fry. The eggs hatch approximately 72 hours after fertilization. Until that time the parents expel intruders and potential egg predators from around the nest. They also fan the eggs, moving water with their fins over the clutch to provide oxygenation. They fan the eggs both day and night; at night they use their sense of smell to recognize the presence of the eggs in the dark, and they keep their pelvic fins in contact with the eggs to remain at the right distance for fanning. In darkness the pair recognizes each other and detect predators using their sense of smell. After hatching, the larvae spend another 72 hours absorb their yolk sacs and developing their fins before they become free-swimming fry. The fry forage during daylight in a dense school and return to the cave or crevice for the night. Like other cichlids, the parents retrieve their young just before dark, sucking up three or four at a time and delivering them into the nest. The parents anticipate night, using a sense of time; in laboratory experiments convict cichlids continued to retrieve young as night approached even in the absence of any signal, such as dimming light. During the night, the fry bunch up at the bottom of the cave or nest, where the parents fan them. Both parents remain involved in guarding the fry from brood predators and engage in behaviors to assist feeding such as moving leaves or fin digging (digging up the substrate with their fins). Brood care of eggs, larvae and free-swimming juveniles in the wild can last 4 to 6 weeks, and occurs only once per season for the majority of females. In contrast, females in aquaria are known to breed many times per year with short intervals of 12 or 13 days between broods, as long as suitable rocks or similar surfaces are available for them to lay their eggs on. Mating system Convict cichlids are serially monogamous, so pair bonds may form first before they establish a territory together, or the male and female may each obtain a territory before pairing with each other. Because the convict cichlids are also substrate-brooding, this territory will include a breeding site for the deposition of eggs. Sexual selection The effect of population density on sexual selection for convict cichlids has been studied. When nest density was greater, the females tended to be larger, which is more