Burrow Dimensions of Ground Squirrels, with Special Reference to the California Ground Squirrel

1. What they did

Ground squirrels dig burrows for shelter from predators and bad weather, for raising young, for hibernation and as quick refuges. Where they live on levees, their burrows can let water seep through (“piping”), leave hollows that collapse, or speed up erosion. In California’s Sacramento Valley, the California ground squirrel is common on levees and is seen as a threat to them. To help levee managers judge that risk, the authors reviewed what is known about how long and how deep ground squirrel burrows are, and why.

• Gathering the measurements. They searched the literature for burrow length and depth of North American prairie dogs, ground squirrels and antelope squirrels. They used only typical nest burrows of adults, leaving out juvenile burrows, temporary refuge burrows and burrows reported as the longest or deepest found. Length was the total length of all tunnels; depth was the depth of the nest chamber, or else the deepest point. Some values were measured from scale drawings.
• Comparing species. They tested whether larger species dig longer and deeper burrows, and whether more social species dig longer burrows, using a published index of social complexity.
• Drawing on earlier work. They also gathered earlier findings on how soil and burrow age affect burrow size.

This report was prepared for the California Levee Vegetation Research Program.

2. What they found

Across species

• Mean burrow length ranged from 1.4 m (golden-mantled ground squirrel) to 26.1 m (Utah prairie dog). Mean depth ranged from 0.2 m (thirteen-lined ground squirrel) to 1.8 m (black-tailed prairie dog and Townsend’s ground squirrel).
• Length in particular varied a lot within species: for example 2.9 to 17.4 m in Great Basin ground squirrels and 3.7 to 29.3 m in white-tailed prairie dogs.
• Larger species dug longer burrows (r = 0.53, 17 species) and deeper burrows (r = 0.57, 18 species).
• More social species dug longer burrows (r = 0.71, 13 species).
• Burrow length and depth also went together (r = 0.56, 17 species).

California ground squirrels

• Burrow lengths ranged from 0.9 to 42.1 m, with a mean of 7.5 m and a median of 4.6 m; 79% were under 10 m long. Depths ranged from 0.2 to 1.7 m (mean 0.6 m).
• Some burrows were a short, nearly straight tunnel with one or two entrances and a nest chamber; others were a maze of tunnels with many entrances (one example had 13).
• California ground squirrels hold the records for both the longest and the deepest ground squirrel burrows ever measured. The longest, in San Luis Obispo County, totalled 226 m, had 33 entrances, displaced 2.8 cubic metres of soil and housed 11 adults (6 females and 5 males). The deepest, in Fresno County, went 8.5 m below the surface.

Soil and burrow age (from earlier studies)

• Burrows tend to be longer in soils that are easy to dig but firm enough not to collapse. Squirrels in sandy soil may dig just beneath a hard clay or caliche layer, which acts as a roof.
• New burrows are often shorter than established ones. Fresh soil at entrances shows burrows keep growing; Columbian ground squirrels were estimated to lengthen their burrows by 4 to 7 m a year.

3. What they concluded

• The wide variation is somewhat surprising given the high energy cost of digging, but squirrels often take over empty burrows or inherit them from relatives, so the cost may be spread over generations.
• At least four factors shape burrow size: body size, soil type, time (burrow age) and sociality.
• Larger squirrels likely need more underground space. Why they also dig deeper is less clear; tunnels slope down steeply from the entrance, so a longer system may also be a deeper one.
• Social species may dig bigger systems to house several residents. In less social species, long systems may arise when neighbouring burrows join up at high densities and the residents tolerate the connections.
• California ground squirrels have a low index of social complexity (0.26, the lowest in the study), and adult females are thought to live alone. Yet large shared systems have been found. These may form where dense aggregations (up to 90 adults per hectare) persist over time.

6. Other technologies, skills or innovations

• Burrow lengths were measured from published scale drawings where authors did not report them.
• A published index of social complexity (Blumstein and Armitage 1997) was used to compare sociality across species.

7. Implications for conservation (from the paper)

• California ground squirrels can dig right through a levee, although most burrows are far shorter than that. Short burrows on opposite sides of a levee can also come close to meeting, raising the risk of piping.
• Winding burrows and many burrows close together can leave hollows that are prone to collapse.
• Levees built from loose material dredged from the river bottom may be easy to dig, and harder layers inside a sandy levee may give squirrels a “roof”.
• The longer squirrels live on a levee, the bigger their burrows are likely to get. Burrows were shorter where squirrel numbers were regularly controlled (Berentsen and Salmon 2001). Populations that stay dense over time may produce longer and more interconnected burrows.