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Testosterone levels in deer change with the season — peaking during the rut when competition for breeding rights reaches its climax. Take a look at how deer hormones affect behavior and physiology.
by Dr. Steve Ditchkoff and Monet Gomes, Auburn University Deer Lab
It all started this past spring, when you found that pair of massive sheds in the woods. From there, you scouted heavily to try to learn more about the buck that accompanied those antlers. You set up some scouting cameras and looked at pictures all summer — your excitement growing as you viewed bucks growing their antlers. Finally, you catch him on camera, just before hunting season. His velvet is shedding, revealing antlers just like the beautiful set you found in the spring.
Leading up to hunting season, you begin seeing scrapes and rubs around your trail cameras. Could it be him? As it gets closer to the rut, you capture more images of the buck, and you see his body size grow, his neck thicken, and finally, you see the trophy buck behind the antlers. Nothing can stop you from getting into the stand opening weekend.
When that day finally arrives, you get in the stand, and not too long after, you hear the crash of antlers smashing together. You wonder if that’s your guy — surely, he’s the dominant buck in the area.
The season progresses and you still haven’t seen him. The rut is on and you see bucks chasing does, forgoing food and expending energy to maximize their chance of passing their genes to the next generation. As the season comes to a close, you feel somewhat dejected. Despite your best efforts, you never once crossed paths with that buck. But you start to wonder: Was your buck among the successful breeders? Did he sire many offspring? As time goes on, and you drift into shed hunting season, you begin your pursuit of the buck’s sheds, and the annual cycle of the white-tailed deer fanatic continues.
The behavioral and physiological shifts we observe in deer throughout the year are triggered by a myriad of hormonal signals, but behind every great buck there lies testosterone. This androgenic hormone is present in all vertebrates and plays a critical role in reproduction. Yet so many more aspects of a buck’s life are driven by testosterone. This hormone is essential to the development of the testes, spermatogenesis (sperm production), muscle building and antlerogenesis (antler production). Although most widely recognized for stimulating breeding behavior, testosterone plays a role in many traits and behaviors that deer hunters witness throughout the year.
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Testosterone levels in bucks change with the season and are driven by changing day length, and go hand-in-hand with the antler cycle (see graph below). From a purely physiological perspective, the complex cascade of deer hormones that drives the antler cycle is fascinating. It involves multiple deer hormones released from multiple organs located in various regions of the body. In its simplest form, the testosterone cycle could be described as a rhythmic change in testosterone that is driven by the changes in the season — and a failure to describe some of the complexity involved in the hormonal cascade that occurs during the testosterone cycle would be irresponsible on our part. (NOTE: If you don’t enjoy academic descriptions of biological/physiological mechanisms, skip the next three paragraphs.)
Annual Changes in Adult Buck Testosterone Levels
Changes in the length of day drive a complex set of reactions in the hypo-pituitary-gonadal axis (HPG axis), a pathway of chemical communication between the brain and the organs that drives reproduction. Decreasing day length as summer turns into fall triggers increased production of the hormone melatonin from the pineal gland located in the brain. This hormone is responsible for regulating sleep and wake cycles and serves as an internal calendar for animals.
In the HPG axis, melatonin sends signals to the hypothalamus, the region of the brain involved with controlling the pituitary gland and nervous system. In the presence of greater melatonin, the hypothalamus releases gonadotropin-releasing hormone (GnRH), another intermediate hormone on the pathway of communication between the brain and reproductive organs.
GnRH acts on the pituitary gland, a small gland located at the base of the brain, responsible for controlling the development of other endocrine glands (hormone-producing glands) within an organism. In males, GnRH causes the pituitary gland to produce luteinizing hormone (LH), the driver of testosterone production in the testes. From here, testosterone can act on hormone receptors throughout the body, initiating changes in the physical development and behavior of bucks, thereby preparing them for the breeding season.
Testosterone in the Fall
During early fall, when hunters deploy their trail cameras while scouting for the hunting season, bucks can often be observed shedding velvet as antlers harden. When antlers begin hardening and velvet sheds, anticipation of hunting season and buck testosterone levels both begin to climb.
