Giant panda scent-marking strategies in the wild: role of season, sex and marking surface

1. What they did

Leaving scent marks costs an animal time and energy, so animals should place their marks where they last longest, carry furthest and are most likely to be found. Giant pandas live alone and rely heavily on scent to communicate. They mark trees with a waxy secretion from an anogenital gland (AGS), rubbed vigorously on the bark, and with urine, sprayed onto trees, sometimes from a handstand. The authors asked whether wild pandas choose their marking trees, and whether marking changes with season and sex.

From October 2007 to October 2008, in Foping National Nature Reserve in China’s Qinling Mountains, they:

• Set up transects along ridge trails, where panda scent marks are mostly found. On eight transects (6.9 km in total, 3 m wide), they measured every tree over 5 cm diameter within 1.5 m of the trail: bark roughness, moss, diameter at breast height (DBH) and distance to the trail.
• Checked every tree once a week for new marks. AGS marks were recognized by flaked bark and a thick waxy substance; urine marks by slight darkening of the bark and a strong, musky smell that could be detected even when the mark was hard to see.
• Told old marks from new ones. They cut a small piece of bark from the middle of each AGS mark with a clean knife; if the lighter patch had darkened by the next visit, the tree had been marked again. Urine smell faded after about 5 days, so a fresh smell meant a new mark.
• Sexed the markers from hair. Where fresh hair was left on a tree with an AGS mark, they collected it (once per transect per day) and used DNA to tell males from females.
• Compared marked trees with all available trees, and marking rates across months.

2. What they found

Marks recorded

• 2186 trees were available along the transects. The team recorded 315 AGS marks on 219 trees and 34 urine marks on 34 trees; five trees had both.
• Pandas marked 11.5% of the available trees during the year, and 38.3% of these scent trees were marked more than once. One tree received five AGS marks.

Which trees pandas chose

• Bark: pandas put 67% of their AGS marks on rough-barked trees, though these made up only 37% of trees. Urine marks went most often on the roughest bark. Smooth-barked trees were avoided for both kinds of mark.
• Moss: 94% of AGS-marked trees were free of moss. Urine marks were placed on mossy trees about as often as those trees occurred.
• Size: urine-marked trees were larger than both available trees and AGS-marked trees. AGS-marked trees were of ordinary size.
• Distance to the trail: both kinds of marked trees were closer to the trail than available trees.

Season and sex

• No marks were found in July and August, when pandas moved to higher elevations to feed on bamboo shoots.
• Marking varied clearly through the year, peaking in February (15.0 marks per km) and November (10.8 marks per km).
• Of 28 hair samples, 19 were from males and 9 from females. Female marks were found mainly in the mating season (about 78%), while male marks were spread across the mating and non-mating seasons (Figure 3). The authors call these data limited.

3. What they concluded

• Wild pandas place their scent marks in ways that fit signal detection theory: on surfaces and in places that help the signal last, spread and be found.
• Rough bark may hold urine in its crevices at nose height rather than letting it run down the trunk, give more surface for the scent to spread from, and resist rain. Large trees may simply be easier targets when spraying urine backwards from a handstand.
• AGS marks carry individual identity and are used as long-lasting range marks; pandas respond to AGS more than 3 months old but to urine only up to about 2 weeks. Moss may cover the bark’s sap, which is thought to help scent last; this could explain why pandas avoid mossy trees for AGS.
• Marking close to the trail may help other pandas find the marks, though it may also just be convenient.
• The February to March peak matches the mating season at Foping. The November peak matched a second, autumn mating season, and the team saw a November mating during the study.
• Males appear to mark year-round, probably as a kind of range mark that shows they are present and their competitive status; females mark mostly in the brief mating season. Pandas do not seem to be territorial.
• The results should be treated with caution, because marks could not be assigned to individuals.

4. How tracking was or was not used

The study is built entirely on signs. Observers walked ridge trails weekly and identified panda scent marks on trees: AGS marks by flaked bark and a waxy substance, urine marks by darkened bark and their distinctive musky smell. They aged and separated marks by cutting a small window in the bark of AGS marks and by the fading of urine smell after about 5 days. Hair left at fresh AGS marks was collected for DNA sexing. No trailing was reported. The authors note that pandas are hard to see in dense bamboo, so signs like scent posts are a way to learn about their behaviour, and that they likely missed some marks, so true marking rates are probably higher.

5. Tracker or observer certification

Not reported. The paper does not say who checked the trees. It notes that the urine odour “is so distinctive to the experienced nose” that fresh urine marks could be detected even when not clearly visible, but gives no detail on observers’ experience. Field staff of Foping National Nature Reserve are thanked.

6. Other technologies, skills or innovations

• A simple field method for telling new AGS marks from old ones: a small cut in the bark reveals lighter wood, which darkens when the tree is marked again.
• Using smell to detect and age urine marks.
• Sexing hair from scent trees with a giant panda Y-linked DNA marker, with three PCR reactions per sample and zoo panda blood as controls.

7. Implications for conservation (from the paper)

• Trees, especially old growth, are already known to be important for pandas. This study adds another service trees provide: a platform for communication.
• Pandas live alone for most of the year and rely on efficient scent communication to come together for mating and to manage competition. Human activities that reduce the availability of, or access to, scent trees, particularly large trees with rough bark, may further harm panda conservation.