Spawning begins with the male excavating a shallow pit. The female lays into it, the male fertilises the eggs, and she immediately takes them up into her buccal cavity. There they are turned and aerated by rhythmic mouth movements for roughly a week to ten days depending on temperature, during which the female eats little or nothing.
After the fry become free-swimming she continues to shelter them, opening her mouth for their return at any disturbance for a further period. The cost is direct: brooding females lose condition, grow more slowly and spawn less often than males. That asymmetry is one reason commercial tilapia farming favours all-male stock, which grows faster and avoids uncontrolled breeding in ponds.
Not all tilapias share the habit. Some related genera are substrate spawners that guard eggs on a surface instead, and the loose commercial use of the word tilapia hides real differences in reproductive behaviour between genera.