All digital images need a sharpen when processing them, whether it's Sigma, Tamron, Nikon, Canon etc. lenses being used.
You'll see that fracster's effort has bought out a lot more detail in the fur.
A big tip when shooting wildlife is to get the subject's eye/eyes sharp. When us humans look at an image, we always look for eye contact first and foremost and so the impact (sharpness) of the eye is paramount when judging the impact that a photo has.
Looking at your image, it looks like you've missed focus on the eyes, so that's the first thing...
Also, the Sigma isn't best known for sharpness at the 500mm end. Reason being is that it covers quite a range and most of these kind of lenses lose sharpness at their maximum. For massive sharpness capability, fixed focal length lenses (primes) are the uber sharp ones, followed by zoom lenses with not a lot of range on them (usually
).
You're right to use the lens "wide open", using the widest aperture to get the fastest shutter speed. Most wildlife 'togs almost always use their lenses wide open, for this very reason and also to control depth of field, so that the background is as out of focus as much as possible, so well done there!
This is probably quite near as you can get this particular lens, sharpness wise. Fracster has done a good job with brining out the detail and others may well have a go, obviously with your permission
A good start though
This forum IMHO is really awesome in helping newbies improve. Quite a few have said it is due to Talk Photography and its members and some are really churning out superb quality stuff now. This forum has helped me massively in developing, in conjunction with books and self teaching in the field.
Thank god they don't charge commission