When the manatee (Trichechus manatus) appeared between 5 and 24 million years ago, conditions favoured freshwater plants in South American coastal rivers but these plants contained a lot of silica. This wore down the manatee's teeth so it adapted by continuously replacing its molars, old ones being shed from the front of the jaw as new ones sprang up at the back. The manatee, which prefers living in large, shallow estuaries, now eats mostly seagrass, consuming up to 9 per cent of its body weight each day. Despite being protected this slow, docile and friendly sea cow is still hunted for its meat in some countries and is often killed or injured in collisions with boats. Its numbers are declining and the most significant populations are now in Florida where habitat degradation and human pressure threaten extinction. Conservation programmes have had limited success.
