WHERE DID THAT BUCK GO?
Death in the Brush Country
by Mickey W. Hellickson, Ph. D.
As soon as I dialed in the radio frequency to buck #0083 and picked-up the antenna, I
knew the 6.5-year-old buck was dead. Instead of hearing the characteristic
“beep....beep....beep” from his collar, I heard a signal that was twice as fast, “beep, beep,
beep, beep.” I jumped down from the truck bed and walked in the direction where the
signal was strongest. After walking only 100 yards, there he was. His tall-tined rack, with
trailing skeleton, stood out like a sore thumb in the open brush country.
It was still March, only three months into my telemetry study, and already nine of the 44
bucks that we had captured the previous October had died! None of these bucks were
killed by hunters, so why were they dying? Most hunters would be surprised to learn that
10-30% of all bucks in south Texas die each year due to natural causes alone, which does
not include bucks dying from hunting-related causes. Dr. Charles DeYoung, at Texas
A&M University-Kingsville, was one of the first scientists to discover this alarmingly
high natural mortality rate.
Dr. DeYoung and his assistants captured 282 bucks on four different ranches in south
Texas from 1984-87. On two of these ranches, 53 bucks were radio collared with special,
mortality-sensing transmitters that allowed the researchers to tell if the buck was dead or
alive by the pulse rate of the radio signal. These bucks were monitored an average of
once every 11 days from an airplane.
Whenever a buck’s radio signal indicated that he was dead, researchers walked in on foot
to locate the buck and try to determine what caused the buck’s death. Twenty-three of
these 53 bucks died during the study. The cause of death could not be determined for 10
of the bucks. Coyotes killed three bucks and two bucks were killed by mountain lions.
The majority of the bucks that died on one of the ranches, died during the post-rut
between late December and March.
Shortly after this study was completed, a second study was started to determine if a
coyote-control program could increase buck survival during the post-rut. This study
began in 1987 and lasted three years. I arrived in Texas to complete the third year of
research.
Again, we captured and radio collared bucks on four different areas of two ranches.
However, on two of these four areas, we did everything we could to remove as many
coyotes as possible from January-June of each year. We removed an average of 107
coyotes from each of the two areas each year of the study.
We then monitored the survival of the bucks on the four areas to see if our coyote control
had any benefit. Coyotes killed seven bucks. Five of these bucks were killed on the two
areas of each ranch where coyotes were not controlled and removed. However, we did
not see any difference in the number of bucks, or the age structure of bucks, during fall
helicopter surveys. If our coyote removal was keeping bucks alive, it wasn’t increasing
buck survival to the point that additional bucks were appearing in our annual helicopter
surveys. The final conclusion was that buck survival did not increase enough to justify
the expense related to the intensive coyote removal.
Shortly after completing this second study, I began work on a third telemetry study on
one of these same two ranches toward my doctorate degree at The University of Georgia.
The purpose of this additional study was to measure home range size, behaviors, and
movement patterns among bucks of different ages. We captured and radio collared 125
bucks, ranging in age from 1.5-11.5 years, from 1992-94. Eleven different assistants and I
used radio telemetry to track the movements of these bucks on a daily basis year-round
for three years.
During the first year of my study, 11 bucks died of natural causes. During 1993, 10 bucks
died of natural causes, and during 1994, eight bucks died of natural causes. Twenty-nine
of the 125 bucks (23%) died of natural causes over the three years of the study.
When I looked at these data based on the mortality rates by age class, I found that young
bucks and old bucks had the highest natural mortality rates. Yearling bucks (1.5 years
old) had the highest natural mortality rate at 37%. The next highest natural mortality rates
were found in 9.5-year-old bucks (33%), followed by 8.5 (27%) and 7.5-year-old bucks
(25%). The lowest natural mortality rates were found in 4.5-year-old bucks (1%) and 5.5year-old bucks (6%). Bucks 2.5-years-old (7%), 3.5-years-old (20%), and 6.5-years-old
(8%) had intermediate mortality rates. Amazingly, 77% (20/26) of these bucks died
during the post-rut between January and March of each year, with most deaths occurring
in January (11).
WHY DO SO MANY BUCKS DIE DURING THE POST-RUT?
The most accepted theory is that bucks are extremely malnourished at this time of the
year due to the rigors of the rut. During the course of the rut from November through
early January, bucks lose up to 30% of their body weight. They are so intent on spending
every possible moment in search of does that are in “heat” that they basically quit eating
in early November. By the time the rut has ended many of these bucks have lost so much
weight that they cannot recover and either die of malnutrition and disease, or they are
killed by coyotes.
Surprisingly, in penned deer studies, where bucks are isolated in small pens and provided
unlimited, highly nutritious feed, bucks still voluntarily decrease food intake and lose
weight during the rut!
Coyotes, at this same time of the year, travel in packs. Working in combination with
other coyotes, they drag these worn-out bucks to the ground. Even mature bucks are
susceptible to coyotes during the post-rut. Dr. DeYoung, while conducting telemetry
studies on the Welder Wildlife Refuge in the 1970's, actually had coyotes kill one of his
radio-collared bucks as he was tracking it!
Another theory that may explain some of the deaths was investigated by the Southeast
Wildlife Disease Unit at The University of Georgia. These researchers discovered that
around 10% of bucks in the Southeastern U.S. die each year from brain abscesses during
the post-rut. Bucks that repeatedly fight with other bucks, as well as bucks that make high
numbers of rubs during the rut, eventually rub their foreheads to the point that the skin is
broken open. This break later causes an infection that allows bacteria through the skin.
