Aging through growth marks in teeth of Spanish red deer

1. What they did

Like tree rings, the cementum on a deer's tooth roots lays down a wide growth layer each spring and summer and a thin, dark "rest line" each autumn and winter. The researchers wanted to know when the first marks form in different teeth, and how well counting them predicts age.

They collected teeth from Spanish red deer killed in sport-hunting seasons and management culls in Sierra Morena from 1993 to 1997. They examined 141 teeth: 55 first lower molars from 51 deer, and 64 first incisors and 22 canines from 61 deer. The deer were 4 to 44 months old, with ages worked out from known date of death and birth season (most births are in May and June). Thin, stained slices were read under a microscope. Age was estimated two ways: a standardized count of rest lines, and linear regression of actual age against the number of rest lines.

2. What they found

• First marks: the first rest line appeared at 6 months in molars and 15 months in incisors. The second appeared at about 18 months in both. After that, each rest line meant one more year.
• Simple rule: age in years equals the rest lines counted, minus 0.5 years.
• Accuracy: molars aged 75% of deer correctly and 99% to within a year. Incisors aged 49% correctly and 86% to within a year.
• Errors: most were underestimates, usually under a year. No error over 2 years occurred with molars, while incisors had errors of up to 3 years.
• Canines: rest lines showed no relationship with actual age.
• Sex differences: fits were better when males and females were separated, and errors tended to be greater in females, more often underestimating age.
• Month of death: deer killed at different times can show the same number of rest lines, so the width of the last growth layer can help refine the age.

3. What they concluded

The authors recommend the first lower molar for aging deer in Sierra Morena. They consider regression preferable to simple counting when only incisors are available, which needs confirming with animals older than 44 months. Canines should not be used. Aging methods need assessing for each tooth type and species, or even for each population. Their error rate is probably underestimated, because the deer used were under 4 years old and marks become harder to count in older animals.

4. How tracking was or was not used

The researchers did not follow or read tracks and signs in the field. They read growth marks in teeth from dead deer in a laboratory.

5. Tracker or observer certification

Not reported. The paper says several people could read a section together on a shared video screen, but does not say who read the marks or what their experience was.

6. Other technologies, skills or innovations

• Decalcified, paraffin-embedded tooth sections, 4 to 5 micrometers thick, stained with haematoxylin and eosin
• Light microscope at 10 and 40 times magnification
• Video camera linked to a computer, so several people could read a section together and reduce subjective errors

7. Implications for conservation (from the paper)

Underestimating age can lead to overestimated population growth rates, cohort misidentification and misread life tables, with a risk of poor management decisions and serious negative effects on wildlife populations and the environment.