Krogan are a battle-loving race of reptilianoid warriors. The most well known of the living Krogan is Urdnot Wrex, who served alongside Commander Shepard. Another Krogan, Urdnot Grunt, after Shepard displayed proper loyalty to him on Tuchanka.
Krogans are arguably one of the toughest organic known amongst the myriad of races that form the Citadel Council. Their bodies are literally a powerhouse of organic strength, durability and adaptability. They have effective, dense musculature, a large mass, and skin 'only penetrable by a molecular blade'. They have eyes on the side of the head to give a greater field of vision, which allows them to better see those sneaking up on them. With most krogan, their body possesses primary, secondary and even tertiary organs and backups, even going so far as to have a redundant nervous system, their primary one being a system of electrically conductive fluid rather than a nervous one. Capable of cellular regeneration unseen in most races, (perhaps explaining their longevity, though they must also counter genetic aging), rivaling even the Vorcha, Krogan are resilient warriors.
They can live for centuries, and it can only be speculated as to how long exactly, though an asari on Illium comments that "Krogan have long lives" and that "It isn't like being with a human where you can stick it out for a century for them to die... no offence."
Though Krogan used to have a very high birthrate - indicating that their species are r-strategists, which caused their expansion and the Krogan Rebellions, they were practically sterilized by the salarians during the closing days of the Krogan Rebellions after the salarians created the genophage and the turian armies deployed it. The genophage is considered by most (incorrectly) to be a sterility plague, when in fact it is a bioweapon that simply alters krogan hormonal balances to ensure that krogan birthrates stay at the ratio of 1:1000. The salarian scientist, Prof. Mordin Solus described this as being the perfect ratio for Krogan as their birthrates before the genophage were too high for the galaxy to be safe, and inaction would mean allowing their population to rise unchecked or letting them fall to eventual extinction.
Krogan are still extremely adaptable, even so far as to naturally work around the genophage for a while, by bombarding it with genetic junk code to protect the important genes. A new modified version of the genophage was reintroduced to keep them in check.
Culture and society
Krogan love the thrill and glory of a messy, up close and personal battle and go to lengths for it, with most krogan off world from Tuchanka being mercenaries and bounty hunters. Due to their physiology Krogan are also popular bodyguards and shock troops, with Saren Arterius having tried to breed an army of Krogan for use by Sovereign (or Nazara as it is known by the Geth) on Virmire, only for the breeding facility to be destroyed by a converted mass effect drive core that had been hastily made into a crude thermonuclear weapon.
Krogan place emphasis on quality over quantity, especially now that the genophage limits their numbers. To prove themselves, all krogan undergo various rites and rituals. The only particularly well known is the Rite of Passage on Tuchanka. During the Rite, the aspiring krogan and his krantt must survive waves of various beasts, including varren, klixen and even a Thresher Maw. However, only Wrex and Shepard have been known to have killed the Thresher Maw on foot.
Rise of the krogan
Roughly 2000 years before 2183 (~183AD), the krogan were a primitive tribal species trapped on a world suffering through a nuclear winter of their own making. They were liberated from this state by the salarians, who "culturally uplifted" the krogan by giving them advanced technology and relocating them to a planet not cursed with lethal levels of radiation, toxins or deadly predators.
But the salarian intervention was not without an ulterior motive. At the time the Citadel was engaged in a prolonged galactic war with the rachni, a race of intelligent space-faring insectoids. The salarians hoped the krogan would join the Citadel forces as soldiers to stand against an otherwise unstoppable foe. The plan worked to perfection: within two generations the rapidly breeding krogan had the numbers to not only drive the advancing rachni back, and capable of doing so due to their ability to endure the harsh conditions of the rachni worlds. They were able to pursue them to their home worlds, find the rachni queens, and eradicate the entire species.
Saviors of the Galaxy
For a brief period the krogan were hailed as the saviours of the galaxy and were given not only the conquered rachni worlds, but other planets in Citadel space to colonise, in gratitude for their help. The Citadel Council even commissioned a statue for the Presidium—the Krogan Monument—to honor the krogan soldiers who died defending Citadel space.
But without the harsh conditions of Tuchanka to keep their numbers in check, the krogan population swelled to unprecedented numbers. Overcrowded and running out of resources, the krogan spread out to forcibly claim other worlds—worlds already inhabited by races loyal to the Citadel. There was always "just one more world" needed. The final straw was when the krogan began settling the asari colony of Lusia. When the Council ordered them to leave, Overlord Kredak, the krogan ambassador, stormed out of the Chambers, daring the Citadel races to take their worlds back. War broke out soon afterward. The resultant Krogan Rebellions continued for nearly three centuries. The krogan sustained massive casualties, but their incredible birth rate kept their population steadily increasing. Victory seemed inevitable. In desperation, the Council turned to the recently discovered Turian Hierarchy for aid. The turians unleashed the genophage on the krogan home worlds: a terrifying bio-weapon engineered by the salarians. The genophage caused near total infant mortality in the krogan species, with only 1 birth in every 1000 producing live offspring.
No longer able to replenish their numbers, the krogan were forced to accept terms of surrender. For their role in quelling the Krogan Rebellions, the turians were rewarded with a seat on the Citadel Council. The krogan, on the other hand, still suffer from the incurable effects of the genophage.
Over the last millennium krogan numbers have steadily declined, leaving them a scattered and dying people. Some try bizarre treatments for the genophage, including testicle transplants. But, faced with the certainty of their extinction as a species, most krogan have become individualistic and completely self-interested. They typically serve as mercenaries for hire to the highest bidder, though many still resent and despise the Citadel races that condemned them to their tragic fate. Wrex comments disdainfully that clubs, bars and brothels often try to hire krogan bouncers, reducing his people to some kind of status symbol.
Despite an announcement about failed, krogan-funded research into the genophage, which can be heard while on a Citadel elevator, most krogan have not worked toward a cure as they are more interested in combat than science. Unless one is discovered and used, the extinction of the krogan seems inevitable.
Mordin Solus eventually explained to Commander Shepard that the krogan were in fact evolving to bypass the damage of the genophage. To prevent overpopulation, Mordin's Special Tasks Group team created and applied a modified genophage, which he claims will keep the population down but still allows for a viable population, indicating the krogan race isn't as doomed as they believe. Mordin states that both the original and new genophage were designed to stabilise the krogan birth rate at pre-industrial levels—one viable birth per thousand. Given the prodigious krogan birth rate, it would appear that the enormously violent nature of krogan culture is actually responsible for their dwindling numbers. Ironically, this means that if the krogan were not so convinced that they, as a species, are doomed, and consequently spent less time roaming the Galaxy spoiling for a fight, their numbers might increase—they are essentially being killed by their own fatalism.