing it for so long they can barely digest anything else. They even produce a biological antibiotic to kill off anything that might compete with the fungus.

It would be easy to see a strong evolved disgust response against even trying to eat other food in such a species. They farm something to feed the fungus which can be fed a variety of things, then only eat the fungus, that would allow them to evolve behavioral plasticity since they have to find things in new environments to feed the fungus. Plus fungus agriculture has evolved at least three separate times in insects so you completely justify it on an alien species.

One of Ian Bank's crazier ideas is a sentient species that grew up as a parasite (think tick/flea) of another. This kind of scenario fits your need very well.

For food sources, it's usually next to impossible that you wouldn't find another source of food that more or less does what your main food source does. But parasitic creatures are different. I'm not sure what exactly causes this, but the evolution of fleas closely follows that of their host and fleas were used to work out when humans and primates went their separate ways through evolution, since the flea species also diverged.

I'd guess that with parasite creatures, it's not just the food they get, but also how they get the food. The method of feeding might be peculiar, with only a certain special way to get under the skin and into the blood of the animal they feed on. Or maybe there are only certain special chemicals that cause hunger or various bits of the feeder's anatomy to be stimulated to "eat". In the way of milk, they may know all the chemical constituents, but they might not be able to synthesize it.

Anyway, that avenue is worth a bit of research.

The evolution of stomaches is much slower than the evolution of technology. There is evidence of the use of fire going back 400,000 years or more, but humans kept their large seed-grinding teeth and smaller skulls up until about 250,000 years ago… That's a big gap, but presumably we could not have evolved our bigger brains until after fire gave us food that was already partially broken down. We are still not very good at digesting uncooked protein, even though it's now considered essential to our diet.

Our brains are also hungry organs that consume almost 1/4 of all our calories.

So it's not much of a stretch to have an intelligent species that became dependent on a certain type of food (or how that food is processed). However, your problem is: with all this technology why haven't they invented "space food"?

In Jared Diamond's book Collapse, he talks about many cultures that died out simply because they stubbornly refused to adapt. He specifically talks about Viking colonists in Greenland who refused to change their diet to fish. They knew the indigenous people ate fish, but the Norse instead tried to keep their "grain and sheep" diet which was unsustainable. They insisted on keeping their own cultural identity and starved.

With all the allergies and biodiversity on our planet, the main reason certain foods are taboo is because of culture and religion… Maybe there were practical reasons to reject certain foods as "unclean" and that got woven into religious law, but those reasons are no longer true and seem arbitrary today. However people dedicated to their faith continue to reject them. There is also the weird case that the tomato was thought to be poison for 200 years.

I suggest you combine some aspect of their evolution that makes them dependent, but emphasize their cultural taboos or religious laws that prevent them from making necessary adaptations.

If your space-travelling-species didn't start this way it should be possible to create a scenario in which they rely solely on one food-species.

The problem with only one type is: it normally does not contain enough nutritions. You will rarely find everything you need in one type of food, especially if your space-travelling-species is supposed to be a highly difficult organism, which it probably is, if they want to go to space.

But: there are quite a lot of animals on our good old earth, that have (nearly) only one source of food. Therefore your species could quite easily start with one primary food-source that makes up most of their diet.

When they evolved they realised, that their primary food-source is far better for them than any other food they could find:

• the food-species was very fertile
• they were very resistant to environmental problems and changes
• there weren't many other predators trying to hunt the food-species
• they are very nutritious, so that your space-travelling-species only needs small amounts of other food
• they didn't take up much space compared with other foods
• it was easy to keep them confined and fatten them
• they contained lots of useful things: a lot of meat to eat, bones to make tools, etc.
• easy to guard from other predators

Because of these reasons they started breeding the food-species more and more. They made them fatter, more resistant, etc. Everything we did with animals like chicken, pig, cow, etc.

You have a space-travelling-species, so their technological level should be beyond ours, so at some point they started bio-engineering. They added whatever they needed to the food-species and started breeding them more than ever before. Soon your species had an animal that could give them everything they needed. They relied only on this one kind of animal. And because their organism was already adapted to this source of food the following generations started to be unable to use other sources of food.

And this was the point, when your species changed from one, that was relying heavily on one type of food to one that relied solely and exclusively on one type of food.

After this they needed to spend some time on details, like how to best store the food. But I imagine if your species has always heavily relied on this one source of food they have evolved very refined methods of keeping their food intact and transport it in the most efficient way possible.

It's highly unlikely that an intelligent species strictly depends on one single "food species". Two main reasons:

1. Evolution: The more narrow a species ecological niche, the less resilient the population is. So almost every species can deal with at least a small spectrum of nutritional sources. If it could not, it would not have survived.

2. Chemistry: Even if your species relied on a single food species, technically the food species is the result of bio-chemical processes. If these processes in an organism lead to an appropriate food source for your species, the same processes would lead to appropriate food sources if artificially initiated, e.g. in a lab or an industrial environment. Likely it is only the mix of a few parameters, maybe chemical compounds, that make the food species appropriate. I cannot think of any reason, why it would be impossible, to get these right in a controlled environment such as a lab.

The idea of intelligent parasites is interesting... however, the most intelligent parasites I can think of, are insects. And insects, as we all know, tend to be rather small animals, which also limits the size of their "brains" or ganglia. Furthermore, the dependence on an appropriate host brings severe limitations to the extend of development. This is because the environment a parasite has to adopt to, is its host. And this environment is really limited, in every way.

In my opinion, your idea is rather unconvincing.

Well, your species obviously has an extensive food production industry on the home planet, the question of whether a spacefaring species could exist with a dependency on such a specific food source boils down to one question, how hard would it be to take the production cycle with them into space?

Maybe, instead of trying to figure out how to artificially produce the food, maybe their technology focused on making the industry mobile.

i.e. Farms in space

The tricky part of this question is why they can't synthesize suitable food, as single-food dependent animals are pretty well known on Earth - giant panda, snail kite, etc. (Many of these animals do eat something else occasionally, but they are definitely totally dependent on one food species for survival. Similarly, the larval-food dependence of many moths/butterflies.)

Some possibilities:

-non-native technological development: the space travel technology doesn't come from a generally high tech base but was obtained from aliens or uncovered from the ruins of a previous destroyed civilization on their world.

-impaired biological development: something about them or their world makes it extremely hard to develop biology relative to the other sciences. It could be some misleading fact (introduction of alien species in the distant past or really high rate of radical mutations) leading to wrong theories of biology becoming dominant, or something cultural (a biological-weapons war almost made them extinct so genetic research is forbidden, long period of dominance of a world dictatorship pushing something like Lysenkoism).

-aberrant means: their space travel tech uses something like FTL or teleportation that doesn't fit well with our physics, and turns out not to require a high general level of technology.["The Road not Taken" by Harry Turtledove] Possibly their brains are wired fundamentally differently from humans', so something is obvious & simple to them that isn't to us.