Testosterone: Jet Fuel for the Whitetail Rut

testosteroneI’ve been interested in whitetail behavior for more than five decades. As a teenage hunter, I had many questions about why bucks and does do what they do and look the way they look at various times of year. As I progressed through life as a hunter, photographer and writer, many of those questions were answered. However, it wasn’t until I began raising whitetails more than 20 years ago that my understanding of them went to the next level, especially when it came to rutting behavior.
Each year, hundreds of articles are written explaining why bucks throw caution to the wind, expand their home ranges, rub, scrape, fight, chase and breed. Though rutting behavior is fascinating, I believe few hunters truly know what drives a buck to do what he does during the rut.

By Charles J. Alsheimer, Deer & Deer Hunting contributor

There isn’t one aspect of rut behavior that stands on its own merits. Rather, all are part of a recipe driven by environmental factors and hormones. Everything leading up to breeding happens for a reason and has a purpose. To understand the whitetail’s journey to the rut requires an understanding of how whitetails exist 365 days a year.

Photo Period and Hormones

The process of photoperiodism is responsible for the seasonal and behavioral changes we witness in the natural world; everything from bird migrations to season changes. In the whitetail’s case, when the vernal equinox leaves winter behind, day length begins to increase rapidly. This triggers many physical changes in whitetails, one of which is the shedding of winter fur. In addition, spring green-up brings an explosion in the quantity and quality of food available to deer. But the most noticeable change in whitetails in spring and summer is the effect lengthening daylight has on antler growth. Though it is believed hormones have a role in antler growth, a deer’s health and day length are the big drivers.

From January to mid-August, a buck’s testicular volume and serum testosterone levels are low. By late August, shortening day length becomes more noticeable, triggering an uptick in hormones in bucks and does. For bucks, the testosterone increase is slow at first because nature is careful not to give them their full arsenal of rutting behaviors until the time is right.

Between mid-August and mid-September, a buck’s testosterone level will increase nearly 100 percent from what it was in June. And from mid-September to Nov. 1, it will double again, reaching its peak by the first week in November in the North. A doe’s estrogen level will also peak around Nov. 1, setting the stage for breeding to occur. Here in western New York, velvet peel is full-blown around Sept. 1. From that point on, rut behaviors begin increasing until reaching their apex in November.

Physical Change

Throughout summer, the physical appearance of bucks is noticeably different from what it will be in November. Because their testosterone is low in summer, the bodies of bucks will be sleek and slender, particularly in the neck. With little testosterone to drive aggressive tendencies, bucks assimilate into bachelor groups throughout summer, bedding together, feeding at the same food sources and often grooming each other. However, by the time velvet-clad antlers are ready to peel, testosterone begins to flow. When velvet peel has occurred, bucks begin to spar with each other. With each passing day, the testosterone valve opens a little more, and with it comes the first signs of rutting behavior. By mid-September, a Northern buck’s winter fur is nearly grown in, and his body begins to fill out. In some mature bucks, this increase in body mass almost gives the impression that the buck is being blown up with an air hose.

In human terms, the effects that testosterone therapy has on athletes is well documented and rivals what occurs with whitetail bucks when their testosterone level increases. For example, when a human male athlete undergoes testosterone therapy, his muscle mass begins to increase, especially if he is involved in a weight-training program. In addition, an athlete’s muscle twitch greatly improves, meaning his reaction time is quicker, and he can run faster. During the 1990s, many baseball players began taking performance-enhancing drugs (primarily testosterone) to boost performance. Players who normally hit 20 to 25 home runs a year were hitting 40 and 50. In the process, home run records that had stood for decades were shattered. Fortunately, Major League Baseball took steps to rid the sport of performance-enhancing drugs, but some players continue to cheat the system. Though steroid use helped records fall, the dirty little secret is that there is a dark side to improper steroid use. Side effects can include making an athlete hyper-active and very aggressive, in some instances dangerous to themselves and others.

I share the baseball comparison to say this: In all my years of raising whitetails, I’ve witnessed very similar behavioral traits and patterns in bucks when their hormones increase unabated. From mid-December to mid-September, when testosterone levels are low, Northern bucks bed a great deal, for the most part keep to themselves and show little aggression to other deer in their home ranges. However, when September begins inching toward November and hormone levels increase and peak, bucks turn into different creatures.

Next week, we’ll look at the physical and behavioral effects of elevated testosterone.

Live the deer hunting lifestyle year-round!