Co-operative strategies to enhance breeding performance are employed intrasexually as well as intersexually. For example, males may co-operate with other males to maximize their access to females. In contrast, females may collaborate with other females to maximize the quantity or quality of offspring they produce. When co-operation with the other sex increases the reproductive success of each individual, they may work together to rear offspring. A male or female uses individuals of the other sex like any other object in their environment. Intersexually, we most often see co-operation when biparental care of offspring is required. Despite this collaboration between males and females, each is trying to optimize their lifetime fitness. As a result, conflict between the two sexes often arises, even during periods of co-operation.