It is well known, however, that among many different species, a resident pair is especially likely to copulate just after an intrusion into the pair's territory. It appears-although it is not yet proven-that this response to intruders is initiated by the in-pair male. This would make biological sense, since intruding males in such cases are unlikely to have simply dropped by to borrow a cup of sugar or to sell Girl Scout cookies. Why should the female go along with this, agreeing to copulate with her mate just because some other male has recently been hanging around? Perhaps it is simply less costly for her to acquiesce than to resist her mate's importunities. Or it may . pay her to permit copulations-especially when her fidelity is in question- so as to persuade her mate of his paternity, in order to assure his assistance in rearing the young. (As we shall see, the loss of paternal assistance is a major potential cost to females of EPCs, if discovered by the in-pair male.)
In any event, there are few things as sexually stimulating to socially monogamous animals as the possibility that the mated female might have had an EPC. Among orioles, males will copulate with their mates immediately after hearing a recorded song from another oriole. One might say that in the oriole world, the song of a male is sexually arousing to other males; the evolutionary significance of this would be that the nearby song of another male suggests that someone might have recently copulated with the in-pair female. If so, paired males who are "turned on" by this telltale signal and who introduce their sperm as quickly as possible to compete with the extra-pair male would be favored by natural selection over those paired males who were indifferent to such cues.
—David P. Barash, The Myth of Monogamy, p. 38