大象传媒

Once a year in autumn, American elk gather together to breed. It is called the rut and is a spectacle where stags compete for breeding rights. But this rut is played out in a most unusual location: the manicured lawns of Yellowstone National Park's Mammoth area. Here, humans get a ringside seat. Each stag has two aims: to impress the females and to frighten off rivals. Although the males are much bigger and more dominant, it is the females who really run the show. Like females the world over, they will be left with the babies so they are very choosy about who they mate with. Only the real studs - those with the most impressive antlers and fighting ability - will do. This forces the males to prove themselves by clashing antlers. It is a knock-out contest where only the winners get the right to breed. Over generations the battles get harder and the stags get stronger. Female choice can shape the bodies and behaviour of their mates, so they are driving evolution. The males get so charged up when they are fighting over females that they will perceive just about anything as a threat.