The rut plays a key role for whitetail deer hunters every fall, from Minnesota to Texas. Why is it so important? Lou Cornicelli, Department of Natural Resources wildlife research manager, explains.

Q: So what is the rut?

A: It's the one time of year that deer breed. In southern states they can breed over six or seven months, but here it's a narrow window in November.

Q: What triggers it?

A: Photoperiod, or changing amount of daylight. Hot or cold weather can slightly interrupt things, but it's the length of day that triggers the response to breed. Our peak breeding is around Nov. 12-14.

Q: Why November?

A whitetail buck stood near a snow-covered Iowa farm field. A change in the amount of daylight triggers the rut for whitetails. Left: Bucks fight during rut “to assert their dominance, perhaps thinking there’s a chance to do a majority of the breeding, even though they don’t really get to do that,” Lou Cornicelli of DNR said.

A: Deer come into estrus in northern states at this time of year for a reason, and the reason is survival. They need 195 days for gestation, which means the fawns will be dropped in May. That puts them at a reasonable size so they can survive the winter. You can't have a fawn born in August, because they won't survive the winter. It's evolutionary adaptation. They have evolved to do this over millions of years. One Canadian researcher says whitetails are the oldest deer species in North America; they have been around for about 6 million years.

Q: So what happens?

A: Males start to build the hormone testosterone; that causes the neck swelling on bucks that hunters notice and the start of fall breeding activity. The females go into estrus.

Q: So how does this affect hunters?

A: Because the bucks are focused on breeding females, they are not paying attention to other things. They are really vulnerable to people and cars. They run out in roads, chasing deer. They bumble in front of tree stands. Their vulnerability goes up as their awareness of their surroundings go down.

Q: So when is the best time to hunt deer?

A: The last week of October to early November is probably the best time of year to hunt for bucks here, because the males have been ready [to breed] for some time, and the females aren't. So the males are chasing them around. When the females start standing [allowing males to breed them], there will be less activity.

Q: Some states, like Minnesota, time their deer seasons to coincide with the rut, and other states, like Wisconsin, avoid it. What's the thinking?

A: All of the states do something different. Our season was specifically designed to be held when bucks are most vulnerable. The idea was to put a lot of pressure on males and little on females, which would increase the success rate and not impact the population. That was done in the 1970s when we had 300,000 deer hunters. Now we have a half-million hunters. When you put that many hunters on the landscape for two days, the timing of the season really doesn't matter. Even if our season was held a week earlier or a week later, it wouldn't really change the outcome.

Q: So one buck will breed more than one doe?

A: Yes, he'll breed as many as he can. But not every single deer gets bred. Those females that aren't bred will come back into estrus in December, and males will go through a smaller, almost imperceivable, rut.

Q: Is there an effect, genetically, on the herd by focusing on bucks?

A: No. Genetics is largely a black hole. It's not something we really can measure or influence in a natural system. Deer, in general, are pretty diverse individually. Killing one buck might impact one genetic line, but it won't have a population-level impact.

Q: Are bucks territorial?

A: No. They don't stake out territory. They do leave scrapes, where they leave scent, and they check those mainly at night. They're marking where they've been. Females will also check scrapes, mostly at night.

Q: Bucks often fight each other during the rut. What's going on?

A: They're fighting to assert their dominance, perhaps thinking there's a chance to do a majority of the breeding, even though they don't really get to do that. If a spike buck happens on the trail of a receptive doe, he will breed it. It's who gets there first.

Q: Does all of the chasing, fighting and breeding take a toll on bucks, leaving them weakened?

A: Absolutely. They are not eating for 30 days, and they have depleted their fat reserves, big time. The first thing you lose in a significant winter are fawns and older males. You look at a buck harvested the first week of the season, and it looks nothing like one harvested at the end of the muzzleloader season [in December].