Krogan are large, heavily built humanoids of reptilian ancestry. Krogan are strong, durable, and adaptable creatures with life spans measured in centuries. Prior to the release of the genophage biological weapon, krogan birth rates were extremely high: a single krogan female could give birth to more than a thousand young in a year.
A key aspect of krogan biology is their multiple redundant organs. Krogan can survive multiple shots from assault rifles and continue fighting. In addition to a "normal" nervous system, krogan have an electrochemical second circulatory system that acts as a redundant nervous system; thanks to this system, a krogan can never be permanently paralyzed, as the circulatory system will heal over time. Krogan can also enter into a "blood rage", a state of rage that makes them unresponsive to pain and other debilitations; the blood rage is such that even a clinically brain-dead krogan with its limbs severed will continue to aimlessly thrash around with its teeth until it expires. To top it all off, krogan have a tough hide, stated to be so tough that nothing short of a "molecular blade" can penetrate it.
The krogan homeworld of Tuchanka was home to a wide array of aggressive fauna, requiring the krogan species to breed aggressively to survive. Against all the predators of Tuchanka, only one krogan newborn in a thousand could expect to survive to maturity.
The predator threat subsided only after the krogan developed advanced weapons technology. With the predator threat reduced, the krogan turned their weapons on each other, eventually devastating their planet in nuclear war. The harsh post-war ecology continued to keep their population in check until the salarians transported krogan off their homeworld to assist in their war against the rachni. The krogan won, but they became a problem themselves when removed from the hostile environment of Tuchanka, as their population began to grow rapidly. Krogan began to expand beyond the planets granted to them by the Citadel Council, even invading planets populated by Citadel member races. The turians used superior technology and strategic intelligence to devastate krogan armies, but despite massive casualties, the krogan population continued to grow. To mitigate the krogan threat, the salarians created a genetically engineered disease called the genophage to reduce krogan reproduction to its pre-industrial level, and the turians unleashed it on the krogan population. No longer able to replace their staggering losses, the krogan were finally defeated.
Afterwards, the surviving krogan mostly gave up on political aspirations of empire. Instead their society fragmented along clan lines. That said, krogan society remains highly martial, and many (though by no means all) krogan now seek employment in mercenary companies. Krogan tactics have also evolved from the rebellion period. Previously, they relied on attritional warfare, but being unable to replace losses so rapidly, their methods of warfare became geared more towards maximising the individual skills of each combatant, giving rise to the krogan Battlemasters.
Krogan of Note
- Urdnot Wrex
While individual krogan are typically strong and incredibly durable, as a political faction the krogan are of marginal importance in the Mass Effect universe. The krogan are divided into numerous armed camps that feud with each other, and their reduced reproduction rate is insufficient to replace losses from inter-clan conflicts. Consequently, the krogan race is slowly but steadily dying out.
By the year 2186, Clan Urdnot had reunited many of the krogan clans under their leadership. The united krogan clans are able to muster a formidable ground combat force, but (being still demilitarized by the Council) they lack an interstellar navy. If the genophage were cured, their potential threat level would likely increase thanks to a population explosion, which makes certain Council races extremely wary of any efforts to create such a cure.