Comparing simple methods for ageing roe deer Capreolus capreolus: are any of them useful for management?

1. What they did

Managers of deer populations need to know not only how many animals there are, but also their ages and sexes, because these shape how a population will grow or shrink. The most reliable way to age a deer is tooth sectioning: counting yearly growth layers in the tooth cement, a bit like tree rings. But this needs laboratory equipment, or sending samples away at about 10 euros each. The authors wanted to know whether simpler methods could do the job well enough for management.

They used jaw bones, eye lenses and skulls from 77 female and 81 male roe deer, all at least one year old, collected in the Lier valley in southern Norway between 1985 and 2001. Fawns were left out because they can already be aged by which teeth have come through. Each animal’s age from tooth sectioning was treated as its “known” age. They then tested three simpler methods against it:

• Molar height (tooth wear). The height of the second molar, measured to 0.1 mm with a caliper. Teeth wear down with age.
• Eye lens weight. Both lenses from fresh heads were fixed, dried and weighed, and the average used. Lenses grow with age.
• Pedicle diameter (males only). The width and length of the pedicles, the bony bases the antlers grow from, on both sides of the head.

They also tried combining tooth wear with pedicle diameter. For each method they built a statistical model of how the measure changes with age, used it to set a range of values for each age, and then checked how many animals each method placed in the right age.

2. What they found

How each measure changed with age

• Molar height was the only measure that changed steadily (in a straight line) with age.
• Eye lens weight levelled off and did not increase after about seven years of age. Females had heavier lenses than males.
• Pedicle diameter seemed to stop increasing at about four to six years of age, although this rested on only two data points.

How well each method aged the deer

• Apart from one-year-olds, no method placed even half the animals at their correct age (but see the note in the detailed keywords: the paper’s own table shows a few exceptions).
• Allowing an error of one year either way, tooth wear was right more than 70% of the time up to four years of age, but tended to overestimate age.
• Eye lens weight, allowing one year either way, was right more than 60% of the time up to five years in both sexes, and for ages up to five it did better than tooth wear.
• Pedicle diameter, allowing one year either way, was right more than 60% of the time up to four years.
• Errors from tooth wear and from pedicle diameter were not linked, so combining the two improved accuracy slightly. Errors from eye lens weight and pedicle diameter were linked, so combining two body size measures adds little.
• Only three males and six females were six years or older, so the authors could not judge accuracy for old animals.

3. What they concluded

• None of the simple methods can age roe deer precisely, even up to five years old.
• If moderate precision is enough (one year either way, right about 70% of the time up to four years), molar height is a very simple and objective method that the authors consider suitable for most routine studies. Because it is objective, it also suits long-term monitoring where the people doing it often change. Scoring tooth wear by eye depends on the observer’s experience: inexperienced people tend to overestimate the age of young animals and underestimate the age of old ones.
• Eye lens weight performs quite well, but preparing lenses needs equipment most managers do not have, so it is less practical for large yearly samples.
• Pedicle diameter is the least useful method, with no clear advantage over tooth wear, and it works only for males.
• Tooth wear can differ between areas, so these scales may not work as well elsewhere. The authors clearly recommend calibrating them for each area.
• Methods that age live animals without pulling teeth could also help capture-mark-recapture research, although measurement errors on live animals would likely be higher.

6. Other technologies, skills or innovations

• Tooth sectioning (cementum layer counts) by the Matson Laboratory in the USA, used as the reference age.
• An objective tooth wear measure (molar height with a caliper) instead of a subjective wear score.
• Eye lenses fixed in 10% formalin for 10 days, dried at 80°C and weighed to 0.001 g.

7. Implications for conservation (from the paper)

• For managers, the main need is to sort a population into stages: young, subadult, adult and old. Harvested populations tend to be young on average, so high precision for old animals may matter less.
• Simple, objective ageing such as molar height can support yearly monitoring of a population’s age structure, as long as the scale is calibrated for each area.
• More information is needed to predict how molar wear rates vary from place to place, to avoid biased age estimates.