This is a common question that comes up all the time in discussions of Tyranids, along with asking why they bother with land invasions at all instead of just breaking them apart in orbit, so this theory is my attempt to answer both of these questions.

My apologies if this has similarity to any other theories, or if there are any major mistakes. And yes, I do know at the end of the day the main reason is to sell toy soldiers, but that doesn’t really answer the in-universe reason of why, and this my try to answer that.

Tyranids are controlled by a collective psychic/warp entity called the Hive Mind, which is the combined souls of all Tyranids into one thing. This is different than being like a warp god, like the Emperor or the Chaos Gods, but this does mean that Tyranids are being controlled not by instinct, or pure real space practicality like many other factions, but instead by what a warp entity feels is most important. IE the chaos gods don’t care if some backwater planet in whocaresville is conquered, even though their followers might, they care if actions done there empowers them, for example Slaanesh wants excess, Khorne wants blood, etc.

Where am I going with this? Well the Tyranids seek out biomass, as it feeds them, keeps them going, lets them evolve. But if that was true, why would they go through the trouble of making big bugs to go down and physically eat them, if they could instead just send down flesh eating microorganisms to do the job just as well, if not more effectively. Why rely on rippers to individually chew down trees if you could just send a cloud to dissolve it into slurry? Well, what if Tyranids don’t just care about getting the biomass, but how they get it.

Going back to the warp entity angle, the warp is a place of symbolism and actions. Ie, Khorne does not care from where the blood flows, but he does care how. See how he disdains backstabbing, favors violent melee combat and carnage over ranged combat (though he will allow it), and loves brutal fights. If all Khorne wanted was blood, he could probably have planets set up for blood farming, where he would get more blood over a long period rather than in one big massacre.

Well, Tyranids are also run by a warp entity, what if they are similar? What if they don’t crave just biomass, but the psychic act of eating, of devouring? An alternate name for the Hive Mind is the Great Devourer, almost all Tyranid bioforms have prominent, massive, slavering, toothy mouths, regardless of if they can even digest food on their own. One of the smallest bioforms we know about are Rippers, who are just swarms of creatures that are pretty much just 90% mouth and stomach. What advantage does a swarm of rippers have over flesh eating bacteria? They are physically eating, devouring. They are so small because that way more of them can devour at a time, as opposed to just one thing eating, or just melting it down.

This would also explain why they bother with ground invasions, instead of only bothering with orbital bombardments. If you invade on the ground, you can more directly eat them, devour them, before still getting the biomass.

Obviously sometimes needs must, and sometimes Tyranids don’t always take their time eating, because at the end of the day, they are physical beings that need sustenance, but the Great Devourer probably prefers, well, devouring over simply getting biomass.

What do you guys think, any concerns, or holes in this theory?