Antler Morphometry in a Colorado Mule Deer Population

1. What they did

Antlers are often used to judge the condition of a deer herd, but little had been measured for Rocky Mountain mule deer from one large population of known ages. The authors asked whether left and right antlers are the same size, how antlers change with age, whether they vary from year to year with rainfall and winter food, and which simple measurements best predict antler weight.

They worked with the migratory mule deer of the Cache la Poudre River drainage in north central Colorado, from 1960 to 1965:

• Check station sample. Antlers of hunter-killed deer were measured at a voluntary check station over six hunting seasons. Deer were aged by tooth replacement and wear and grouped into three age classes: 16 to 18 months (yearlings), 28 to 30 months, and 40 months or older.
• Collected sample. Deer shot twice a month throughout the drainage from 1961 to 1965 were taken to the laboratory, aged to the nearest month from tooth cementum, and weighed. Their antlers were sawn off within 8 hours of death and weighed.
• Measurements. Beam diameter 2.5 cm above the burr, beam length, inside spread, the number of points (2.5 cm or longer, not counting brow tines), and the presence and length of brow tines.
• The environment. On three winter-range study areas they measured yields of the three main winter browse plants, weekly precipitation and soil moisture. They estimated deer numbers by counting and clearing deer fecal (pellet) groups twice a year on permanent 100 square foot plots.

2. What they found

Left and right antlers

• Of 36 paired tests on beam diameter, length and number of points in the two younger age classes, only three showed a significant difference, close to the two expected by chance. Left and right antlers were essentially symmetrical.
• In 25 sets of antlers, left and right beam weights did not differ.

Points, spikes and brow tines

• Among 646 yearlings with symmetrical, normal antlers, 14.9% were “spikes” with one point per beam. The yearly share of spikes ranged from 6.3% (1964) to 22.6% (1961), a significant difference.
• About 6% of 820 yearlings had six points and 0.1% had eight.
• Brow tines became more common with age: 5.9% of 845 yearlings, 43.3% of 374 deer aged 28 to 30 months, and about 77% of deer 40 months and older (Table 2). Among 28 to 30 month old deer, brow tines varied from 26.7% (1961) to 55.6% (1965).
• Antler weight was closely related to brow tine length (r = 0.80).

Year-to-year differences

• Beam diameter, beam length and number of points all differed significantly between years. In general they were lowest in 1961 and similar in the other years. Together with the many spikes and few brow tines that year, this suggests antler development was relatively poor in 1961.
• From 1961 to 1964, rainfall, soil moisture and browse yields mostly fell, with three- to fivefold drops in mountain mahogany and bitterbrush. Deer numbers stayed stable or rose slightly. Antler development did not appear to follow these changes.

Predicting antler weight

• Beam weight was the most variable measurement and beam diameter the least.
• In yearlings, beam length was the best single predictor of beam weight (r = 0.82); in deer about 27 months and older, beam diameter was best (r = 0.82). The number of points was the poorest predictor in both.
• Combining diameter, length and points improved prediction only slightly for yearlings and not at all for older deer. The measurements were only fair predictors of the highly variable antler weight.

Weight, age and body size

• In 23 collected deer, total antler weight rose in a straight line with age, by about 24 g per month (r = 0.94). Variation grew with age, especially after 40 months.
• Antler weight rose steeply (curvilinearly) with eviscerated carcass weight (r = 0.85).
• Antlers made up 0.2% to 1.9% of bled carcass weight. One set still in velvet, from a 73-month-old deer, weighed 4.33 kg, 3.9% of bled and 5.2% of eviscerated carcass weight.

Sample size

The yearly yearling samples were large enough for the chosen precision (5% of the mean), but samples of 28 to 30 month old deer were often too small. About 210 yearling beam weights would be enough.

3. What they concluded

• Left and right antlers can be treated as essentially the same.
• The environmental factors they measured were not strongly related to average antler development. This may be because the measurements were not precise enough, or because other factors masked the effect: the winters were mild and other foods were available, overall diet quality (protein, phosphorus, calcium) may matter as much as browse quantity, and the deer spend late antler growth on what appears to be excellent summer range.
• Year-to-year differences in antler development may be hard to detect in migratory mule deer, and more evidence is needed. Antlers may still be useful for comparing areas with large differences in habitat or deer density.
• The weight of a single yearling antler beam may be the most reliable and sensitive measure of male condition or environmental change: yearlings respond more to stress, are easily aged, are plentiful and have little trophy value, and a single beam is a direct and precise measure.
• The relationships between antler weight, carcass weight and age need testing in other populations, which may need their own prediction equations. Possible uses are estimating the carcass weight of antlered males when other body parts are missing, and estimating the age structure of antlered males on a winter range by weighing a sample of shed antler beams.

4. How tracking was or was not used

The study is about antlers, not tracking. The only use of signs was to estimate deer numbers: fecal (pellet) groups were counted and removed twice a year on permanent 100 square foot plots, and the counts were converted to deer per square mile using an assumed 13 fecal groups per deer per day.

5. Tracker or observer certification

Not reported. The paper does not say who counted the fecal groups. These counts were used only as a background index of deer density, not for the main findings.

6. Other technologies, skills or innovations

• Ageing deer to the nearest month with the dental cementum technique, assuming a June birth date.
• Standard antler measurements adapted from the Boone and Crockett Club scoring system, with beam diameter used instead of circumference because it was easier to measure accurately at a check station.
• Most analyses were run on an IBM 1401 digital computer at the Colorado State University Statistical Laboratory.

7. Implications for conservation (from the paper)

• If antlers are used to judge male condition or environmental change, the weight of single yearling beams may be the most reliable and sensitive measure.
• Antler development may still help compare regions with large differences in habitat or deer density.
• Data from shed antler beams may help assess the effects of habitat management, harvest density or climate on the age structure of males.