How common is sex between different species of animals?

The original question as it was written:

How common is sex between different species of animals?

Specially interested in sex between species which cannot produce hybrid offspring together.

So, this another question posted on Quora. I understand that the poster stressed the point that the two species weren't able to produce hybrids together because their intention was to find how common was sex for the sake of sex in the case of different animals.

And I opened my big mouth, I mean I started to type this answer looking for being clear and open minded. Enjoy...

____________________________________________________________________

No, I am not telling that there is an evolutionary reason to find goats sexy, and I would not sympathise with anybody that would use this as an excuse. I am just talking about facts recorded in the wild, but never, never will admit they are an excuse for human behaviour.

But, interspecific intercourse is more common than I used to think.

I thought this event took place in human designed enviroments (planned interbreeding as in mules, and sex for the sake of having fun among animals not very stressed by the need to runaway predators, this was my mental scenario), but not...animals, stubborn creatures always find a way to cheat the rules. Why?, because they are stubborn rulebreaker maniacs, this is my current hypothesis. Anybody wants to defy it?...
The best clue that some interspecific sex has been there, are hybrids. And there some weird examples of hybrids in the wild:

• Natural hybrids populations of lemurs in Magadascar: DNA analysis indicates that the population  at Andringitra is a hybrid population between the two species. -> Source: http://messybeast.com/genetics/h...

• Hybrid wild dolphins (yes, dolphins are horny, but it would be more logical that they could only produce offspring in captivity, but...again stubbornnes won logic): In 1933, three strange dolphins were  beached off the Irish coast that were apparent hybrids of Risso's Dolphin and Bottlenose Dolphin, and this hybridation was later proven to be possible in captivity.There are also reports of possible hybrids of bllue Whales, Fin Whales and Humpback Whales in the wild. Source->http://messybeast.com/genetics/h...

(If you feel more curiosity about hybridization, there are hundreds of them compiled in Sarah Hartwell´s site - http://messybeast.com/genetics/h... it is amazing, and she is very strict with the material she uses, I recomend it).


But the conclusion is that yes, different species mate in the wild.

• Most of the time we can not keep man's influence off the equation, loss of habitat has impossed many animals an added difficulty to fin a mate, and has push together species that used to be isolated.  This is explained under the epigraph "Introduced species and habitat fragmentation" in http://en.wikipedia.org/wiki/Hyb....

• But this does not explain all cases of interspecific matings in the wild. There are some extremely bizarre cases that are not even taken into account in this article in the wiki (a suggestion, include them, they are amazing, or sexy, or whatever, but deserve a place in this wiki article): Sexual parasites, forced hybrids. Sexual parasites offer unique insights into asexual and sexual  reproduction. They mate with a ‘host’ whose genetic contribution is  discarded either immediately (in androgenesis or gynogenesis) or after a  delay of one generation (in hybridogenesis). The discarded genome can  be maternal or paternal, implying that not only females but also males  can reproduce asexually. The resulting lineages are often older than  ecological or evolutionary theory predicts.-> http://www.sciencedirect.com/sci....

And all this talking only about hybrids (I would say that in the last case not the type of hybrid we should expect, only clever animals hacking sex for reproducing breaking the laws of sexual reproduction. Stubbornnes and animals again).

______________

Well, reading my answer again I realize that I have talked about hybrids once and again. Well, take into account that for needing that two species different generate an third hybrid species (the case I've talked about beneath these lines), we need first two different species. On many ocassions these species mate because they were iniatilly one species that diverged into two, and they were different enough to produce a third one together. See this interesting article here: http://arstechnica.com/science/2016/10/what-happens-when-you-end-up-with-an-extra-genome/.

 

But on many ocassions animals mate with animals of different species just because it happens:

Some animals opportunistically mate with individuals of another species. This is more commonly observed in domesticated species and animals in captivity, possibly because captivity is associated with a decrease in aggression and an increase in sexual receptivity.^[52] Nevertheless, animals in the wild have been observed to attempt sexual activity with other species.^[53] It is mostly documented among species that belong to the same genus, but sometimes occurs between species of distant taxa.^[54] Alfred Kinsey cites reports of sexual activity involving a female eland with an ostrich, a male dog with a chicken, a male monkey with a snake, and a female chimpanzee with a cat.^[55]
A 2008 review of the literature found 44 species pairs that had been observed attempting interspecies mating, and 46 species pairs that had completed interspecies matings, not counting cases that had resulted in hybridization. Most were known from laboratory experiments, but field observations had also been made.^[54] It may result in fitness loss because of the waste of time, energy, and nutrients.^[54]
Male sea otters have been observed forcibly copulating with seals,^[56][57] and male seals have been observed forcibly copulating with penguins.^[58] Male grasshoppers of the species Tetrix ceperoi often mount other species of either sex and even flies, but are normally repelled by the larger females.^[54] Males of the spider mite species Panonychus citri copulate with female Panonychus mori mites almost as often as with their own species, even though it does not result in reproduction.^[54]
 (https://en.wikipedia.org/wiki/Non-reproductive_sexual_behavior_in_animals#Cross-species_sex).