Human
In the human female, her fertile period in the oestrous cycle is hidden, and mating can take place at any time. That is quite unusual. In mammals generally the fertile period is very noticeable. Mating only takes place when the female signals her fertility. Think about cats, for example. The human cycle is unusual, and it is thought that there is a reason. Humans band together in tribes which have many people. It helps the tribe if the father of a child is not known for certainty. Men live together and work together in much larger groups than do chimpanzees (our nearest living relatives). They have a collective interest in the tribe. It is thought that the human mating system helps this.