Selection of Trees for Rubs by White-Tailed Deer in Maine

1. What they did

Male white-tailed deer make “buck rubs” by stripping bark from small trees and shrubs with their antlers and then rubbing the frayed patch with their scent-producing foreheads. Earlier work suggested rubs mostly serve to show dominance, and a study in Georgia found deer chose aromatic trees. Along field edges in central Maine, the authors noticed that rubs did not seem to follow the Georgia pattern, so they tested which trees deer chose and how rubbed trees differed from those on offer.

In March and April 1986, after the rut, they searched 17 randomly placed plots (30 by 125 m) along forested field edges for rubbed trees. Only rubs made just before, during or just after the rut were sampled. For each rubbed tree they recorded:

• species
• trunk diameter at the middle of the rub, and at breast height
• height of the first branch above the ground
• smooth or rough (rugose, scaly or spiny) bark
• whether the tree was alive and whether it had aromatic substances obvious to them
• distance to the nearest other rub

To see what was available, they recorded the same details for every tree and shrub in 34 small plots (5 by 5 m) inside the large plots. They also tested whether rubs were spread at random.

2. What they found

Which trees

• Deer rubbed 12 species of trees and shrubs (103 trees in all). No tree had more than one rub.
• Selected (rubbed more than their share of what was available): Populus tremuloides (33.0% of rubs versus 15.6% available), sumac Rhus typhina (31.1% versus 5.2%) and Salix sp. (10.7% versus 1.0%).
• Avoided: chokecherry Prunus virginiana (1.0% versus 16.7%), black cherry Prunus serotina (4.8% versus 13.6%), Betula papyrifera and Acer saccharum (both never rubbed, 4.2% available).
• When the analysis was repeated using only trees of the trunk size and branch height that deer rubbed, the same species were selected and avoided.

What the rubbed trees looked like

• Smooth bark: 90.3% of rubbed trees versus 61.5% of available trees.
• Thin trunks: rubbed trees averaged 1.7 cm across at breast height (largest 8.5 cm), against 6.8 cm (largest 40.7 cm) for available trees. At the middle of the rub, trunks averaged 2.3 cm.
• Higher first branch: on average 94.2 cm above the ground on rubbed trees (lowest 30 cm), against 78.6 cm on available trees.

Aromatic trees

• The aromatic Prunus species were rubbed far less (5.8%) than their availability (30.2%), even though most of them (82.8% of 29) had smooth bark.
• Aromatic conifers made up 14.6% of available trees and 7.8% of rubbed trees, a difference that was not significant.

Where the rubs were

Rubs averaged 10.2 m from the nearest other rub (from under 0.1 m to 134 m), much closer than the 12.4 m expected if they were spread at random. Rubs were strongly clumped.

3. What they concluded

• White-tailed deer in Maine were strongly selective: they chose smooth-barked trees with small trunks (8.5 cm or less at breast height) and no low branches (first branch 30 cm or higher). Studies in Georgia and Ohio also found smooth-barked trees were chosen, and several studies found rubs clumped as here.
• Unlike in Georgia, deer in Maine avoided aromatic cherries. The authors suggest this may relate to how common these trees are: black cherry and chokecherry made up 30.2% of available trees in Maine, against 3.1% for black cherry in Georgia. Aromatic trees may draw more attention to a rub where they are scarce.
• Rubs are presumed to be both visual and scent marks linked to male dominance, so placing them on trees other deer find easily makes sense. Large trees may not be rubbed because their thick bark is hard to remove.
• Some of the variation in which species deer rub in different areas may be explained by how common those trees are and how likely other deer are to find the marks on them.

4. How tracking was or was not used

The whole study rests on finding and identifying deer rubs, a sign, along field edges, and on judging which rubs were made around the rut. No tracks or trailing were used, and the paper does not say how rubs were told apart by age.

5. Tracker or observer certification

Not reported. The paper does not say who found and identified the rubs or how rub age was judged, though all the results depend on it.