When a South Dakota couple began bottle-feeding an orphaned litter of kittens just two weeks ago, they didn’t expect to become the center of a viral cat sensation.

But the striking size difference between two littermates—one a fragile runt, the other described as “an absolute brick of a kitten”—has captured viral attention online.

On Reddit‘s popular r/cats forum, the foster posted a picture of Fleece, the smallest of the litter, alongside her much-larger brother, Flannel. Their sisters, Cotton and Sherpa, fall somewhere in between, but the picture of the size difference between Fleece and Flannel melted hearts online.

“One is a runt and the other is whatever the opposite of runt is,” the caption said on the Reddit post that now has more than 70,000 upvotes. At just 4 to 5 weeks old, the kittens are still adjusting to life without their mother after being fostered through A Place for Meow Rescue, a rescue based in Rapid City, South Dakota.

Volunteers stepped in to provide round-the-clock care after the litter was orphaned. The foster caregiver, who wished to remain anonymous, said that, while Fleece prefers to curl up in blankets, Flannel is already more adventurous. “I love watching the two of them together, not only because of their size but also their different personalities,” the volunteer told Newsweek.

What Is a Runt?

Veterinary science offers more-formal terms for what pet lovers call a “runt,” including low birth weight (LBW), intrauterine growth restriction (IUGR), and small for gestational age (SGA). Research shows these kittens face unique challenges in early life.

A large 2024 French study of 5,504 kittens found that felines with a low birth weight often grow faster in the first week, a phenomenon known as compensatory growth.

Despite this, those in the lowest 25 percent of early growth had higher mortality rates than their larger counterparts. The study also highlighted that failure to gain weight, or any weight loss between birth and day 2, is a critical red flag for survival risk, regardless of initial birth weight.

Internet Reacts

The size difference between the pair delighted internet viewers, prompting a series of responses on Reddit.

Lots of pet owners shared their own pictures of little-and-large siblings: “I too am the proud owner of the Alpha and the Runt. Same litter, he’s twice her size but she’s twice as fluffy,” said one commenter.

Another Reddit user said: “Ours grew opposite. My wife was so concerned with how small the runt is that she bottle fed him and gave him wet food a lot. Now he he is 22lbs, his big bro is 18 only.”

“They are so freaking cute!!!! I gasped at the level of cuteness,” said another reply.

This isn’t the first time the “runt” of the litter has gone viral. Earlier this year, one woman shared just how big her tiny kitten grew, going from the smallest of the bunch to weighing almost 30 pounds. While another feline’s owner was told that her runt would “stay small,” but the reality was unexpected when she grew into a huge cat.

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