Characterisation of Eurasian lynx Lynx lynx den sites and kitten survival

1. What they did

Many young lynx die in their first year, and the place where a mother gives birth may be the first thing that affects their survival. Over almost two decades of radio-tracking Eurasian lynx in Switzerland, the researchers found 70 dens belonging to 26 radio-tagged females in the Jura Mountains and the northwestern Swiss Alps (1983 to 2000). They compared natal dens (where the kittens were born) with maternal dens (later places the mother carried the kittens to).

• Finding dens. When a female was located again and again at the same spot in the denning season, this pointed to a possible den; it counted as a den once kittens were confirmed. In the early years the den was never approached while the family was there. From 1992, natal dens (sometimes the first maternal den) were visited 3 to 5 weeks after birth to count, check and ear-tag the kittens.
• Measuring the dens (summer 2000). They went back to the old dens, using GPS locations and photos, and recorded the den itself (type, size, entrances, soil, nesting material, camouflage, cover inside and out) and its surroundings (forest type and structure, aspect, visibility, lookout terrace, hiding places, accessibility for humans). Of the 70 dens, 44 could be revisited; 26 no longer existed or could not be found.
• Mapping the setting. With topographic maps and a Geographic Information System (GIS), they worked out slope, altitude and distances to roads, settlements and forest edges, and compared the slopes and aspects of den sites with what was available in the females’ home ranges.
• Linking dens to survival. They related den features and the mother’s age to whether kittens survived to the end of September and to the following March.

2. What they found

The dens

• Most dens were in rock: 26 in rock, 13 in wood (such as tree roots, root stocks of fallen trees or piles of branches) and 2 in underground caves. Block heaps among boulders were the most common type (10 of 41).
• 71% had a closed structure, completely covered by rock or vegetation. Half had a single entrance.
• The main chamber averaged 0.97 m² (0.03 to 8.0 m²). Maternal dens varied more in size than natal dens.
• 72% had permeable humus soil, and 64% had nesting material such as leaves, branches and pebbles.
• Protection from sun and rain inside the den was complete in 83% of cases, even though cover in front of the den was often poor. Almost two thirds of dens were cooler inside than outside (the temperature measurements were not analysed).
• Natal and maternal dens did not differ significantly in structure.

The surroundings

• 71% of den sites were in mixed forest, mostly dominated by conifers; no natal den was in deciduous forest. Natal dens were only in open or half-closed forest. The more open the forest, the better the cover inside the den compared with outside.
• Maternal dens were more often rich in visual contrast (good camouflage) than natal dens (82% versus 46%), and had more hiding places nearby.
• 62% had a terrace or resting place with a view in front of the den. Visibility averaged 5 m uphill, 15 m to the sides and 20 m downhill.
• Half of the dens could easily be reached by people. Steeper sites were less accessible.

Slope, altitude and people

• Females in both regions chose dens on significantly steeper slopes than were generally available. In the Jura, most dens were on slopes of 26 to 45°, while most available terrain was 0 to 15°.
• Natal and maternal dens did not differ in slope, distance to roads or distance to settlements. Natal dens were on average about 100 m higher, but not significantly.
• Jura dens were closer to roads than Alps dens, and Alps dens were closer to forest edges, explained by altitude and the more broken forest high in the Alps.
• In the Jura, dens mostly faced east or southeast, as did the slopes on offer; in the Alps, dens did not face east or southeast at all. The authors suggest sunny slopes in the Alps have largely been turned into pasture.

Kittens and mothers

• Of 89 kittens in litters with a fairly complete record, 53 (60%) died before the next spring, but only 14 to 20 disappeared before the end of September. Survival did not differ between the regions (41% in the Alps, 40% in the Jura). The heavy losses came after the denning period, not during it.
• Only two whole litters (not counting two lost when the mothers died in accidents) and one kitten (with bite wounds) were known to be lost during denning.
• Of known causes of death, 58% were human-related (traffic or illegal killing), 19% were orphans, and 15% were unknown.
• Den features were not linked to kitten survival, and the mother’s age was not linked to survival. Older mothers chose dens with better cover inside.
• Females moved kittens from the natal den to one or two maternal dens, never more than 500 m away. Four females used exactly the same, very hard to reach, site in two consecutive years. Daughters did not choose dens like their mothers’.
• When researchers approached to tag kittens, mothers generally moved away while the kittens hid where they were.

3. What they concluded

• A good den in these areas is a closed rock structure of about 1 m², with few entrances, permeable soil and nesting material, in fairly open mixed forest on steep slopes that people find hard to reach.
• Natal and maternal dens differed little. Females seem very selective about natal dens but cannot choose as carefully when they must find another den in a hurry. Hiding places and camouflage around maternal dens may matter more because older kittens move around inside and outside the den.
• Contrary to expectation, the two den types were equally exposed to human disturbance and lay in terrain that could be dangerous for kittens.
• Den quality did not appear to limit kitten survival. Suitable sites seem so plentiful in the Alps and Jura Mountains that females have no trouble finding them, so den availability is not a conservation concern there, unlike for Iberian lynx in flat country.
• Small structures such as an overgrown block heap do not show on maps or in GIS habitat models at the available resolution.
• The study was retrospective, so some vegetation had changed and sample sizes varied. Building a convincing sample of large carnivore dens takes careful, consistent recording over many years, which the authors say they missed in their early field years.

6. Other technologies, skills or innovations

• Radio telemetry of females, with daily locations, to find dens and follow litters.
• A standardised field protocol for den measurements, with old photos used to relocate and reconstruct den sites.
• Topographic maps (1:25,000) and ArcView GIS, with Swiss GEOSTAT land-use data, to compare used and available terrain.

7. Implications for conservation (from the paper)

• Recovery programmes need suitable habitat with all relevant resources, including den sites for lynx living among people.
• In these mountainous areas, den sites are not a limiting resource, and den limitation is of no conservation concern for Eurasian lynx, unlike for Iberian lynx in flat habitat.
• GIS habitat models based on available baseline data cannot capture den microstructures.
• Most known kitten deaths were human-related (traffic and illegal killing).