At about 7 a.m. on the first day of buck season, the first shot rings out, and a dozen more echo across the valley over the next 15 minutes. It amazes me such vigilant animals are so easily killed.

But then I remember, deer season coincides with the rut -- mating season. And that explains why so many bucks fall so quickly on opening day. Their hormones are raging, and they're preoccupied with a quest for receptive does.

Estrus lasts just 24 hours, so a doe in heat is a valuable resource. In a mating system where one male can mate with many females, competition among males is intense. Typically, older bucks dominate younger males, but still they compete. Sparring, antler to antler, is common, and young bucks learn to evaluate the competition. Sometimes actual fights ensue.

Rarely, antlers lock, and a pair of bucks can die a slow death. More typically, though, jousts, snorts and stamping feet establish the dominance hierarchy.

The bigger challenge is wooing females. After a buck has established a breeding territory, which may encompass hundreds of acres, he makes many scrapes, which he marks with urine. When a doe finds such a scrape irresistible, she also marks it with urine and leaves behind a scent trail. If the buck returns and finds a fresh scent, the chase is on. The pursuit may last for several days before the doe actually comes into heat. After mating, the buck moves on to find another receptive doe.

If a doe does not breed, she will recycle in 28 days. Because all does do not mate during their first cycle, the rut can last from October through December. It's a particularly exhausting time for bucks, and the onset of winter doesn't help.

Throughout the fall, it's sensory overload: sights, sounds and odors of other bucks, does, and now, hunters. That's one reason so many bucks fall on opening day. But those that survive, get older, wiser and bigger for next year.