After reading the question on inbreeding (Could fictional species benefit from limited inbreeding?), I started to wonder if there could be any reasons why an species (intelligent or otherwise) would benefit from intercourse with a non-compatible species (aka bestiality). This could be for any reason, be it social, economic, biological, political,... Any reason.
The obvious reasons are:
- Social bonding (for example two intelligent species may use such relations to help bridge gaps between themselves even with no offspring possible)
- Establishment of dominance between species
- Some other benefit for one or other species.
You may not be compatible DNA wise but you may still supply something useful for the process. For example intercourse with a member of species A may help increase the fertility of species B even though it doesn't directly lead to offspring.
You can have one species plant an egg during the intercourse in the partner who will then raise or be eaten by the offspring (think Alien breeding mechanics).
Killing the host is called Parasitoid behavior but the act of implanting can be viewed as intercourse and is seen in current insects. Though the egg is usually fertilized beforehand though external fertilization is not impossible.
Expanding my comment into an answer:
Animals and in particular humans have sex for many reasons other than procreation and essentially all of these may apply to your question:
- Recreation, fun and sports
- Satisfying psychological needs
- Financial and other materialistic advantages (prostitution)
- Controlling a partner dependent on sex
- Acquiring status
- Traumatising one participant or destroying his status, e.g., for psychological warfare
- Ritualistic reasons, including the establishment of trust or partnership, religious rituals, coming-of-age rituals
- Communication – something which can be drastically extended for fictional beings
Some possible fictional expansions of these concepts:
- While the human impulse for sex is psychological, it could be physiological for another species. With other words, members of this species need to have sex in order to avoid physiological damage or death. As having an urge for sex is a strong evolutionary advantage, this is still plausible. (Of course, there is masturbation, but that may be religously forbidden to said species or actually technically difficult.)
- Relatedly, exchanging bodily fluids may be advantageous for reasons other than procreation. To give you some ideas, human sperm also contains nutrients and hormones.
For intelligent species, the benefits are exactly the same as in intercourse with birth control - no offspring are produced, but people still seem to get something out of it. Even for non-intelligent creatures, the usual non-procreative reasons still apply (pleasure, dominance, social cohesion and conflict resolution, prostitution, etc.).
As far as the biological factors, this rather depends on your definition of 'no DNA compatibility'.
A horse and a donkey are not compatible in the sense that they can successfully produce fertile offspring. Though they even have a different number of chromosomes, they can produce sterile hybrids (though a few fertile female hybrid have been born, it is astonishingly rare, and no fertile males have been recorded), but they can still produce mules.
Getting a little more unusual, depending on what exactly is involved in intercourse, it could involve sharing anything aside from genetic code - intentionally swapping bacterial colonies, administering particular hormones, or providing a nutritional supplement from the ejaculate.
I feel like people have failed to point out some of the interesting complexity that occurs in evolution; looking instead only at humans sexuality.
In fact there are species that benefit from 'sex' without being genetically compatible, in fact species that would not exist without having 'sex' with other species; though admittedly most of these species would not be expected to be sapient.
Send in the lizard clones
I'm a little too lazy to link to each example I'll make, but lets start by talking about probably the most sited example whiptail lizards. This is a species of all female lizards that still have sex. In their case it's sex with the same species (usually...), but since their all female there is no DNA recombination going on; yet sex is still very important for them! Females who are mounted by other females are more likely to lay eggs and more likely to lay fertile eggs, the mounting makes them better able to reproduce, even though no DNA is exchanged.
There are similar groups of lizards (not just lizards, but seems the majority are lizards, due to their DNA structure) that are all female but mate with males of other lizard species. The female will then conceive a female that is a 'clone' of the mother (I put clone in quotes because there are a few ways to produce that clone, depending on approach the DNA of the male may exist within the child but not affect the child's phenotype or be carried on to it's children; making it still effectively a clone). These females may not be able to produce young at all without mating with the males.
The reason for these lizards usually comes down to the fact that evolution isn't guided, and can take some strange turns. If your excuse the shorthand of personifying evolution you could say that these lizards started out sexual but then 'decided' to become asexual later so they could spread their genetics twice as fast without wasting time with males. However, they still have a sexual reproductive system that evolve for tens of thousands of years to expect sex, and that is hard to 'get over'. So while the lizards want to stick to making clones they still need sex to help reproduce because they haven't gotten around the fact that they were evolved for sex. In essence the whiptail lizards haven't gotten around to fully evolving away sex and so in the meantime use mounting to simulate sex to help ensure they have young.
