Another week, another round of viral animal stories that need a thorough fact-checking.

Today, a photo has been doing the rounds of a lion getting his testicles bitten by a lioness. This one has been spreading around the Internet in some form since it was first taken in January, due to the amazing expression being pulled by the adult male.

The tweet has been shared 77,000 times at the time of writing, with people making jokes along the NSFW lines. 

The images are undeniably funny, but is the caption in any way true? As usual with these viral photos, not quite. 

Lions do have somewhat intense mating behaviors. All cats, including lions, have barbed penises (or "penile spines") that rake the walls of the female's vagina as the animal withdraws at the end of copulation, which is thought to be a way of inducing ovulation. Mating appears to stimulate ovulation in lions (though ovulation can occur spontaneously in some lions). 

Asiatic lions will mate in a marathon session of 3-7 days. During this time, they will stay in close proximity to each other, with the male guarding against other potential mates. The end of mating is abrupt with "some amount of aggression directed towards the male." They will then rest, before resuming mating at frequent intervals over a number of days.

Though the female is usually observed initiating the mating with growling vocalizations, there is no evidence to suggest that lionesses will bite the male on the balls to get things going. That part appears to have started as a joke, before being passed on as "fact", as is the way of the Internet.

What's more, the images of the Asiatic lions were taken by Safi Kok at Rotterdam Zoo in the Netherlands. As the rest of the photo series makes clear, the lioness is a juvenile, one of the three that were born at the zoo in August 2018 to mother Lalana and father Aapel.

The distress seen in the male lion is not a tired lion being prompted for another round of mating, but a father lion being bitten on the testicles by a cub. According to the zoo, this is not the first instance of this behavior for cubs Asha, Reena, and Mette.

"Lion Aapel has taken up his role as a father well," the zoo wrote about the lions back in April 2019. "The playful cubs first studied him from a distance, but in the meantime his daughters are challenging him, playing with his tail, snapping into his mane or butt."

So there you have it, it's still an amusing image, just perhaps more so for any father that has been accidentally kicked in the testicles while entertaining their kids.