>>My sister owns female brindle black rat snake. The snake was 2 years old when she bought her in the winter of 2009-2010. She is my sister's only snake and has not interacted with any other snakes since my sister bought her. She is at least 5 feet long. In the spring of 2011, she laid a total of 16 eggs over a period of a couple weeks. Is there a precedent for this? What would cause this to happen? We sure were confused!

two possibilities. Rat snakes can retain sperm for a couple of years.They can also lay eggs without ever being introduced to a male. The later will usually be infertile but I THINK there have actually been a couple cases of fertile eggs.
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Corn snakes and rat snakes...No one can have just one.
"Resistance is futile"
Jimmy Johnson
Draybars Snakes