In most white-tailed deer, testosterone levels typically begin increasing in August and peak during the breeding season, but timing can vary from region to region. In Northern regions, testosterone peaks earlier in the fall, around November. By contrast, in Southern regions, this peak may occur later in December (or even in January in some locales). Ultimately, this peak is dependent on the timing of the breeding season for that region. Increasing testosterone causes antlers to mineralize and harden, leading to a restriction of the blood supply to the antlers, and ultimately to the death of the antler and shedding of the velvet.
Testosterone During the Breeding Season
As you trek off to your best hunting spot, you see that the woods are littered with scrapes and rubs — all signs that a buck (maybe your big buck) is around and beginning to make his presence known. Your excitement continues to build as deer activity increases, leading up to the peak of the breeding season.
When hunting during this time of year, hunters often observe a myriad of specialized breeding behaviors in deer. For a buck, everything done around this time is aimed at one goal: finding a doe and maximizing the chances that his genes are passed on to the next generation. This is the time of year when testosterone levels reach their climax, as bucks are driven to maximize their reproductive output. With this influx in testosterone comes other physiological changes hunters associate with a buck getting “rutted up” in preparation for breeding.
Most hunters are familiar with the increased body size and neck swelling that occurs during this time of year. This is partly due to testosterone driving protein anabolism — increasing muscle mass and overall body size. With greater body mass comes the potential to successfully compete against other bucks in the population and obtain more breeding opportunities.
Aggression and Risk Taking
Testosterone’s effects on behavior are well documented across many species, and this hormone’s reputation for increasing aggression and risk-taking behaviors is well earned. As testosterone levels rise, bucks increase sparring frequency and intensity to establish dominance. Bucks with greater levels of testosterone rank higher in dominance hierarchies, and dominant bucks are better able to find and tend does.
After successfully breeding with a doe, tending becomes an important strategy to ensure paternity. While tending a reproductively receptive doe, other less dominant bucks are often lingering close by, and if given the opportunity might breed with that doe. Tending helps a buck decrease the chances of other bucks breeding with that doe and helps to guarantee that the fawns produced by her belong to him. Being a larger, more dominant buck leads to better success at fending off other bucks and increases reproductive output.
Walking to your tree stand during this time of year, you see rubs and scrapes. Testosterone levels, along with dominance status and age, promote this seasonal change in behaviors. These signposts serve a role in olfactory communication with other deer. Testosterone production influences secretions from glands in the forehead, preorbital (on the inside edge of the eye socket), tarsal, metatarsal and other regions of the body. All of the glands play a role in marking scrapes (e.g., rubbing branches above a scrape on the forehead and preorbital area, urinating down the inside of the hocks across the tarsal glands) and advertising the presence of a buck to potential mates, as well as competitors.
During the breeding season, variants of testosterone, also known as testosterone metabolites, are detectable in glandular secretions. Thus, a buck’s testosterone level influences the chemical signals he advertises to other deer.
Ultimately, the physiological and behavioral changes that testosterone induces during the breeding season serve the purpose of increasing reproductive output. Most critical to reproduction, increasing testosterone levels prior to and during the breeding season increase testes size and sperm production. Although bucks possess the ability to breed throughout other times of the year, these greater levels of sperm production serve to maximize the chances that a buck can successfully reproduce when does become receptive to breeding.
In addition to sperm production, other morphological factors are associated with breeding success for a buck. A study conducted in our deer research facility here at Auburn looked at factors that influenced reproductive success of bucks. In determining maternity and paternity of deer in the population, we assessed which physical characteristics were more likely to be present in bucks that sire more offspring. We found that age, body size and antler size were all associated with greater numbers of offspring produced in a given year. Since previous research has shown that greater testosterone levels drive greater antler and body size, it is likely that big-bodied bucks also have greater levels of testosterone and are more successful during the breeding season.
Although testosterone benefits a buck in many ways, there are also significant trade-offs associated with maintaining elevated levels of testosterone. To begin, dominance behaviors put an individual at greater risk of injury or death. Bucks are competing for breeding opportunities, and aggressive individuals might frequently be involved in fights that can cause serious injuries (impalements, broken bones, etc.).
