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.