Crocodiles reproduce by laying eggs, which are either laid in hole or mound nests, depending on species. A hole nest is usually excavated in sand and a mound nest is usually constructed out of vegetation. Nesting period ranges from a few weeks up to six months. Courtship takes place in a series of of behavioral interactions that include a variety of snout rubbing and submissive display that can take a long time. Mating always takes place in water, where the pair can be observed mating several times. Females can build or dig several trial nests which appear incomplete and abandoned later. Egg laying usually takes place at night and about 30–40 minutes.[59] Females are highly protective of their nests and young. The egg are hard shelled but translucent at the time of egg-laying. Depending on the species crocodile, a number of 7-95 eggs are laid. Crocodile embryos do not have sex chromosomes, and unlike humans, sex is not determined genetically. Sex is determined by temperature, where at 30 °C (86 °F) or less most hatchlings are females and at 31 °C (88 °F), offspring are of both sexes. A temperature of 32 °C (90 °F) to 33 °C (91 °F) gives mostly males whereas above 33 °C (91 °F) in some species continues to give males but in other species resulting in females, which are sometimes called as high-temperature females.[60] Temperature also affects growth and survival rate of the young, which may explain the sexual dimorphism in crocodiles. The average incubation period is around 80 days, and also is dependent on temperature and species that usually ranges from 65 to 95 days.[61] At the time of hatching, the young start calling within the eggs. They have an egg-tooth at the tip of their snouts, which is developed from the skin, helps them pierce out of the shell. Hearing the calls, he female usually excavates the nest and sometimes takes the unhatched eggs in her mouth, slowly rolling the eggs to help the process. The young is usually carried to the water in the mouth. A group of hatchlings is called a pod or crèche and may be protected for months.[