females. 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. The eggshell structure is very conservative through evolution but there are enough changes to tell different species apart by their eggshell microstructure. Scutes may play a role in calcium storage for eggshell formation. 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, and that helps them pierce out of the shell. Hearing the calls, the 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. She would then introduce her hatchlings to the water and even feed them. The mother would then take care of her young for over a year before the next mating season. In the absence of the mother crocodile, the father would act in her place to take care of the young. However, even with a sophisticated parental nurturing, young crocodiles have a very high mortality rate due to their vulnerability to predation. A group of hatchlings is called a pod or crèche and may be protected for months. Cognition Crocodiles possess some advanced cognitive abilities. They can observe and use patterns of prey behaviour, such as when prey come to the river to drink at the same time each day. Vladimir Dinets of the University of Tennessee, observed that crocodiles use twigs as bait for birds looking for nesting material. They place sticks on their snouts and partly submerge themselves. When the birds swooped in to get the sticks, the crocodiles then catch the birds. Crocodiles only do this in spring nesting seasons of the birds, when there is high demand for sticks to be used for building nests. Vladimir also discovered other similar observations from various scientists, some dating back to the 19th century. Aside from using sticks, crocodiles are also capable of cooperative hunting. Large numbers of crocodiles swim in circles to trap fish and take turns snatching them. In hunting larger prey, crocodiles swarm in, with one holding the prey down as the others rip it apart.