A male cat's paternal instincts kicked in after the mother of their litter of kittens died unexpectedly.

Viewers' hearts have been left broken by a short video shared to TikTok account @the3amigotuxedos, dedicated to sharing updates on three formerly feral kittens who were adopted into their now-owner's home. And their origin story is as sweet as the kittens' names: Riley, Izzy and Piper.

The kittens' mother was first seen with the trio from the window of their future adopter's house, with the man initially touched that the mama cat trusted him enough to bring them close.

However, in the devastating viral video shared in April, which shows a different black-and-white cat curled up with the kittens, the man wrote: "After their mom passed away their dad kept the babies safe until we brought them inside."

It is unusual for a male cat to feel protective over kittens, and they have been known to kill those that they haven't fathered themselves, according to a vet-reviewed report from Catster.

This behavior, devastating as it may seem, is an evolutionary instinct that may allow the male cat to pass down his own genes through the mother. It can also be caused by felines' territorial nature.

However, some males can be nurturing toward kittens they have fathered themselves, the report adds, although it is unusual in feral or colony cats.

In the TikTok video, the three kittens are curled up asleep beside their father, who stretches and nuzzles against them—clearly not acting territorial in any way.

Users commented in their droves on the video, which has close to 15,000 likes and more than 136,000 views; one wrote about the "whole herd of pretty cow kitties," with another declaring it "tuxedo heaven."

"So beautiful that you kept all of them," another viewer posted, with one adding: "Thank you for caring for them all."

A viewer, writing alongside a series of crying emoji, wrote: "Rip to mommy cat. God bless daddy for caring for babies and protecting them, thank you as well."

The cats' owner wrote in a comment that he never learned how their mother died, saying "I wish we knew."

"She disappeared for a couple days and then we found her dead by some flowers," he said, adding they believe she may have eaten poison put out by a local farm.

He also said that the father cat is "doing great and very happy," living as a part-indoor, part-outdoor cat.

"I spent many, many months working with him! He was feral but the sweetest boy now," the man added.

Newsweek has contacted @the3amigotuxedos via TikTok for comment.

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