Lopsided?

Looks like this calf may have a leg up on its bovine pals. When it was born in 2003 in Cambodia, its owner was shocked to find an extra set of legs hanging from the calf's neck. Fearing the oddity would bring him bad luck, the farmer donated the calf--named "Cham Leck," or "strange" in Khmer language-to monks.

What caused the extra appendages? Some scientists believe that the dangling limbs are remnants of a twin calf that never fully developed, says Alison Van Eenennaam, an animal geneticist at the University of California-Davis. Like most animals, Cham Leck got its start when a female sex cell, or egg, joined with a sperm, or male sex cell. That produced a zygote, or a single cell that holds the genetic material from both parents. When this cell divides over and over, it eventually forms a mass of cells called an embryo.

But in Cham Leck's case, scientists think the single zygote split in half before it started dividing. This split formed two separate zygotes, which began to divide side by side. The resulting embryos were on course to become twin cows.

As the embryos developed, certain genes (units of hereditary material) sent vital directions to cells, Van Eenennaam says. During this process, called differentiation, various cells got instructions for turning into parts of the body--including legs.

Apparently, both embryos formed healthy legs. "You can easily see legs at about the 42-day point on an embryo," says Tom Geary, a reproductive physiologist who studies the body's vital functions at the U.S. Department of Agriculture. But as Cham Leck grew, for some reason its twin didn't survive-and Cham Leck ended up with its legs.

How? Geary explains that if the two embryos were touching, the twin's just-forming legs may have fused with cells from the other embryo. Result: The twin's legs got vital nutrients and blood from Cham Leck's blood vessels, and continued to mature-eventually attaching to Cham Leck's neck.

The extra limbs may look functional, but they probably can't move. That's because the legs don't contain any movement-signaling nerves.

So even if Cham Leck can't outrun its pals, having additional appendages makes this calf unique. Says Geary: "The chances of something like this happening are one in a million."