The krogan are a species of large reptilian bipeds native to the planet Tuchanka, a world known for its harsh environments, scarce resources, and overabundance of vicious predators. The krogan managed to not only survive on their unforgiving homeworld, but actually thrived in the extreme conditions. Unfortunately, as krogan society became more technologically advanced so did their weaponry.
Four thousand years ago, at the dawn of the krogan nuclear age, battles to claim the small pockets of territory capable of sustaining life escalated into full scale global war. Weapons of mass destruction were unleashed, transforming Tuchanka into a radioactive wasteland. The krogan were reduced to primitive warring clans struggling to survive a nuclear winter of their own creation, a state that continued until they were discovered by the salarians two thousand years later.
With the help of the salarians, the krogan were 'uplifted' into galactic society, and lent their numbers and military prowess to bring an end to the Rachni Wars (see below). Ironically, after the rachni were eradicated, the rapidly-expanding krogan became a threat to the galaxy in turn, starting the Krogan Rebellions and forcing the turians to unleash the genophage. This genetic 'infection' dramatically reduced fertility in krogan females, causing a severe drop in births and, ultimately, population, eliminating the krogan numerical advantage.
Due to the brutality of their surroundings, natural selection has played a significant role in the evolution of the krogan. Unlike most species on the Citadel, krogan eyes are wide-set - on Earth this is distinctive of prey animals, but in this case it gives the krogan 240-degree vision, giving them greater visual acuity and awareness of approaching predators. Prior to the genophage, krogan could reproduce and mature at an astonishing rate.
Their large shoulder humps store fluids and nutrients, enabling them to survive extended periods without food or water. A bigger shoulder hump is seen as a sign of high status, showing how successful an individual krogan is at hunting. Their thick hides are virtually impervious to cuts, scrapes or contusions, and they are highly resistant to environmental hazards, including toxins, radiation, and extreme heat and cold. Younger krogan have yellow or green markings on their hides. These markings darken to brown or tan over time, showing their age.
Biotic individuals are rare, though those who do possess the talent typically have strong abilities. The most amazing physiological feature of krogan biology is the multiple instances of major organs. These secondary (and where applicable, tertiary and quaternary) systems are capable of serving as back-ups in the event of damage to the primary biological structures. They have, for example, four testicles, which reflects in their slang, where they often speak about a "quad" where a human would use the words "balls" or "pair". Krogan also have a secondary nervous system using a neuroconductive fluid, meaning they are almost impossible to paralyze. Krogan bleed a yellow or orange fluid when shot, which may be this fluid or actual blood.
Having redundant systems makes krogan difficult to kill or incapacitate in normal combat scenarios. Sheer physical hardiness means an individual krogan can expect to live for centuries. Based on Okeer's dossier, krogan can actually live for well over a thousand years (his dossier states he has millennia of combat experience).
Matriarch Aethyta, bartender at the Eternity lounge, tells how her father, a krogan, fought in the Rachni Wars. If this is true, he would have been about 900 years old at her birth. Warlord Okeer states "let 1000 die in a clutch" this means that each krogan birth can potentially birth 1000 krogan infants, at this rate a handful of females could breed the modern equivalent of a division(10,000-15,000).
According to Zaeed Massani, if one lodges a knife at a certain spot on their crest, you can rip it off. It is apparently the one thing they do fear.
Krogan typically stand over 7 feet and weigh roughly a ton in armor.
RISE OF THE KROGAN
Roughly 2000 years ago 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 insects. 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, but the 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 saviors 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.
A hardsuited krogan
The so-called 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.
In Mass Effect 2, Mordin Solus explains that the krogan are in fact evolving to undo the damage of the genophage. To prevent overpopulation, Mordin's STG team created and applied a new 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.
