The role of den quality in giant panda conservation

1. What they did

Giant panda cubs are born almost naked and helpless; the authors note they are the most altricial (least developed at birth) of all placental mammals. Pandas do not hibernate, but mothers raise their cubs in dens for at least three to four months after birth, in tree hollows or rock caves. Earlier work in Foping Nature Reserve, where old-growth trees have almost all been logged, looked at how pandas choose rock dens. This study looked at tree dens in old-growth forest, and at how well dens shelter cubs from the weather.

• Tree den survey. In Fengtongzhai Nature Reserve, Sichuan, the team searched on foot for tree cavities big enough for an adult panda to enter and stand on all fours. A cavity counted as a used den if a mother and cub were found there during monthly monitoring in the cub-rearing season (2005 to 2010), or if it held panda signs: fur, scratch marks or bedding material.
• Den features. At each cavity they measured 19 features of the den and its surroundings, such as slope, tree size, entrance and chamber size, and how well the den was concealed. They compared used dens with unused cavities, and compared tree dens in a general way with the rock dens described earlier in Foping.
• Den climate. In ten tree cavities in Fengtongzhai and ten rock cavities in Foping, they placed temperature and humidity loggers inside each cavity and 3 to 5 m outside it. These recorded every 1.5 hours from July to December, while the cavities were empty. A bigger difference between inside and outside meant better “buffering” against the weather.

2. What they found

Which tree cavities pandas used

• Of 54 tree cavities surveyed, 22 had been used as dens.
• Nine of the 19 features differed between used dens and unused cavities. Used dens were better concealed, on gentler slopes (about 16° against 28°) and in larger trees (average diameter at breast height 1.24 m against 1.10 m).
• Used dens had wider entrances, closer to the ground (about 71 cm against 113 cm), and larger chambers. Compared with the chamber, though, the entrance of a used den was relatively narrow.
• Slope, entrance width and concealment together correctly sorted 86.4% of cavities into used and unused.

Tree dens compared with rock dens (qualitative)

Tree dens in Fengtongzhai were at lower elevation, on steeper slopes, farther from water and showed more signs of use by other species than rock dens in Foping. Rock den entrances and chambers were wider and shorter, and tree dens were roomier inside relative to their entrances.

Climate inside the dens

• All dens, tree and rock, buffered against outside conditions. Inside, the average and lowest temperatures were warmer, the highest temperatures cooler, the air drier and conditions much steadier through the day and night.
• The difference was largest at night: at midday and early afternoon dens were only somewhat cooler than outside. Humidity followed the same daily pattern.
• Used dens buffered better than unused cavities. Used tree dens were on average 4.51 °C warmer than outside, unused tree cavities only 1.47 °C. Used tree dens were also drier, with the biggest difference in the colder winter months. Used rock dens also buffered better than unused rock cavities, though less strongly than tree dens.
• Used tree dens buffered better than used rock dens: by 1.44 °C more for temperature and 4.24% more for humidity, and they were also better against the coldest and most humid days.

3. What they concluded

• Den quality, including structure, surroundings and microclimate, decides the conditions in which panda cubs are raised.
• Mothers choose dens with a small entrance and a roomy chamber, which likely protects cubs from predators and weather, and concealed dens, which likely protect against predators and human disturbance.
• In Foping pandas chose steep slopes for rock dens, but in Fengtongzhai they chose gentle slopes for tree dens. The authors suggest this is because large old trees grow better on gentle slopes, while caves are more common in steep terrain.
• Tree dens give more stable and favourable conditions for rearing cubs than rock dens. The authors suggest that forests with old-growth trees may support more pandas than logged forests, which may help explain why pandas sometimes favour old-growth forest. Whether den microclimate affects cub survival still needs study.

4. How tracking was or was not used

Used dens were identified by finding a mother and cub during monitoring surveys and/or by panda signs at the cavity: fur, scratch marks and bedding material. The authors consider these signs a reliable sign of a maternal den, because pandas do not hibernate and are only known to use dens for raising cubs. How the monitoring surveys found the dens is not described.

5. Tracker or observer certification

Not reported. Who identified the signs at the cavities is not described; four people are thanked for help with fieldwork. This matters because the split between used dens and unused cavities rests partly on sign identification.

6. Other technologies, skills or innovations

• Temperature and humidity data loggers placed 1 m above the ground in the centre of each cavity and 3 to 5 m outside it, recording every 1.5 hours, day and night.
• Comparing inside with outside readings for each den let the authors compare tree and rock dens in two reserves with different climates.

7. Implications for conservation (from the paper)

• Large old trees may be a limiting resource for denning mothers, possibly helping cubs survive. Preserving them may raise the number of pandas a reserve can hold, and managers and policy makers are encouraged to prioritise them in panda habitat.
• Current panda monitoring does not include systematic surveys of large old trees; future monitoring and management plans should include them. Declines in cavity-dependent species can lag behind the loss of tree hollows.
• Where large old trees are scarce, artificial dens could be trialled, following the structure and microclimate features of preferred dens and tested for how well they buffer the weather. Otherwise they risk becoming “ecological traps” that attract pandas without giving cubs what they need. Trials are planned in Liziping Nature Reserve.
• Tree cavities and old-growth forest may become even more important as refuges under climate change.