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 William R. Pritchard Veterinary Medical Teaching Hospital at the University of California, Davis. "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 Magnolia Springs, Ala.
At Stray Love, smaller rescue dogs are adopted five times faster than the larger ones. 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," 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 center 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, 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. All of these things tend to press on the mother in many of us to protect them," Guthrie 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.
Cheddar, the runted kitten of an abandoned litter that Kristin Ramsdell fostered for the Black and Orange Cat Foundation, now weighs more than 7 pounds. He weighed less than half a pound when he was found in June 2011 with the rest of his 8-week-old littermates.
At 8 weeks, a kitten should weigh between 1.5 and 2 pounds, Ramsdell said.
"I stayed up for three straight days with him, giving him fluids and antibiotics, warming him with IV bags heated in the microwave, using a humidifier and watching him round-the-clock. I didn't think he would make it," she said. Cheddar and one of his siblings, Colby, have been adopted by a Philadelphia family and are thriving, Ramsdell said.
That special attention required to bring some runts to health can create a special bond. Cat owner Melissa Hadaway took the runt of a litter and its sister to her home in Winder, Ga. She recalled how six years ago, Annie, the runt, "was the littlest and bravest. She fought very hard to get her share."
Kathy Covey of the Cat Adoption Team in Sherwood, Ore., said a kitten runt weighed 11 ounces when he arrived in August at 6½ weeks old.
"His eyes and ears were too big for his face, he had a kidney infection. He was on fluids, syringe feeding, pain meds and antibiotics. When you picked him up, you could feel each of his ribs. But he was a lover, snuggling in to you whenever you showed any affection and purring the whole time," she said.
Little Big Burger worked hard and gained a pound in two weeks, Covey said. He has to stay on antibiotics for his kidneys but his prognosis is improving.
"He's not giving up, so I'm not," she 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, Meadows explained.
Meadows also 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.