Of course the whiptail lizards are seen mostly as an evolutionary dead end, odds are they will die out, likely without any genetic relatives to take their place, because giving up sexual reproduction is a short term boon at the cost of any long term adaptivity which will kill the species in the long run.
However, the other lizards, the ones that mate with males of related species, are a bit more successful. There are actually a multitude of these strategies, which vary mostly in rather or not they keep the male's genetics at all or simply use mating to trigger development of eggs, but these lizards have lasted quite a bit longer and better then we would necessarily expect from asexual species. It's believed this is because they aren't entirely clonal.
You see genetics is, again, kind of random at times. It's believed every now and then the lucky males with the offer of free sex manage to be a bit more successful then expected and their DNA manages to make it's way into the resulting child's genome, and the genome of it's children, despite the mother's 'intent' to not use it. These genetic 'accidents' are rare, but common enough to allow some slight genetic influx and thus variation amongst the otherwise clonal lizards. The lizards benefit from still using males by getting some rare genetic variation and thus stick to the seemingly 'worthless' sex.
So yes, species absolutely benefit from sex with otherwise non-compatible species and have been doing it for tens of thousands of years!!
Of course none of the above lizards will achieve sapience. Without sexual reproduction and new genetics they don't 'evolve' much at all, and the odds of a species like this managing sapience is slim to none. So the above example will meet your requirements, but may be boring to write about. lets move on...
Accidental sex is fun
Of course there is even simpler explanations, mating 'accidents', which can be quite useful. Many species will mate with another species in hopes of reproducing without realizing it won't succeed. (and I realize the use of word 'species' here is going to get complicated by the varying definitions of that word, here I'm including closely related but not necessarily genetically compatible animals). I refer to this as an accident, but animals aren't really thinking about children when they have sex, they are driven by sexual desire and to them they mate because it was enjoyable and scratched an itch. They see it as pleasurable interaction, not as an attempt to produce a child that is doomed to fail. They are both happy with this mating.
I also should not refer to this as an 'accident' because it is not necessarily genetically undesirable either. Again, genetics is complex and seemingly impossible things can happen at times. It's believed that many species have DNA in them from otherwise genetically incompatible species. This is because even if a species is usually not compatible on rare occasions they may manage to produce a fertile child, which may then go on to mate with one of the parent species. This can result in DNA from the 'donor' species to be introduced into the gene pool whatever species the child ended up mating with.
This is a huge advantage actually. Genetic variation allows for evolution, but usually genes can only vary through rare mutations. If, however, a rare fertile hybrid is produced and manages to mage with one of it's parent species it will introduce a significant amount of novel DNA into that species. Much of that DNA may very well prove mal-adaptive an be quickly cut out of the gene pool by good old survival of the fittest, but some of that DNA may prove advantageous and manage to be passed on. Thus these rare fertile offspring result in benefiting the overall DNA of the species it mates with by allowing novel beneficial genetics to spread into the gene pool much faster then mutation would allow.
Don't have a cattalo man!
In fact I feel like pointing out the most obvious example of this, modern cows, or more exactly beafalo. Crossing cows with Bison was not unheard of, due to it's producing cattalo that survived in certain climates better. However, to quote Wikipedia
The female offspring proved fertile, but rarely so for the males. Although the cattalo performed well, the mating problems meant the breeder had to maintain a herd of wild and difficult-to-handle bison cows
This was solved when, by random mutation chance, a fertile bull->bison male hybrid was produced. This does not usually happen, it was very rare fluke of genetics. However, this bull was a godsend, and was quickly studded out to as many females as possible, producing the beefaloo. An entire new species due to a rare successful mating of 'incompatible' species; it now makes up a large part of meet you buy in the store.
The bull mated a bison purely for satisfaction, and the bison female was only interested in satisfying her own itch. Neither side thought even about the child they may produce, much less their species as a whole. Yet this 'fun' but 'pointless' mating produced an entire new species that benefited from their parent's 'fun'.
To further generalize the point of the two above examples I would say that mating between supposedly incompatible species may prove adaptive if there is even the slightest chance of genetic flow. So lets come up with another example of a more sapient species with 'non-reproductive' sex as an adaptive advantage using some theoretical primates based off of our closest genetic relatives.
