A black cat wanders in, sits, curls his tail around his toes and gazes up at you with amber eyes. You pet; he purrs. You turn to go, and there he is again, sitting and blinking, waiting to be adored. Are you seeing double? No you're seeing twins! Identical twins are a fascinating phenomenon, and for cat lovers, a pair of twins can be double the pleasure and the fun.

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When is a twin a twin? That depends. Human twins occur any time two babies are born from the same pregnancy. They may not be identical, but they are still twins.

Cats are a different story. Jessica Sylvester, a cat breeder and rescuer in Rhode Island, says, Because its normal for cats to have multiple births, there is no such thing as fraternal twins in cats. There are only identical twins. What makes them different from other littermates is that they were conceived with the same single egg and same single sperm.

[Twins] share the same exact DNA, says Michael H. Andress, a veterinarian at Gate City Animal Hospital in Greensboro, N.C. Twins are formed when a fertilized egg splits into two separate embryos. Other litter mates develop from a separate egg and a separate sperm.

The scientific term for identical twins is monozygotic, and the same process occurs in cats as in humans. No studies have been done on twinning cats, but according to human twin research, 23 to 33 percent turn out to be mirror image twins, which exhibit reversed characteristics, such as markings found on the right side of one kitten appear on the left of its twin. This happens when the fertilized eggs splits later in the process (around days nine to 12). If the division takes place too late, conjoined twins occur. Conjoined twins are identical twins that did not completely separate and are therefore joined in some way, such as sharing organs or limbs. These are often referred to as Siamese twins in humans, named after the famous brothers. However, with cats, the term Siamese twins can be misleading, as this phenomenon can happen in any breed, not just Siamese. The exact percentage is not known, but conjoined twins are estimated to occur once in ever 85,000 human births.

**For the full article, pick up the July issue of CAT FANCY**