In a recent study, a team of researchers presented evidence that male alpacas impregnate their partners by pushing their penises into the female's uterus - a technique not documented in any other mammal species to date.

The results of the study, conducted by scientists from Mount Holyoke College in Massachusetts and the North American Camelid Studies Program, were recently published in PLOS One.

Camelids are a group of large, four-legged, even-toed mammals that include camels, llamas and alpacas.

Intra-horn insemination in the alpaca Vicugna pacos: Copulatory wounding and deep sperm deposition
Alpacas (Vicugna pacos) are reported to be the rare mammal in which the penis enters the uterus in mating. To date, however, only circumstantial evidence supports this assertion. 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. All female alpacas sacrificed one hour or 24 hours after mating showed conspicuous bleeding in the epithelium of some region of their reproductive tract, including the hymen, cervix and the tips of each uterine horn, but typically not in the vagina. Unmated females showed no evidence of conspicuous bleeding. Histological examination of mated females revealed widespread abrasion of the cervical and endometrial epithelium, injuries absent in unmated females. Within one hour of mating, sperm were already present in the oviduct. The male alpaca’s cartilaginous penis tip with a hardened urethral process is likely responsible for the copulatory abrasion. The entire female reproductive tract interacts with the penis, functioning like a vagina. Alpacas are induced ovulators, and wounding may hasten delivery of the seminal ovulation-inducing factor beta-NGF into the female’s blood stream. There is no evidence of sexual conflict in copulation in alpaca, and thus wounding may also be one of a variety of mechanisms devised by mammals to induce a beneficial, short-term inflammatory response that stimulates blastocyst implantation, the uterine remodeling associated with placental development, and thus the success of early pregnancy.
PLOS ONEPatricia L. R. Brennan ,

Alpacas are an important livestock animal for farmers in Peru and neighboring countries, as well as in some other parts of the world. They are bred mainly for their soft fleece, but also for their meat.

Insemination in mammals normally takes place deep inside the vagina, with the male's sperm having to pass through the cervix and then the uterus to reach the female's egg in the oviducts. In humans, this place 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 the sperm directly into the uterus. However, even in these animals, the penis only seems to penetrate the vagina.

But alpaca breeders have noticed that alpaca males penetrate their partners a lot during mating. Males are also known to have very long, thin penises that end with a hard cartilage tip, almost like a spear.

These two observations have led many people to conclude that insemination in these mammals must take place directly in the uterus, but until now, notes Gizmodo, no clear proof of this claim had been presented.

The authors of the study were able to dissect female alpacas, which were being raised for meat, immediately after they had had sexual intercourse with males - one to 24 hours after the act. These females were compared with those that had not recently been mounted by males.

Even within an hour, the team managed to find sperm inside the oviducts of the females. Compared to the unmounted alpacas, the researchers also found clear signs of penile penetration (hemorrhaging) throughout the females' reproductive tract, reaching as far as the uterus.

"Using female alpacas slaughtered for meat, we determined that the alpaca's penis penetrates all the way to the tips of the uterine horns, eroding the tract and rupturing fine blood vessels," wrote the authors. "The entire female reproductive tract interacts with the penis, functioning like a vagina."

Although the bleeding found in alpacas after sex may suggest a less friendly mating process, there is no evidence of sexual conflict in the species.

Among other things, female alpacas have to cooperate voluntarily and adopt a prone position for penetration and mating to take place, sometimes for an hour.

Instead, the researchers believe that the bleeding caused by alpaca sex may actually help the females become pregnant, perhaps by triggering a short-term inflammatory response that helps the fertilized egg adhere to the uterine wall.

So, as strange as alpaca sex may seem, it apparently works very well for them, say the study's authors.