Buck Testosterone: Why His Routine Changes During the Rut

STEP 1 · Antlers

Antler growth happens before peak testosterone

Much of a buck's antler growth takes place while circulating testosterone is relatively low. The later rise helps bring the antlers into hard condition as breeding approaches.

Researchers demonstrated part of that relationship by blocking androgen action in captive whitetails. Antler mineralization was severely disrupted, and the antlers continued growing through the year. The hormone system involved in breeding also helps govern how the antler-growth season ends. Antiandrogen experiment.

Antler size reflects growth from an earlier part of the cycle. It cannot reveal the buck's current testosterone concentration from a photograph.

The seasonal timing system is sensitive to light and darkness. Melatonin, produced during darkness, helps coordinate the hormonal changes that prepare deer for the coming season. SAMPLED: in an experiment involving four treated male whitetails, researchers altered the melatonin signal and advanced antler mineralization, velvet shedding, and rutting behavior by about 50 to 55 days. Testosterone rose earlier too. Photoperiod research, melatonin experiment.

A cold front can change the activity you see. Seasonal preparation for breeding is already underway by the time much of the chasing becomes visible. A busy morning does not establish that testosterone suddenly surged overnight. Mississippi State Deer Lab.

STEP 2 · Seasonal Levels

The rise can be substantial, and the peak is local

Auburn researchers studied a freely breeding Alabama herd inside a large enclosure. They had known-age deer, repeated blood samples, and a breeding season that peaked in January.

SAMPLED: the study included 228 blood samples from 88 bucks, collected between September and March during 2007 to 2017. Testosterone peaked in January alongside the herd's main breeding period. Older bucks had a higher January mean than the younger age class. Gomes and colleagues.

Here, ng/dL means nanograms per deciliter of serum. Standard error describes uncertainty around an estimated mean, rather than the range of individual bucks' levels. The herd received supplemental food and was not hunted, so these concentrations belong to that study population. The age-group difference was detected during breeding, but not outside that period.

An earlier wild-deer study also recorded a strong seasonal change. SAMPLED: mean serum testosterone was 210 ng/dL during August through November, compared with 34 ng/dL during December through July. Those were broad seasonal averages from a different population and laboratory method. They illustrate the annual change, but should not be joined to the Alabama numbers to make a single monthly curve. McMillin and colleagues.

The January peak is as useful to a hunter as the concentrations. These Alabama bucks were at the height of their reproductive season when a November-centered rut calendar would have put them well past it.

Biologists can estimate local breeding dates by aging fetuses from does harvested later in the season. Those estimates describe when breeding occurred. A run of camera photos describes activity at a location, which can peak at a different time. Mississippi State Deer Lab.

STEP 3 · Buck Age

Young bucks can give you a different view of the season

The ages you are watching can affect your impression of the rut.

SAMPLED: a study of 292 hunter-harvested bucks in west-central South Carolina found that bucks 3.5 years and older had their highest sampled testosterone during the first week of the study. Younger bucks peaked approximately four weeks later. Sampling ran from late October through late December, which means the older bucks' actual peak could have occurred before sampling began. Miller and colleagues.

Those were hormone peaks in a particular population, so the gap should not be read as a universal delay in chasing. Observations of young bucks may give an incomplete view of mature bucks' reproductive condition.

STEP 4 · Searching and Breeding

A buck can be ready while most does are not

Once a buck is in breeding condition, he still needs a receptive doe. Early in the local breeding season, relatively few may be available. That can put bucks on the move as they search for an opportunity.

The searching has more structure than it sometimes appears to have from a stand. GPS research in South Texas found that rutting bucks increased movement while repeatedly visiting relatively small focal areas. Their travel was consistent with returning to places likely to hold females. This was a movement study, so it describes searching behavior rather than measuring the testosterone behind each trip. Foley and colleagues.

As more does enter heat, a buck may find a mate and spend more time with her. He can still be in strong reproductive condition while covering less of the ground where you were detecting him. That is how peak breeding can produce a quieter stretch for a hunter. Mississippi State Deer Lab.

Your busiest day of buck sightings therefore need not be the day when the most does are breeding. Searching and mating put deer in different places and occupy their time differently.

STEP 5 · Scent Marking

Rubs and scrapes show part of the change

Fresh sign gives you another view of the season. A rubbing buck can leave more than scraped bark behind. Researchers examining forehead glands found increased activity during the rut, especially in dominant males in their study, supporting the role of gland secretions in scent deposited on rubs. Forehead-gland study.

Scraping also has a social component. In captive groups, age, dominance, and testosterone were related to scraping behavior. Mature dominant bucks produced the most substantial scraping activity in that setting. Scraping study.

This makes a fresh scrape useful as a place to observe deer communication. Repeated visits can reveal how its use changes as the season progresses. The scrape itself cannot tell you an exact hormone concentration or the date when most does will conceive.

STEP 6 · Feeding

Food can be available and bucks can still eat less

Rut weight loss is easy to picture as the cost of chasing. Research shows that reduced food intake can be part of it too.

In a captive South Dakota experiment, researchers kept feed available while tracking intake and body condition through the breeding season. Bucks ate less during the rut despite that availability. Higher testosterone was associated with lower intake during the sampled pre-rut week.

SAMPLED: adult males lost about 20% of their body mass during the study's autumn period. Yearlings lost about 12%. These were results from captive experimental groups, not expected losses for every wild buck. Monteith and colleagues.

The results show a seasonal reduction in feeding with food still available. They do not establish testosterone as the sole cause of the weight loss.

A food-plot camera records use of that feeding location. Fewer photographs may reflect a change in feeding priorities, a different food source, or time spent elsewhere. A summer feeding pattern can lose its usefulness while the buck is still using the surrounding area.

STEP 7 · Reproductive Success

The highest hormone level does not guarantee the most offspring

It would be easy to assume that the buck with the most testosterone also does the most breeding. A later analysis of the Auburn herd did not support that simple ranking.

Researchers measured annual reproductive success through offspring sired that survived to recruitment into the breeding population. Body size and antler size were positively associated with that outcome. Testosterone was not a significant direct predictor in the analysis. These were overlapping animals and years from the Alabama work, rather than an independent test in a separate herd. Gomes and colleagues.

Testosterone is central to reproductive physiology, but a sampled concentration does not provide a tally of mating opportunities or surviving offspring. It also cannot tell you how often that buck will pass your stand.

STEP 8 · Late Breeding

Breeding opportunities can continue as the season fades

A doe that did not conceive may cycle again. A young female may become ready to breed later than the adults. Either can create more searching and chasing after the main concentration of breeding has passed.

Those events help explain what hunters call a second rut. Later chasing does not, on its own, demonstrate a second testosterone peak across the entire buck population. Ecology of the rut.

As bucks move out of peak reproductive condition, testosterone declines. The antler cycle eventually reaches casting, when bone is resorbed where the antler attaches to its pedicle. In a captive study, dominant bucks retained their antlers longer than subordinates, and the authors discussed different findings from other regions. Even the end of the cycle needs local and individual context. Seasonal hormone research, antler casting, dominance and antler retention.

A buck's seasonal hormone cycle helps explain the change from velvet growth to breeding condition and eventually antler shedding. His opportunities to mate depend on the does, and those opportunities can change how often a hunter sees him. A quiet stand during peak breeding can fit a buck spending time with a receptive doe instead of searching the routes where you had been watching him.