In on corner we have our pseudo-chimp, I believe chips are well known and need little introduction. In the other corner we have our pseudo-bonobo. You may not know about Bonobo, which is a shame because they are fascinating species from evolutionary standpoint, and most important for this example make humans look like prudes when it comes to non-reproductive sex. For them sex is social and engaged in for reasons such as bonding, building female alliances (women rule in bonobo land!), resolving conflict peacefully, or as a trade off to get a nice piece of fruit.
Now in real life bonobo and chip are quite similar genetically, they are separate species only because they are separated by the Nile, making it difficult for them to meet and reproduce. They are fertile, a male bonobo in a zoo fathered multiple hybrids on female chimps; so they are not non-reproductive. However, for our example lets imagine our pseudo-primates are not as genetically compatible, they can meet up in the forest but usually are not capable of producing young.
Now our male chimps may look at that sexy bonobo tail (bonobos have actual tails!, get your mind out of the gutter :P ) and may think it's pretty attractive. Female bonobo's are pretty lax with their matings so it's not too hard to convince them to mate with our males. Perhaps when chimps and bonobo meet up the females offer up sex as their usual conflict avoidance technique and the otherwise far more aggressive chimp males decide that sex is more tempting then attacking, or even killing, the bonobos (which is quite possible, chimps can be violent and territorial).
So females offer sex because they don't want a fight and that's a good way to avoid it, the males take sex because...their males; and as it happens they are distracted enough that violence doesn't happen. Conflict is avoided and everyone walks away happy; this already would be a benefit if it worked (though frankly chimp aggression would still be a risk to the bonobos).
However, what if the chimps and bonobos weren't entirely non-reproductive. Once in a great while the females manage to produce young with the chimps. Now the females, on average, will benefit from increased genetic diversity in their tribe from these very uncommon matings. The males will have successfully spread their genetics; though unless those genetics filter back into the chimp gene-pool the males doesn't really 'care' from an evolutionary stand point. Perhaps bonobo-chimp hybrids females are more likely to join up with chimp tribes then bonobo tribes when they inevitably migrate from their home tribe; so the hybrid genetics enter chimps as well. Even rare successful matings like this could work to benefit both species.
Or lets go from the other way. Maybe chimp females run into bonobo tribes often. Chimps generally are less open to mating so the female usually isn't going to be interested in mating with the sexually excited bonobo males. However, bonobos sometimes offer 'bribes' to females for mating (only small things), so perhaps female lone female chimps have realized that meeting up with bonobos could mean protection and food for a little while, and all they have to do is allow matings that won't lead to reproduction so it doesn't 'hurt' much. This would require a specific evolutionary adaptation for female chimps to realize this, but once realized it's possible lone females may even join up with bonobo tribes briefly while looking for chimps to live with (not that many loan females will be met up with), or the female may offer herself simply for small food bribes.
The male bonobo do this because they have a strong drive to mate with females as often as possible and they don't 'realize' the female is not compatible, she's sex and bonobo males like sex. The female does this for brief protection or freedom to accept bonobo's food, she's likely less interested in sex, but it's a simple sacrifice at little cost for what she needs. of course if the chimp female occasionally does produce a hybrid this can encourage this behavior.
These sort of interactions could result in chip and bonobo allowing matings somewhat often and benefiting from them on average, in the short term by allowing conflict to be mediated safely and pleasure earned, in the long run through spread of genetics. Over time it's even possible that chimps could take on more of the sex-to-mediate conflict approach when interacting with bonobos when they benefit from it.
Now lets say bonobos eventually reach full sapience. They are quite open to matings with chimps and have benefited in it from the past, and their already sexually promiscuous. This could be a standard part of their culture and accepted. It's possible even sapient bonobos won't recognize the occasional chimp hybrid as such for some time, their many matings mean the mother never knows the genetic father and if they hybrid doesn't look too unusual no reason to presume a hybrid instead of just an odd looking normal bonobo.
Who knows, maybe the chimps reach sapience, but domesticate the bonobo early on, as we did wolves. In this case domestication of bonobo would likely result in some level of sexual interaction with them, since so much of bonobo society depends on sexual interaction. The chimps could evolve to a point where 'bestiality' with bonobos is no stranger then enjoying to play tag or wrestle with your dog; that's just how you play with a domesticated bonobo. Bonobo benefit from sex because it's how they learn to work with chimps, it's needed to be domesticated and they benefit from domestication; and chimps benefit from having sex with bonobo because it helps them to train and keep the loyalty of a bonobo.
There are many more species involved than meet the eye at the macroscopic level of perception. Additionally, all life on earth shares a common origin and therefore there is always some level of compatibility of genes: this is how viruses work.