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. The end result is that they destroyed their homeworld in a nuclear war that reduced their race into primitive warring tribes.
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. 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 secondary to prenatal and postnatal death and, ultimately eliminating the krogan's numerical advantage.
Due to the brutality of their surroundings, natural selection has played a significant role in the evolution of the krogan. Unlike most sentient species, krogan eyes are wide-set - on Earth, this is common among prey animals, and in this case it gives the krogan 240-degree vision, giving them greater visual acuity and awareness of approaching predators. Krogan eyes have narrow pupils. Their irises may be red, green, amber, or blue in color.
Prior to the genophage, krogan could reproduce and mature at an astonishing rate. Females are known to produce clutches of up to 1,000 fertilized eggs over the course of a year. In present conditions, according to EDI, the odds of a krogan successfully giving birth to two females stand at 1 in 2,000, though krogan biology is slowly adapting to the virus.
Their large shoulder humps store fluids and nutrients in much the same way as an Earth camel, enabling them to survive extended periods without food or water. In particular they require very little water intake; a 55-gallon tank system can sustain an entire colony of krogan for over 10 years.[1] 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. Consequently their diets can include food and drink which would prove very dangerous to other species—a fact reflected in the krogan liquor of choice, ryncol, which is said to "hit aliens like ground glass".
Younger krogan have yellow or green markings on their hides. These markings darken to brown or tan over time, showing their age. Younger krogan tend to have looser plates on their head with soft spots in between. When they grow older the plates grow together to form one whole.
Krogan typically stand over 7 feet. Heavy creatures, krogan have been known to weigh at least 150 kilos, upwards of nearly 200 kilos for above-average specimens. Some particularly large krogan weigh in at about 800 pounds (~362 kilos), estimated independently by themselves and by non-krogan observers. 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) systems are capable of serving as back-ups in the event of damage to the primary biological structures. They have, for example, two hearts, four lungs, and four testicles. This reflects in their slang, where they often speak about a "quad" where a human would use the words "balls" or "pair". Instead of a traditional nervous system, Krogan have a second circulatory system using a neuroconductive fluid, meaning they are almost impossible to paralyze, as any lost fluid is replaced over time similar to blood.[2] Krogan bleed a yellow or orange fluid when shot, which may be this fluid or actual blood.
Aside from redundant systems, the legendary krogan "blood rage" adds to the race's reputation for being notoriously difficult to kill or incapacitate in normal combat scenarios. In this state, krogan become totally unresponsive to pain and will fight to the death regardless of injury level, with the side effect of reducing their capacity for logic and self-control.
Sheer physical hardiness means an individual krogan can expect to live for centuries. Krogan can live for well over a thousand years, as evidenced by Warlord Okeer, a veteran of the Krogan Rebellions who died (of decidedly unnatural causes) well over a thousand years after the Rebellions ended. Nakmor Drack is another example of their long life-spans, a 1,400-year-old veteran of the Krogan Rebellions who joined the Andromeda Initiative to voyage to the Heleus Cluster. Some elder krogan such as Drack can develop benign boney growths as facial exostosis which may resemble a spikey beard.
Despite their natural propensity for toughness, krogan do have physical weaknesses. According to Zaeed Massani, if one lodges a knife at a certain spot near the frontal plate on a krogan's head, the plate can be ripped off. It is apparently the one thing krogan do fear.
Ancient krogan society was once rich with cultural, architectural, and artistic accomplishments. However, the krogan birth rate exploded despite the natural limits of their predatory homeworld once they achieved industrialization. Technology made life "too easy" for them, so when they looked for new challenges they found those in each other. Wars were fought over dwindling resources as the krogan expanded.
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.
The krogan were liberated from their primitive state by the salarians, who "culturally uplifted" the race by giving them advanced technology and relocating them to a planet not cursed with lethal levels of radiation, toxins or deadly predators. The salarians even gave the krogan the means to stabilize Tuchanka's atmosphere by means of the Shroud. 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 not only to drive back the advancing rachni, 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.
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. The resultant Krogan Rebellions continued for nearly three centuries. The krogan sustained massive casualties, but their incredible birth rate kept their population steadily increasing. They also had no compunction about resorting to extreme tactics, such as using redirected asteroids as weapons, almost rendering several planets uninhabitable. Soon their 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 bio-weapon engineered by the salarians. The genophage caused near-total infant mortality in the krogan species, with only one birth in every thousand 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 effects of the genophage.
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. Sometimes, when krogan disagree, one of them will head-butt the other to show dominance. They respect strength and self-reliance and are neither surprised nor offended by treachery. Their worst insult is to call someone "not worth killing". Most krogan trust and serve no one but themselves, living with a generally standoffish attitude focused entirely on personal gain and short-term gratification of their violent urges. Their version of cooperation is reflected in one of their sayings: "Seek the enemy of your enemy, and you will find a friend." The krogan are without any doubt a warrior race.