We are also discounting cases in which animals do engage in sex even though reproduction is an impossibility, because we claim there are other “purposes” (of which the animals themselves are unaware) at play.
(For example, the females of some species of birds will invite males to mate with them even after they have laid their eggs, but we ascribe a purpose to this behavior: this is a biological “trick” to fool males into caring for hatchlings they didn’t father.) We also employ subjective terms such as “willingly” and “regularly” in claiming that bonobos and dolphins are the only other animals who “willingly (and regularly) engage in sex with each other even when there is no possibility that offspring will be produced as a result,” and even then it may be the case that these species have some other “purpose” for doing so that we haven’t yet discovered.
International 304, 304A, 345, 392 Model Engines Service Manual, 68 pgs.
(This manual is redundant if you purchase a complete service manual below.
(Reproduction may be impossible when the female is not yet sexually mature, is not ovulating, is already pregnant, is still caring for young from a previous pregnancy, or is past reproductive age.) Therefore, in most species females produce signs detectable by males that indicate when they are ovulating and fertilization is possible.