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," says Dr Julie Meadows, a faculty veterinarian at the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California, Davis. "That'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," says Gayle Guthrie, founder-director of Stray Love Foundation in Magnolia Springs, Alabama.
At Stray Love, smaller rescue dogs are adopted five times faster than the larger ones. Meadows says that could be a result of the growing popularity of so-called pocket puppies.
"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 says.
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 says.
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 says.
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 (3kg). He weighed less than half a pound when he was found in June 2011 with the rest of his 8-week-old litter mates. At eight weeks, a kitten should weigh between 1.5 and 2 pounds, Ramsdell says.
"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 says. Cheddar and one of his siblings, Colby, have been adopted by a Philadelphia family and are thriving, Ramsdell says.
