A famous zoologist, Caughley, as late as 1974, claimed age ratios were of little value to wildlife workers and challenged their use. He rightly claimed that there has been little interpretation of them, even though collected at great cost, as part of deer composition studies. Perhaps the following uses might be helpful.
It is possible to determine the age of deer by several techniques (e.g., "rings" or annuli in teeth (like tree rings), eye lens weight) and estimates can be made for each year of life.
For many reasons, some statistical, age is usefully re-grouped as merely fawn (1/2 year), 1 1/2 (or yearling), and adults. The half-year convention is related to the hunting season when most data are collected. The fawn is about 6 months old as it enters the first hunting season.
Fawns rarely produce young (there are a few unusual records from early-born animals on superior range). We assume zero. Yearlings rarely produce young on poor range. On good range, over half will bear fawns. Greatest production seems to occur at 3.5 or 4.5 but conditions are highly variable. Ten does will produce 18 fawns. Most will have twins when conditions are good.
What if a population appeared as:
BEING DEVELOPED
The adult:fawn ratio is 100:100. The proportion of adults is the reproductive component of the population. It must reproduce itself if the population is to stay stable. Three-hundred adults with some help from the yearlings (in a good year) must produce 500 young if conditions are OK and objectives are being achieved. The rate of increase is 0.5 (100 animals produce 500). The numbers are needed to estimate the number likely to be produced next year. Change is seasons, in buck hunting only, on doe hunting ... any of these can cause a major change in the population next year. Change in one factor may balance out or mask a change in another factor.
It is possible to get a rate of change in population over a few years, but the data used (e.g., deer spotlight counts or harvest data) are summary data. They are the end result of the complex workings of the population machine. The three age classes consume different amounts of food, have different sex ratios (fawns are usually 100:100 while adults may be 25:100), and each produced different numbers of fawns. Knowing this interplay and working with the parts is the job of the landowner and deer herd manager.
The Lasting Forests Deer program Super Pop allows explorations of these relations and their effects over a 5-year period. Knowing the age structure-at least 3 categories, seems essential for managing any deer herd. Not ratios, but estimates of real numbers in each category seem essential.
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Giles, Jr.
Last revision January 17, 2000.
