Kip's Korner, Sex Ratios, November 2003

Hunters often ask about the sex ratio of the deer herd where they hunt and then compare that ratio to herds in other areas or states. There are a lot of misunderstandings regarding sex ratios and this article will help clarify some misconceptions. First, what is a sex ratio, what animals are used to determine it and when is it measured? The sex ratio is a number describing the number of adult females for each adult male in a population. The number includes deer 1.5 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 or post-hunt ratios as the latter are nearly always heavily skewed towards females.

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, the ratio cannot become more skewed than 5 does per buck. The biological maximum is about 5:1 because even in the absence of female harvest, 15-20% of adult females in the population will die each year from old age, vehicles, disease, predators, etc. Also, about 50% of fawns born each year are male (it's actually slightly more than 50%), thus the sex ratio gets an annual correction when fawns are recruited. This concept is easier to understand with an example.

Let's say a hypothetical population contains 120 adult deer (fawns not included).

Pre-hunt population = 100 does and 20 bucks (this is a 5:1 ratio)

During the hunting season let's say hunters kill 90% (18) of the bucks and 0% (0) of the does.

Hunting mortality 0 does and 18 bucks

Post-hunt population = 100 does and 2 bucks (50:1, heavily skewed after the hunt)

Natural mortality gets added next. Since there are very few bucks left in the population, very few will die from other causes. We'll say 1 of the 2 remaining bucks dies. However, 15-20% of the does will die from natural causes. We'll be conservative and use 15% (15 does).

Natural mortality 15 does and 1 buck

Remaining population = 85 does and 1 buck (85:1, the ratio is still heavily skewed)

We'll be conservative again and say each of the remaining does has only 1 fawn. That means there will be 85 (about 43 buck and 42 doe) fawns. 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 are equal.

Recruitment 42 does and 43 bucks

Pre-hunt population = 127 does and 44 bucks (this is 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. However, from a biological perspective, a 3:1 ratio is heavily skewed and reflects poor management on the deer population. This 3:1 ratio could lead to hunters observing 10 or more antlerless deer (females and fawns) per buck.

The sex ratio by itself however, can be misleading. When discussing sex ratios it's important to determine the age structure of the buck population. For example, you can have 2 populations that both have 2:1 ratios. One population has only yearling bucks, the second has bucks from 1.5-5.5 years making up the ratio. Which population is better managed and which would you rather hunt? 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 state-wide sex ratio was <3:1. The deer population was skewed towards females but an even bigger problem was nearly all the bucks were yearlings. Pennsylvania's new seasons are designed to tighten the sex ratio and increase the age structure of the buck population. You may never get a 1:1 ratio but well managed herds will have <2 adult does per adult buck. The state of New Hampshire has a successful deer management program and its state-wide deer herd has approximately 1.5 adult does per adult buck with nearly 60% of the bucks being 2.5 years and older.
Kip's Korner is written by Kip Adams, wildlife biologist and Northeast regional director for the Quality Deer Management Association (QDMA). The QDMA is a non-profit wildlife conservation organization dedicated to promoting sustainable, high-quality, white-tailed deer populations, wildlife habitats and ethical hunting experiences through education, research, and management in partnership with hunters, landowners, natural resource professionals, and the public. The QDMA can be reached at 1-800-209-DEER or www.QDMA.com.