Members of an individualís own species are also part of its biotic environment. Social interactions are especially important. Selection resulting from interactions associated with getting a mate is called sexual selection. One result of sexual selection is sexual dimorphism, which means males and females look different. In some species, males fight with each other for the chance to mate. In such cases, males are often larger than females and have some feature used for fighting, such as horns. In many birds the female chooses a mate and sexual dimorphism consists of the male being very brightly colored to attract the females.