Culture and Government:
The harsh krogan homeworld conditioned the krogan psychology for toughness just as it did the body. Krogan have always had a tendency to be selfish, unsympathetic, and blunt. They respect strength and self-reliance and are neither surprised nor offended by treachery. The weak and selfless do not live long. In their culture, 'looking out for number one' is simply a matter of course. Young krogan undergo a rite of passage that is overseen by a shaman respective to the clan the krogan wishes to join. This rite consists of battling various wild fauna, ending with an encounter with a thresher maw. Merely surviving for five minutes is considered proof of worthiness. Killing the thresher maw increases the initiate's prestige and standing. Few outsiders have seen the rite of passage take place, even though there are no rules in krogan tradition that state that a non-krogan can't help with the undertaking of the rite by acting as the participants Krantt. Membership of a clan allows a krogan to own property, join the army and apply to serve under a battlemaster. Krogan have powerful territorial instincts which serve them well in combat, but can create problems; when traveling on starships, for example, krogan find sharing quarters nearly impossible.
After their defeat in the Rebellions, the very concept of krogan leadership was discredited. Where a warlord could once command enough power to bring entire solar systems to heel and become Overlord, these days it is rare for a single leader to have more than a thousand warriors swear allegiance to him. It is speculated that their instinctive aggression and territorial nature prevent the krogan from forming any kind of centralized government or parliament that is not based on fear or obedience. Most krogan trust and serve no one but themselves.
This solitary attitude stems in part from a deep sense of fatalism and futility, a profound social effect of the genophage that caused krogan numbers to dwindle to a relative handful. Not only are they angry that the entire galaxy seems out to get them, the krogan are also generally pessimistic about their race's chances of survival. The surviving krogan see no point to building for the future; there will be no future. The krogan live with an attitude of 'kill, pillage, and be selfish, for tomorrow we die.'
Female krogan rarely leave their home worlds, focusing on breeding in an attempt to keep krogan numbers from declining too quickly. The few remaining fertile females who can carry young to term are treated as prizes of war, to be seized, bartered or fought over. Recently, it has been noted that the females of the krogan species live in clans separate from that of the males. Envoys are sent out from the female clans to determine who amongst the males is worthy to visit the female clans. Due to the effects of the genophage and the lack of fertile females, this happens often and many male krogan sire children from one female.
Traditional krogan tactics were built on attritional mass-unit warfare. Equipped with cheap, rugged gear, troop formations were powerful but inflexible. Command and control was very centralized; soldiers in the field who saw a target contacted their commanders behind the lines to arrange fire support.
Since the genophage, the krogan can no longer afford the casualties of the old horde attacks. The Battle Masters are a match for any ten soldiers of another species. To a Battle Master, killing is a science. They focus on developing clean, brute-force economy of motion that exploits their brutal strength to incapacitate enemies with a swift single blow of overwhelming power. This change of focus from mass-unit warfare to maximal efficiency has increased employment demand in the fields of security and 'muscle for hire.' Due to the unsavory reputation of the krogan, most of these jobs are on the far side of the law.
Battle Masters are not 'spit and polish,' but they do believe in being well-armed and equipped, preferably with a gun for each limb. They are callous and brutal, but methodical and disciplined. They use any means at their disposal to achieve their goals, no matter how reprehensible. Hostage-taking and genocide are acceptable means to ensure a quiet occupation with few krogan casualties.
The krogan serving with Saren's forces appear to be returning to the old style of mass attritional combat. They also work in close cooperation with supporting geth units, who fill in the roles occupied by combat drones in other armies.
Biotics are rare among the krogan, especially since the practice of surgically creating krogan biotics has been discontinued (due to the high mortality rate). Those that exist are viewed with suspicion and fear. The krogan see this aura of fear as a useful quality for an officer, and often promote them. Combat drones and other high-tech equipment are likewise in short supply.
Krogan do not have strong religious beliefs. The closest they come is to establish ritualistic burial grounds called the Hollows, where the skulls of their ancestors are displayed to remind them of "where we all come from, and where we all go." The Hollows are as sacred as any krogan place can be, and violence there is forbidden. Wrex also mentions a place called "the Void" which some have speculated to be the Krogan version of the afterlife, however, he may have just been refering to deep space.