The krogan evolved in a lethal ecology. Over millions of years, the grim struggle to survive larger predators, virulent disease, and resource scarcity on their homeworld, Tuchanka, turned the lizards into quintessential survivors. Perhaps the most telling indicator of Tuchanka's lethality is the krogan eyes. Although they are a predators species by any standard definition, their eyes evolved to be wide-set, as any Earth prey species like deer and cattle. Krogan eyes have a 240-degree arc of vision, better suited for spotting enemies sneaking up on them than for pursuit.
Physically, the krogan are nigh-indestructible, with a tough hide impervious to any melee weapon short of a molecular blade. While they feel pain, it does not affect their ability to concentrate. They have multiple functioning examples of all major organs, and can often survive the loss of one or two of any type. Rather than a nervous system, they have an electrically conductive second circulatory system. A krogan can never be paralyzed - they may lose some fluid, but it can be replaced by the body in time.
The hump on krogan's back stores water and fats that help the krogan survive lean times. Large humps are a point of pride; being well-fed implies the krogan is a superior predator.
The most widely-known biological feature of the krogan is their incredible birth rate and rapid maturity. Once freed from the hostile cauldron of Tuchanka, the krogan population swelled into a numberless horde. Only the genophage kept them from out-breeding the combined Council races. Now the rare krogan females capable of bringing a child to term are treated like strategic resources: warlords will trade them at diplomacy or (more frequently) fight wars over them.