The Reality of Doe:Buck Ratios

The Reality of Doe:Buck Ratios

Deer sex ratios are a common topic of conversation among whitetail hunters. Other than deer density, few subjects ignite controversy as quickly as a discussion of the number of does per buck in any given parcel of woods. There are many misunderstandings regarding sex ratios, so I’ll clearly define what they are, how they are measured, and what they mean to your Quality Deer Management program.

The term "sex ratio" can be used to compare the number of bucks and does of all ages in a population, or it can be used to compare the number of antlered bucks and antlerless deer. It can also be used to compare the number of adult bucks and adult does, as well as others. Given the possible uses of the term, it’s important to clearly define what you’re referring to when discussing this subject. The definition I’ll use is the number of adult does for each adult buck in the population. The number includes deer 1½ years and older (all deer except fawns) and describes the population immediately preceding the hunting season. When comparing ratios, make sure you are referring to pre-hunt adult sex ratios. These are the ratios biologists most often refer to, and they should not be confused with observed ratios or post-hunt ratios as the latter are nearly always heavily skewed toward antlerless deer.

Observed ratios are generally skewed toward does because during hunting season antlerless deer (does and fawns) are often more viewable than bucks, and many hunters inadvertently consider fawns as adult does. Also, in areas of high buck harvest, the actual and observed sex ratios truly can dramatically favor does during and following the hunting season. However, this likely was not the case prior to the season.

I often hear hunters, outdoor writers, and even biologists refer to 10:1 or 15:1 doe:buck ratios. These cannot be pre-hunt adult ratios because as long as the deer herd is reproducing and recruiting fawns, the ratio cannot become more skewed than about five does per buck. The biological maximum is roughly 5:1 because even in the absence of doe harvest, a certain percentage of adult does in the population will die each year from old age, vehicle collisions, disease, predators, etc. Also, about 50 percent of fawns born each year are bucks, thus the sex ratio gets an annual correction when fawns are recruited. This concept is easier to understand with an example. Review the chart below, then I'll discuss this example as you continue reading.

 

1) Let’s say a hypothetical population contains 120 adult deer (fawns not included). We’ll skew this unnaturally toward does to show how quickly deer herds can correct the sex ratio – let’s say there are 100 does and 20 bucks (a 5:1 ratio).

2) During hunting season, hunters kill 90 percent (18) of the bucks and none of the does.

3) The post-hunt population is 100 does and two bucks.

4) Natural mortality is considered next. Since there are very few bucks left in the population, few will die from other causes. We’ll say one of the two remaining bucks dies (50 percent). However, at least 10 percent of the does will die from natural causes.

5) The remaining population is 90 does and one buck.

6) For our example, we’ll say each doe recruits 0.83 fawns. In 2008, a QDMA survey showed the average fawn recruitment rate in the United States was 0.83 fawns per adult doe. The rate ranged from less than 0.5 fawns per adult doe in Arizona and Oklahoma to 1.2 fawns per adult doe in Illinois and Iowa. The fawn recruitment rate isn’t the number of fawns born but the number that survive to about 6 months of age and are recruited into the fall deer population. At this rate there will be 75 fawns (about 38 bucks and 37 does; fawn sex ratios often slightly favor bucks). These won’t be added to the adult population until the following year, but last year’s fawns get added this year. For simplicity, we’ll assume last year’s population had the same number of fawns and immigration and emigration were equal.

7) Following fawn recruitment, the population has 127 does and 39 bucks for a 3:1 ratio.

This example is simplified, but it demonstrates that pre-hunt adult sex ratios can’t become as skewed as many think (as long as fawns are being recruited). If fawns are not being recruited due to herd health, significant predation, or other issues, then the annual “correction” shown above is reduced, and the ratio can remain more skewed. However, we started this population with an unnaturally skewed sex ratio, applied an unnaturally skewed harvest to it, and still had a more closely balanced population one year later. Given a deer population’s ability to correct itself, a 3:1 pre-hunt ratio should be considered heavily skewed from a biological perspective and reflects poor management of the deer population in many cases, or factors like predation. This 3:1 ratio could lead to hunters observing 10 or more antlerless deer (does and fawns) per buck during hunting season.

Conversely, just because a herd has a “good” sex ratio doesn’t mean it is properly managed. Prior to antler restrictions and liberalized doe harvests, Pennsylvania was considered to be among the poorest managed states in the country. Even then, Pennsylvania’s statewide pre-hunt adult sex ratio was less than 3 adult does per adult buck. The deer population was skewed toward females, but the bigger problem was nearly all of the bucks were yearlings, just like in our example on the preceding page. Of course, things are much improved in the Keystone State today.

There are a few methods for estimating the pre-hunt adult sex ratio, but of all of them, trail-camera surveys are far superior to the other methods, and in addition they can provide density, age structure and fawn recruitment data. A late-summer trail-camera survey is a great way to estimate the pre-hunt adult sex ratio. To help more hunters become proficient with trail-camera surveys, QDMA published a book, Deer Cameras: The Science of Scouting.

In the QDMA survey mentioned earlier, we also collected sex ratio data from state agencies for 1998 and 2008. The national average was 2.0 adult does per adult buck in 1998 and 1.9 adult does per adult buck in 2008. In 2008, pre-hunt adult sex ratios ranged from 1.1 in Connecticut and Georgia to 3.1 adult does per adult buck in Texas. You may never get a 1:1 ratio where you hunt, but well-managed herds can easily have less than two adult does per adult buck.

Do you enjoy watching bucks chase does or fight for breeding rights during the rut? Do you enjoy hearing bucks vocalize or like to grunt or rattle them in while hunting? If so, then balanced sex ratios and complete age structures have many benefits for you. Balancing these population parameters increases competition for breeding, which improves your opportunity to witness vocalizing and chasing or fighting while afield, and thus increases the likelihood of seeing bucks within range of your deer stand.

Two goals of QDM are to balance deer herds with the habitat and have bucks of many ages in the population. By accomplishing these goals you obtain balanced sex ratios (2:1 or better) with complete age structures for bucks and does. So, the next time you ask about sex ratios, be sure to follow that question up with another about the age structure of the herd.

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by Kip Adams
on April 23, 2013