CBU93 said:
From another thread:
Think of all the hunters that report seeing 20 does per buck, yet that is biologically impossible. Just an example...
As pass-thru pointed out, because of large numbers of button bucks being "recruited" into the adult population as yearling bucks on their first birthdays (a deer is an "adult" when it turns one year old), it is impossible for sex ratios to be highly skewed at the beginning of the hunting season. Now if bucks are slaughtered
during the season the sex ratio can become highly skewed by the end of the season, but often these highly skewed
observed sex ratios are reported even in bow season, before buck harvests could skew the ratio.
Some overly smplistic math shows how the PREHUNT sex ratio can never be highly skewed as long as fawn production is good:
Let's start with a perfectly balanced prehunt herd of 100 does, 100 bucks, and 100% fawn recruitment (for every 100 does, 100 surviving fawns are produced). 100% fawn recruitment means there are 100 fawns in the herd for the 100 does. So you have 100 bucks, 100 does and 100 fawns. Now lets say that during the hunting season, eventually hunters kill every living adult buck, and to make matters worse, hunters killed zero antlerless deer. At the end of the season you have zero bucks, 100 does and 100 fawns. Remember that those 100 fawns are basically 50/50 male/female. By the following summer, all of the fawns are recruited into the adult population by turning one year old. Now before the following season you have 50 button bucks that have become antlered adult bucks (yearling bucks), the original 100 does plus 50 female fawns that have become adult does for a total of 150 does, and those 100% fawn recruitment produces 150 fawns. So with a prehunt herd of 50 bucks, 150 does, and 150 fawns, you have an adult sex ratio of 3 does per buck (150 does divided by 50 bucks).
Now follow the same patterns as the previous year--all bucks are killed and no antlerles deer are killed. At the end of the second season you have zero bucks, 150 does and 150 fawns.
However, again fawns are recruited into the adult population over the following summer (75 button bucks to adult bucks and 75 female fawns to adult does) and the 3rd year's prehunt population is 75 bucks, 225 does and 100% fawn recruitment produces 225 fawns and again you have a prehunt adult sex ratio of 3 does per buck (225 does divided by 75 bucks).
If you keep following these calculations out year after year you will find that the prehunt sex ratio is NEVER worse than 3:1, because the large fawn crops keep swamping the prehunt sex ratio through recruitment of fawns of both sexes into the adult population. And this scenario assumes hunters kill every living buck each year. That won't actually happen.
However, as pass-thru mentioned, this system keep working because of the large fawn crops each year. If fewer surviving fawns are produced each year, you could see the prehunt sex ratio become skewed if few button bucks were available to be recruited as yearling bucks each year. Take the same calculations as above but make the fawn recruitment only 20% (20 surviving fawns for every 100 adult does) and you will find the prehunt sex ratio is consistantly 11 does per buck.
But again, this was an oversimplistic set of calculations. In reality, not all bucks are harvested and some antlerless deer are harvested, so
as long as fawn production is ADEQUATE, rarely are prehunt sex ratios ever worse than 4 does per buck. And most solid census techniques actually find that most deer herds have pre-hunt sex ratios no worse than 2.5 adult does per adult (antlered) buck. This is also what the thermal imaging census data the TWRA is collecting across the state shows as well. In the vast majority of locations censused, rarely is the prehunt sex ratio in TN worse than 2.5 does per buck and some areas are quite good at 1.5 to 1.7 adult does per buck.
So if real prehunt sex ratios can't possibly be highly skewed
if fawn recruitment is adequate, and it is in most parts of TN, how can hunters report such highly skewed adult sex ratios early in the year? The answer to that question demonstrates why hunter observations cannot be used to show what the real herd composition is. The first part of the answer is harvest policies and the second is deers' ability to learn to avoid harvest pressure.
Let's use another oversimplistic example of how harvest policies affect OBSERVED sex ratios. A hunter is asked to hunt inside a 100-acre high-fenced property in Unit B all year. The hunter doesn't know it but there are only 2 adult does (that travel together) and one adult buck in that fenced property. Over the course of the season, the hunters sees the one buck and immediately kills him, preventing that buck from ever being observed again. However, since does are off-limits to harvest, the hunter--over the course of an entire hunting season--ends up seeing that pair of does 5 times. Now total up the hunters observations. He saw the buck once and killed it. He saw the same pair of does five times, giving him 10 total doe sightings. This is an observation ratio of 10 doe sightings to 1 buck sighting, or an observed sex ratio of 10 does per buck. Yet in reality, the real prehunt sex ratio was 2 does per buck.
That's an oversimplistic example, but it points out just how rapidly observed sex ratio numbers can be skewed away from reality by buck-centric harvest policies.
Now take a more realistic view of what takes place. As MZ and gun seasons open, young , inexperienced bucks get hammered hard, yet it doesn't take long for the few surviving bucks to learn that moving during daylight gets them killed, and they learn to become nocturnal for survival purposes. This nocturnal movement greatly reduces how often they are seen by hunters in daylight. Those few remaining bucks are still in the woods, but now they are rarely observed by hunters in daylight. However, with little harvest pressure on the does, they don't need to adapt this nocturnal-movement survival technique, and they walk around in daylight with impunity, increasing how often they are observed by hunters multiple times.
So in this scenario, you have a decreasing number of surviving bucks that no longer move much during daylight hence are not seen by hunters. Yet you have does walking around in daylight and being observed multiple times by hunters. This produces a highly skewed OBSERVED adult sex ratio. Then throw in that many of the fawns in this population have grown to a size (by November) that they are hard to distinguish from adult does and these fawns are often being misclassified in observation data as adult does, increasing the artifical skewing of observed sex ratio numbers.
Observation data is important. It can display very helpful trends. However, it should NOT be used to produce realistic herd composition numbers, only trends. In addition, care must be taken in evaluating these trends if sudden changes in harvest policy are adopted, as they can cause significant and often massive changes in observation data trends that are not necessarily "real."
)
