What is the likely diet of an advanced (spacefaring) alien civilisation? I suspect that all such aliens will be omnivorous.
Firstly, let me explain why we are unlikely to find herbivorous aliens (for brevity, I use aliens to refer to intelligent ones). For intelligence to evolve, it must first be useful to the evolutionary fitness of the creature. Is intelligence useful to early herbivores? I doubt this.
Intelligence is much more useful for meat-eating animals than purely herbivorous animals. Intelligence allows for the development of hunting tools and simple hunting tactics, which would prove highly significant in increasing the food sources of the hunting animal. But for a herbivorous species, intelligence affords no such benefit. If it does not increase the fitness of the herbivorous species, it is unlikely to evolve.
Then, if herbivores are unlikely to evolve into advanced aliens, would the opposite, a carnivorous race, be more likely to do so? Again, I doubt so.
It is quite reasonable to believe that a carnivorous race would become reasonably intelligent. However, as a civilisation, they are inherently limited. The reason - the lack of argiculture. Without agriculture to support a sufficently large population base, civilisation would not form, and technological advancement would be halted. This is why purely carnivorous species are unlikely to become our advanced spacefaring aliens.
From process of elimination, it appears that any advanced alien species must be omnivores!
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