Simply put, no. Guinea pigs don't NEED UVB rays. But they can definitely benefit from exposure to them.
If you own a guinea pig, consider investing in a fluorescent fixture and a UVB emitting fluorescent bulb. These can be purchased from the reptile section of any pet shop and placed about 12-18" above your guinea pig's cage or area where he spends a lot of his time.
Mercury vapor bulbs are alright for large spaces where your guinea pig roams (like a bedroom), but they also emit a great deal of heat, so unless your guinea pig likes it toasty, these aren't the best choice for guinea pigs.
Studies have shown that guinea pigs (and many other pocket pets) have an increase in activity, appetite and overall happiness with the addition of UVB rays. Many guinea pig owners find UVB rays especially useful in cavies who are sick or recovering from an illness (such as ileus), new and stressed out pigs, and guinea pigs who have recently lost their cage mate.
Just be sure your UVB light isn't too close to your guinea pig or he can get sunburn or hurt his eyes. Keep the light on for no more than 12 hours a day, and replace the bulb every 6 months, even if the light is still working since the invisible UVB rays run out after about 6 months, depending on the manufacturer.
