that seems really surprising to me that it's common

Don't think it is common. I wrote "not uncommon". Living in a college town as I used to I would often hear about it and when I started reaching out and learning different things I became curious. On the former AB there were a lot of posts and discussion about it.

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Not uncommon.

Usually starts with teenage girls who are close to and spend a lot of time with their dogs. They see their dogs erections and making sexual motions and smelling them - dogs quite naturally come up to us and stick their noses in our crotch - and they take it from there. So becomes solitary sexual play but not masturbation. They sometimes share it with friends and bring them in on it. Girl play which lasts only as long as they stay interested or with a particular dog. Never heard of anyone becoming fixated on their dogs - because although they can lavish affection on their dogs such a relationship would never be totally fulfilling from a human sexual point of view. But some do become attached to the pleasure they derive from it which of course will lead them to seek it more often. College age and young women sometimes us sex with dogs to bond - Perhaps similar to men sharing an escort - and I know of college social groups and sororities where it was a shared activity. Also young couples with a dog sometimes become curious and experiment. As do swingers of various ages, sometimes paying the dog's owner (often by having sex with him) who supervises. (new paragraph) If we allow ourselves to become smelly dogs become interested and if we get down on hands and knees they will instinctively mount and penetrate us. During which they go very fast and grow in size and swell up. Their penises are warm and slender so capable at full extension of penetrating our cervix so the fucking part although only lasting a few minutes can get us up there fast and bring us to orgasm if we are more sensitive, especially when the swollen part of their penis lodges against and stimulates our G-spot. Providing of course that they don't lose interest and pull out before. But their swelling fixes them up us for the ejaculation phase which can last sometimes as much as twenty minutes during which they stay in place and if we push back against them we can orgasm one or two or three times while they ejaculate which they do very copiously which stimulates us as well. Most women who enjoy canine sex more regularly put down newspapers beforehand to absorb the ejaculate which leaks out during and which comes out quite torrentially once the dog detumesces and breaks the connection. Before there is mostly no breaking it without some effort and discomfort to us. Which is why many of us lean on a chair during for greater comfort. (new paragraph) Dog's behavior is completely instinctual so we hitch a ride for ourselves on that behavior if we engage in sex with dogs. I have heard that some men engage in it as well, including anal penetration but perhaps because men do not share such experiences the way we often do I have never heard from any of them.

surprised more men engage in it

That surprised me, too, but it makes sense. Given the nature of the agricultural labor force, men do more work with farm animals than women. That accounts for some of the difference. More availability means more incidence, on average. (Remember, we are talking worldwide, here.) Further, men can be both submissive, meaning that they can be penetrated, and they can be dominant, meaning that they do the penetrating of a female animal. That therefore accounts, statistically, for some of the difference. One other interesting stat - men who engage in penetration with animals tend to have a somewhat higher incidence of penile cancer than the average man. Correlation is not causation, so there may be other explanations of the cause, but it is an interesting stat.

did that study say what age it starts or what first triggers it ?

Well, not so far as I am aware. (I can ask my dad the next time I talk to him. He might know.) Generally, it is the lack of available sex partners that usually triggers a desire for sex with animals. Human males are instinctively driven by evolution to seek out multiple sex partners. (It's how the species has survived for a million years or so.) When they cannot, the psychological pressures can build and an animal becomes an alternative. As that habit becomes more "normalized" in a person's behavior, they adapt to it. (A similar dynamic tends to develop in all boys boarding schools. Studies conducted as early as the 19th century - when methodology was admittedly not as far developed as it is now - showed that there was a higher incidence of homosexual sex in such schools because young boys who were moving into their sexual prime could not find female companionship and their close guy friends would end up being a viable alternative. That is why sports became such a massive obsession in British boarding schools. It was seen as a distraction by Victorian society, which was appalled at the idea of boys engaging in homosexuality.) For women, the desire is similar but different. Our female ancestors could typically only carry and nurture one child at a time, so they tended to seek mates who could protect them. (This is a whole branch of evolutionary psychology. I will spare you the details.) Anyhow, there is, in women, a desire to become impregnated and - again - when no eligible human males are available, animals become a viable option, albeit to sate the desire to mate, not to actually become impregnated. (This is also another part of the explanation of the lower incidence of bestiality with women. It serves only part of the female sexual instinct.) As far as age, as I said, I can ask what might dad might know the next time I talk with him - we don't live in the same state - but my hunch is that it would tend to be as males and females near their peak sexual development. (About 16 to 24 for men. About 14 to 22 for women. )

Thanks. I don't know that there will be too much to add, but next time I talk to my dad - he's a doctor living in Michigan and I work in politics in DC, so we don't chat every day - I'll raise the issue and see what he says. If you have any other questions related to the topic, let me know.

See below.