Understanding those deer sex ratios

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Rifle season opened over the weekend in Vermont and New Hampshire, and doe:buck ratios will be a hot topic of conversation around the cribbage board at deer camp.

For those who aren't up on their deer-herd lingo, the ratio of does to bucks in the woods is one indicator of how well the deer herd is being managed. The more balanced the sex ratio (the same number of does and bucks), the better. It may be impossible to get to 1:1, but well-managed deer herds will have fewer than two adult does per adult buck, or a ratio of less than 2:1.

Now, right off the bat this year, some hunters, upon seeing a bunch of does and no bucks near their hunting stand on the first morning, will report that the herd is all out of whack. Even people who should know better -- like outdoor writers and biologists -- sometimes talk about 10:1 or 15:1 doe:buck ratios. But these cannot be pre-hunt adult ratios, because as long as the deer herd is reproducing and recruiting fawns into the fall herd, the ratio cannot become more skewed than about five does per buck in northern regions.

In reality, the biological maximum on the first day of deer season is about 5:1, because even if no does are killed during the hunt, 15 to 20 percent of the adult females in the population will die each year from old age, vehicles, disease, predators, and other causes. Also, slightly more than 50 percent of the fawns born each year are male, providing an annual correction.

Let's look at a hypothetical population containing 120 adult deer. Fawns are not included. The pre-hunt population has 100 does and 20 bucks, a 5:1 ratio. Let's say that during the hunting season, hunters kill 90 percent (18) of the bucks and none of the does.

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The post-hunt population is 100 does and 2 bucks, a 50:1 ratio, 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. But we'll say 1 of the 2 remaining bucks dies. Natural causes will claim 15 to 20 percent of the does. We'll be conservative and use 15 percent (15 does). The remaining population is 85 does and 1 buck, 85:1, even more heavily skewed.

For simplicity we'll say each of the remaining does recruits 1 fawn. This is above the national average, but the point is still valid. 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. So, recruitment to adulthood adds 42 does and 43 bucks, bringing the pre-hunt population to 127 does and 44 bucks, a 3:1 ratio.

This example, though simplified, 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 of the deer population. This 3:1 ratio could lead to heavily skewed observation ratios, because many people routinely mistake button bucks and doe fawns for adult does. By their nature and behavior, does are more readily seen than bucks, which also creates a visibility bias.

The sex ratio by itself can be misleading, however, and it's only one part of a well-managed herd. It's important to also consider the age structure of the buck population. For example, you could have two deer populations that both have 2:1 ratios. One population has only yearling bucks, while the second has bucks from 1.5 to 5.5 years. Clearly, the second herd is better managed and offers a wider variety of hunting opportunities.

A herd like that is the goal of Quality Deer Management, a management philosophy that works to balance deer herds with the habitat (reducing herds by removing female deer) and to have all age classes of bucks represented in the population. By accomplishing these goals, you obtain good sex ratios with good age structure on the bucks' side. So, the next time you're discussing sex ratios at deer camp, be sure to have a follow up conversation about the age structure of the herd.

Kip Adams is a certified wildlife biologist and northern director of education and outreach for the Quality Deer Management Association. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: wellborn@nhcf.org


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