Epreuve orale de DNL Sciences Session de juin Baccalauréat 2016 Durée de préparation : 20 minutes Durée de l'épreuve : 20 minutes (10 minutes de présentation orale ; 10 minutes d'entretien)

Theme : EVOLUTION Question: Using the document, Explain why elephant tusks size has halved since the mid-19 th century.

Most predators target the young or the weak. We are different, targeting the biggest and best! Sources: https://www.newscientist.com/round-up/unnatural_selection

Why elephants are not so long in the tusk ? Demand for ivory has seen Africa’s elephant population more than halved since the late 1970s. By Richard Gray, Science Correspondent 2:01AM GMT 20 Jan 2008 Elephants are evolving smaller tusks due to pressure from hunting and poaching for ivory, according to conservation experts. The average tusk size of African elephants has halved since the mid-19th century. A similar effect has been spotted in the Asian elephant population in India. Researchers say it is an example of Darwinism in action, caused by the mass slaughter of dominant male elephants - but whereas evolution normally takes place over thousands of years, these changes have occurred within 150 years. Zoologists at Oxford University fear that poaching and hunting of the largest male elephants, which also have the largest tusks, has changed the natural breeding behaviour of these animals. Their research has shown that the hunting of these large males for their ivory allows smaller males with shorter tusks to produce more calves. Over time the average tusk size decreases. Iain Douglas Hamilton, from the conservation charity Save the Elephants and who was one of the authors of the study, said: "What appears to be the case is that average tusk sizes have decreased greatly since the mid-19th century. The data comes from the trade statistics and from records of hunters around Africa who find that large trophies are very much harder to find. "While some of this may be due to an absence of older animals, it is possible there has been a genetic selection pressure against large tusk size that outweighs their usefulness in contests with other males in winning females." African elephants are larger and have longer tusks than their Asian relations. In males, tusks are often used to intimidate other males, and sometimes in fighting for mates. Those with the biggest tusks are usually the most successful. But the ivory of large African elephants is particularly prized by hunters and traders for its quality and this has seen hundreds of thousands of animals killed. It is estimated that there were 1.2 million African elephants in the late 1970s, but there are now fewer than 500,000. Sources: http://www.telegraph.co.uk/news/science/science-news/3322455/Why-elephants-are-not-so-long-in-the-tusk.html