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Intra -sexual selection
Intra-sexual selection is where one favours the ability of one sex to compete directly with one another for fertilizations. This is usually seen with male rivalry, and this situation is usually solved by fighting.
Red deer ( Cervus elaphus ) ales grow antlers in autumn times to defend their harems from rival males during the mating season. The females have no choice over who to mate with but they do gain because they are looked after by their male, if he can keep his harem then the females are likely to have strong male babies and have more reproductive success.
When the males initiate the fight they start roaring at each other and the roars get louder and faster. If the challenger can not roar as loud or as fast as the other stag, then he would usually leave and not continue to pursue the harem. However, this doesn't always work. In the event of this happening the rival males will size each other up, by walking around each other comparing their physical qualities with their own; for example, body size and size of antlers.
The deers will try and not to fight with each other as the risk of getting hurt is high and the owner of the harem will not want to fight because if he gets injured he is likely to lose some females from his harem as opportunistic males will steal them. A deer will not fight if the chances of him losing are high, so if for example the challenging deer appears to have an advantage the defending one will back down and leave. This is an evolutionary high point (although it may not seem like it for the losing deer) as it means that the deers have become smarter and as a result live longer and in theory be able to produce more offspring, passing their genes for assessment on.
