Sometimes when photographing wildlife with the aid of an electronic flash, your shutter finger can operate faster than your flash can recycle itself. It might not be that noticeable if you were using the flash only for fill light, but when depending on a flash as your main light source, you can end up with a very underexposed (or even black) image capture.
Take for example this shot of a leopard dismounting a tree, shot during poor light in a dim riverine forest in the Mara North Conservancy. Since the recycle time depends on the intensity of the flash used, I had previously that day been able to take fairly rapid shots since I was using it mainly for fill and details in shaded settings. But as the natural light waned in late afternoon, the proper exposure depended almost entirely on the flash, and I ran into trouble with a too eager shutter finger--resulting in some nearly black images.
On a whim, while editing the images back at home, I played with the exposure and fill light settings when converting the RAW captures into a usable TIFF file. Voila, a usable image appeared!
So don't write off an underexposed image as unusable, even if you were depending on available natural light (although you might run into low contrast problems).
CAUTION: Your mileage may vary, depending on your camera and RAW conversion software. This faint image was captured on a state-of-the-art Canon 5D Mark II sensor, with excellent sensitivity and signal to noise ratio, as well as the very expensive Photoshop CS5 RAW conversion software.
So, before discarding an underexposed RAW image, try a little salvage work first. Your results, of course, will depend on your camera sensor and RAW conversion software, and some knowledge of how to use that software.
You are shooting in RAW, aren't you?