For puppies and kittens, size really does matter.
Shelters say smaller animals get adopted faster, and animal experts say the runt of a litter tends to be better protected by the mother.
Pet owners-to-be tend to heap attention on them, since they're attracted to big heads on little bodies.
"Humans are drawn to animals or beings of any kind whose proportion of eyes to head is large," said Dr Julie Meadows, a faculty veterinarian at the University of California.
It's why we all coo when we look at babies, whether they're human or animal.
For runts destined to become family pets, their size is their greatest risk before birth but also their greatest appeal after birth.
"It's the underdog, undercat thing," said Gayle Guthrie, founder-director of Stray Love Foundation in Alabama.
At Stray Love, smaller rescue dogs are adopted five times faster than the bigger ones.
Dr Meadows said that could be a result of the growing popularity of so-called pocket puppies - teacup dogs bred to be small and stay small.
"Pet owners are looking for that really cute runt equivalent, almost like we are selecting for runted creatures because we like those little things that can ride around in our purses and strollers and never weigh more than 5 pounds (2kg)," Dr Meadows said.
A litter has only one true runt, but not every litter will have a runt. Litter-bearing mothers have Y-shaped uteruses. Those at the centre of the Y get the least amount of food and have the greatest chance of being runts, while those closest to the mother's blood supply get the most nourishment and have the highest birth weights, Dr Meadows said.
When runts are born, they have to fight harder because they are small, weak, and others often pick on them or push them away from their food source, Ms Guthrie said.
"All of these things tend to press on the mother in many of us to protect them," she said.
In most cases, if the runt of a litter makes it to six to eight weeks, it will probably survive and likely grow close to full size, experts said.
Runts aren't welcomed everywhere, though. Wilbur, the classic runted pig in the children's book Charlotte's Web, was saved from slaughter with the help of a spider, but animal agriculture and food producers in real life aren't as forgiving.
A pig farmer thinking about Easter hams will probably cull runts from his pens because they will never reach the body size needed for meat production, Dr Meadows said.
She noted that in the wild, only the strong survive. And runts likely won't win sporting awards, since they won't have the muscles or build needed for agility or show ring competition.
Even some animal welfare groups won't champion all runts to families. The Cat Adoption Team in Oregon wants to place as many kittens as possible, but it will draw the line with some runts, said operations manager Kristi Brooks.
"If there are a lot of rambunctious kids, we suggest that a bigger kitten might fare better," she said.