sentences:  In males, a number of theories are proposed to explain extra-pair copulations.One such hypothesis is that males maximise their reproductive success by copulating with as many females as possible outside of a pair bond relationship because their parental investment is lower, meaning they can copulate and leave the female with minimum risk to themselves.Females, on the other hand, have to invest a lot more in their offspring; extra-pair copulations produce a greater cost because they put the resources that their mate can offer at risk by copulating outside the relationship.