Additionally, bucks might lose up to 25% of their body weight during the breeding season because they forego feeding in order to maximize their time spent searching for does. This physical exhaustion can lead to elevated levels of natural mortality following the breeding season.
Some of the early research from our lab found that up to 15% of mature bucks in a population might die from natural causes following the breeding season. Additionally, testosterone can suppress the immune system: Maintaining greater concentrations of testosterone during the breeding season makes a buck more susceptible to parasitism and disease. While the negative consequences of a suppressed immune system in bucks have yet to be documented, some of our early research showed that bucks have greater levels of parasitism than does during the breeding season. Ultimately, bucks have a trade-off during the breeding season. Their goal is to maximize breeding success, but to do so they have to increase testosterone to dangerous levels in order to compete.
For younger bucks unlikely to outcompete mature bucks for breeding opportunities, the costs of maintaining greater levels of testosterone and investing heavily in reproductive efforts are too great. For fawns, testosterone levels do not peak during the breeding season in the same way as older deer. During a buck fawn’s first autumn, a temporary increase in testosterone is crucial for the development of the pedicle, the base from which antlers will grow for the lifespan of the deer. As a buck ages, seasonal testosterone levels increase, generally reaching peak concentrations between the ages of 5 and 7. For mature males, testosterone’s trade-offs are rewarded with greater reproductive output. As a result, older bucks generally maintain greater testosterone levels during the breeding season.
Testosterone During the Post-Breeding Season
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Following the breeding season, testosterone levels drop dramatically. This abrupt and dramatic decline in testosterone causes antlers to shed, ushering in shed hunting season, and testosterone levels will remain low until the initiation of antler growth. When antlers begin to grow, testosterone concentrations temporarily increase. In most regions, this typically occurs in March. This temporary increase is followed by a subsequent drop in testosterone levels, which then remains low during the period of antler growth. Once testosterone concentrations begin to climb during early fall, antlers begin to harden, and testosterone continues to climb as deer once again approach the breeding season.
Conclusion on Deer Hormones
Throughout the year, white-tailed deer hunters are able to observe testosterone’s influence on a range of behavioral and physical changes. During fall, you might see a button buck developing his pedicles in preparation for a lifetime of annual antler growth or a buck shedding velvet to reveal hard antlers. While hunting, you might see scrapes and rubs, or two massive bucks competing for dominance. While shed hunting during the spring, you might run across a buck who just shed his antlers or trophy sheds laying on the ground.
Much of our fascination with deer comes from the plethora of biological processes we observe as hunters — and all of these are driven by the annual testosterone cycle. The next time you catch a big buck on camera, hear the clash of two monarchs locking antlers from your tree stand, or pick up that shed in the woods, you might appreciate the physiology behind this animal we have come to know and love. Each glimpse into a buck’s life gives some insight into his physiological story, and one thing is certain: Testosterone makes the buck.
— Dr. Steve Ditchkoff is a professor in the School of Forestry and Wildlife Sciences at Auburn University. He manages the deer research program at Auburn and has been conducting research on white-tailed deer for more than 25 years.
— Monet Gomes is a M.S. graduate student working under Dr. Steve Ditchkoff in the School of Forestry and Wildlife Sciences at Auburn. Gomes’ thesis research is examining testosterone patterns in white-tailed deer and how they influence reproductive success.
References
Ditchkoff, S.S., E.R. Welch, Jr., R.L. Lochmiller, R.E. Masters and W.R. Starry. 2001. “Age-specific Causes of Mortality Among Male White-tailed Deer Support Mate Competition Theory.” Journal of Wildlife Management 65:552-559.
Ditchkoff, S.S., S.R. Hoofer, R.L. Lochmiller, R.E. Masters and R.A. Van Den Bussche. 2005. “Mhc-DRB Evolution Provides Insight into Parasite Resistance in White-tailed Deer.” Southwestern Naturalist 50:57-64.
Newbolt, C.H., PK. Acker, T. J. Neuman, S.I. Hoffman, S.S. Ditchkoff and T.D. Steury. 2017. “Factors Influencing Reproductive Success in Male White-tailed Deer.” The Journal of Wildlife Management 81:206–217.
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