Alpaca Sex Is Even Weirder Than You Think

Just in case you’d like to know: Alpaca sex is weird, biologically speaking. In a new paper this month, scientists have presented evidence that male alpacas impregnate their partners by thrusting their penises all the way up inside the female’s uterus—a tactic not documented with any other mammal species to date.

The research was led by scientists from Mount Holyoke College in Massachusetts. They also worked with a researcher from the North American Camelid Studies Program, part of the larger Nuñoa Project.

Camelids are a group of large, four-legged, even-toed mammals that include camels, llamas and alpacas, while the project is named after a district in Peru with some of the highest densities of alpacas in the world. Alpacas are an important livestock to farmers in Peru and surrounding countries, as well as a few other parts of the world. They’re primarily raised for their soft fleece, but can be used as meat, too.

Insemination for mammals typically happens deep in the vagina, with the male’s sperm having to make its way through the cervix and then the uterus to reach the female’s egg in the oviducts (in humans, this location is better known as the fallopian tubes). The penis of some mammals, such as dogs and horses, can get very close to the cervix and basically shoot sperm straight through into the uterus. Even in these animals, however, the penis seems to only penetrate the vagina.

But alpaca breeders have noticed that male alpacas thrust very far into their female partners during mating. Males are also known to have very long and thin penises that end with a hard cartilage tip, almost like a spear. And these two observations have led many people to conclude that insemination for these mammals must happen directly in the uterus. The authors of this new study, published last month in the journal PLOS-One, say that no clear evidence for this claim has been presented, though, until now.

The researchers were able to dissect female alpacas soon after they had sex with males (the alpacas were being raised for meat), either one hour or 24 hours following the deed. These females were compared to those who hadn’t been recently mounted by males. Even within an hour, the team was able to find sperm inside the oviducts of the females. Compared to unmounted alpacas, the researchers also found clear signs of penile penetration (bleeding) throughout the females’ reproductive tract, reaching all the way into the uterus.

“Using female alpacas culled for meat, we determined that the alpaca penis penetrates to the very tips of the uterine horns, abrading the tract and breaking fine blood vessels,” the authors wrote. “The entire female reproductive tract interacts with the penis, functioning like a vagina.”

Though the bleeding found in female alpacas after sex could suggest a less than friendly mating process, the authors note that there is no evidence of sexual conflict in the species. Among other things, female alpacas have to willingly cooperate and adopt a prone position (laying stomach side down) for penetration and mating to occur, sometimes for up to an hour. Instead, the researchers believe that the bleeding caused by alpaca sex might actually help females become pregnant, perhaps by triggering a short-term inflammatory response that helps the fertilized egg stick to the uterine wall. So as weird as alpaca sex sounds, it seems to work just fine for them, the authors say.

“Thus a peculiar copulatory mode in alpaca may improve the odds of successful fertilization and pregnancy,” they wrote.

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