Once inside, these bacteria literally eat their way through the brain case of the buck. The
bacteria most often tunnel through one of the fissure lines in the skull, eventually leading
to the buck’s death. Unfortunately, this disease affects predominantly older-aged bucks
because they often make rubs in higher numbers than younger bucks.
In order to determine if brain abscesses were causing mortality in south Texas bucks,
graduate student Chris Baumann traveled from Georgia to examine as many skulls as
possible from bucks that had died of natural causes. Chris examined all of the buck skulls
from my study, as well as 100’s of additional Texas buck skulls but did not find a single
incidence of brain abscesses. Thankfully, the researchers concluded that Texas’ warm
and dry climate made it difficult for the bacteria to survive in our state.
More recently, graduate student Gabriel Karns traveled from North Carolina State
University to the King Ranch to examine live bucks captured during our annual
helicopter-net gun deer capture. Gabriel used cotton swabs to swab the forehead area of
each captured buck. These swabs were later tested to determine if any brain abscess
bacteria were present. Fortunately, Gabriel found no evidence of the bacteria from the
swabs, confirming the results of Chris’ earlier study. Amazingly, 35% of Gabriel’s radiocollared bucks on Chesapeake Farms in Maryland died from brain abscesses and 62% of
mortalities in bucks four years old and older were caused by brain abscesses. South Texas
landowners, managers, and hunters should be thankful the bacteria cannot survive in our
environment otherwise successful trophy management would be even more difficult!
WHAT DOES ALL OF THIS RESEARCH MEAN? !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
First, managing for mature, trophy bucks is not very efficient because many bucks will
die each year of natural causes. To illustrate this, let’s start with a population of 100 sixmonth-old buck fawns. Our buck mortality research indicates that an average of 46% of
buck fawns will die of natural causes from December-May, reducing the buck population
to only 54 1.5-year-old bucks. Our data shows that an average of 37% of 1.5-year-old
bucks will die of natural causes so, after the second year the buck population has
decreased to only 34 2.5-year-old bucks. Our research indicates that an average of 7% of
2.5-year-old bucks die each year, knocking the population down to 32 3.5-year-old bucks.
We found that 20% of 3.5-year-old bucks die of natural causes, so we are now left with
26 4.5-year-old bucks. Natural mortality in 4.5-year-old bucks is the lowest of any age
class (1%) and all 26 bucks survive to age 5.5. Of the 26 5.5-year-old bucks, 6% will die
before reaching age 6.5. Therefore, only 24 bucks, out of the original 100 buck fawns,
will survive to age 6.5, the age at which antler growth peaks. This shockingly high rate of
natural mortality means that without any hunting at all, 76 of 100 buck fawns will die
before age 6.5! Ranch owners, managers, and hunters interested in managing for mature,
trophy bucks need to realize that in south Texas the majority of bucks will die of natural
causes before reaching maturity.
WHAT CAN BE DONE TO INCREASE BUCK SURVIVAL?
What can ranchers and managers do to increase buck survival, especially during the postrut? Past research clearly shows that intensive coyote control methods can increase fawn
survival. Therefore, as a minimum, as many coyotes as possible should be removed prior
to and during the peak fawning months of June-August. However, our past research
indicates that coyote control does not significantly increase survival of adult bucks, so
coyote control during the post-rut probably will not help.
An additional likely solution to the high rate of natural mortality during the post-rut, is to
increase the quantity and quality of the nutrition available to bucks from January-March.
The nutritional level of the natural vegetation reaches a low point during late winter,
further stressing bucks that are already worn out from participating in the rut. These
bucks then have to suffer through late winter eating browse that is often low in nutrition.
Not until spring “green-up” are these bucks able to regain the weight lost during the
previous rut.
LATEST BUCK MORTALITY STUDY
More recently, graduate student Stephen Webb monitored the survival rates of 48 radiocollared bucks on the Callaghan Ranch in Webb County. This ranch provides
supplemental feed on a limited basis to the deer herd. In addition, during the last two
years of Stephen’s study the ranch received above-average rainfall.
> Apparently, the presence of supplemental feed combined with the wet conditions
greatly increased buck survival. Stephen found an average annual survival rate of 88%
(only 12% mortality rate). In addition, he found that 52% of bucks radio collared as
yearlings, survived to six years old. These encouraging results clearly indicate that
improved nutrition, either through providing supplemental feed or increased rainfall,
results in higher buck survival. Stephen and his co-investigators concluded that a large
percentage of bucks could reach the mature age class under trophy management and be
available for harvest.
REFERENCES
Baumann, C. D. & W. R. Davidson. 1998. An evaluation of intracranial abscesses among
white-tailed deer. [Abstract] Southeast Deer Study Group 21:34.
DeYoung, C. A. 1989. Mortality of adult male white-tailed deer in south Texas. Journal
of Wildlife Management 53:513-518.
Karns, G. R., R. A. Lancia, C. S. DePerno, M. C. Conner, & M. S. Stoskopf. 2009.
Intracranial abscessation as a natural mortality factor in adult male white-tailed deer.
[Abstract] Southeast Deer Study Group 32:21.
Webb, S. L., D. G. Hewitt, & M. W. Hellickson. 2007. Survival and cause-specific
mortality of mature male white-tailed deer.