Tyranids have consumed multiple galaxies

• 5th Ed Codex [Tyranid]:

"The Tyranids are the most alien of races to infest Imperial space, for they come from the void itself. Their chitinous bio-ships drift from star system to star system in brooding silence. Behind the Hive Fleets lie the barren husk of a dozen galaxies already consumed. Once the remorselessly hungry shoals of bio-ships detect the presence of a prey world, they begin to teem with life, closing upon their target like a set of impossibly large jaws. There can be no escape from their fatal embrace. For this reason the Tyranids are known as the Great Devourer."

• White Dwarf #145

"Humanity will be absorbed, broken into strands of DNA to be used to create a new generation of bio-technology. It will be the death of the human race, but to the Tyranid hive mind this is of no more consequence than the mining of ores or the harvesting of crops. For the Tyranids have no sense of pity or compassion, they are as utterly beyond human understanding as humans are beyond their comprehension. To them man is just an inefficient and primitive lifeform, something to be consumed and turned to a higher purpose. Such has been the fate of a thousand galaxies, of millions of intelligent species, since time immemorial."

• Hive Fleet Horror:

"And then he became aware of the psychic presence of the tyranids. It was weird, like nothing else he had ever experienced: an implacable, ferocious sentience which was ancient beyond imagining. It stood alone; no one would ever be able to speak to it. Suddenly he felt as though his psyche had been torn apart like the human body on the floor of the chamber. The scene before his eyes vanished. He was somewhere else. Somewhere dark, but filled with a seething and a rustling. He had entered the hive mind. And now he understood what the tyranids were.

The Tyranids were what ants and termites would be if they could evolve further and become intelligent. What made such intelligence incomprehensible was that the tyranids had never evolved emotions. They were aware that concepts such as sympathy and honour existed in the species they harvested, but they viewed them only in the abstract and dismissed them as evolutionary mistakes. Gene coding for emotion was never made use of by the hive fleets.

Yes, the tyranids were intelligent, but intelligence was not a quality particularly prized by the hive mind. A tyranid creature could reason, but it never did so out of self-interest. Intelligence, like everything else, served only tyranid hive instincts - or rather, it served the single great tyranid instinct the one overwhelming, compulsive urge.

SURVIVE! AND SURVIVE FOREVER!

When the tyranids invaded a galaxy they took aboard vast amounts of foodstuffs and raw materials, but those were not what they came looking for. They knew that every system, whether mechanical or biological, eventually runs down. Most species lasted only a few million years. A few - like some Earth ants - managed to survive for up to a hundred million years. But sooner or later they perished as their DNA either failed to adapt or simply deteriorated through natural wear.

The tyranids had found the only possible remedy for this. They moved from galaxy to galaxy, harvesting fresh, newly evolved DNA with which to renew and reinvigorate their own. They were the universe's ultimate life form. Quite possibly they had exisited forever and would continue to exist forever. Quite possibly the universe contained an infinite number of hive fleets.

The Imperium of Man had beaten off one hive flee. Perhaps it could beat off others. It would be a rare reversal for the tyranids, but that did not matter at all. Ina few million years the Imperium would be gone, the human race would be gone, and some other hive fleet would arrive, meeting weaker resistance, and would leave the galaxy lifeless and desolate. Then, a few billion years later, life would evolve all over again, on millions of planets. And again a hive fleet would move in....

Jaxabarm did not think the hive tyrant was at all aware that he was eavesdropping on the hive mind. He was not worthy of notice. They tyranid did not respect human intelligence - tyranid did not respect any intelligence, not even their own. All they saw in the human race was a species possessing young, vigorous DNA...

He would try to persuade Drenthan Drews to join the Imperial Guard and help defend the Imperium. Hive Fleet Kraken had to be repelled or humanity was doomed. Not that the outcome was of any importance to the tyranids. To them, species evolved and perished like blades of grass. Galaxies condensed, blazed, then guttered out. The supposedly immortal Chaos gods would not even last that long. They would perish when the psyches which sustained them died out. Only the tyranids lasted forever."