Burrow System of Richardson's Ground Squirrels
(also known as gophers)
Richardson's ground squirrels spend the majority of their lives in underground burrows. Depending on age and sex, hibernation lasts for 4 to 8 months, during which time the squirrel is underground 24 hours a day, 7 days a week, week after week, month after month. In the active season, the burrow system is used for sleeping, copulation, raising young, and as a refuge from inclement weather and most predators.
Burrow System Architecture
The burrow consists of a network of underground tunnels that are inter-connected to underground chambers and to surface openings and which extend for up to 10 m and go as deep as 1 m. A single burrow system commonly has 5-10 surface openings, 2-5 grass-lined sleeping chambers, and a latrine.
Burrow System Use in the Active Season
Each adult female Richardson's ground squirrels owns at least one burrow system, and commonly uses two different systems during the active season. Although burrow systems can persist for years, they frequently undergo renovation and modification when squirrels create new tunnels and chambers or, sometimes, close off unwanted sections.
During the active season, an adult female usually sleeps in 8 to 10 different underground chambers. If a mother changes chambers during the lactation period, she moves her litter, carrying young one at a time in her mouth. Usually the mother moves the litter from one chamber to another within the same burrow system, but sometimes mothers move the litter to a different burrow system - in which case she must carry each infant above ground to the new site and keep returning until she has moved the entire litter to the new location.
During pregnancy and lactation, females usually use the same sleep site for 10-20 consecutive nights. About 6 weeks before hibernation starts, their pattern of use changes such that females spend only 2-5 nights in a sleep site before switching to another one, often returning to a previously used chamber.
Male Richardson's ground squirrels tend to use burrow systems not occupied by females.
Richardson's ground squirrels are very conscious of their surroundings. If a burrow system becomes vacant, nearby squirrels begin investigating it within hours of its owner's disappearance. Consequently, population reduction is often rapidly followed by reinvasion.
The hibernaculum is the underground chamber in which a Richardson's ground squirrel hibernates alone. The hibernaculum system consists of a hibernaculum chamber with a single entrance connected to a tunnel that divides into a drain tunnel to carry away moisture and an exit tunnel that reaches almost to the surface. The hibernaculum system is constructed off the main burrow system. However, when the squirrel enters hibernation, it severs all connections to the main system and the surface by blocking any inter-connecting tunnels with soil. Thus, the hibernaculum system is a closed system during the hibernation season. To emerge from hibernation in the spring, the ground squirrel must extend the exit tunnel by excavating to the surface.
During the 4-6 weeks before hibernation, Richardson's ground squirrels collect nesting material, such as dried grass, to line the hibernaculum. Many male squirrels also collect seeds and cache them under the grass lining in the hibernaculum; however, these seeds will not be eaten until many months later, between termination of the last torpor bout and emergence in spring. Female Richardson's ground squirrels do not cache food in the hibernaculum.
The hibernation systems of male and female Richardson's ground squirrels have similar architecture, but males have a larger hibernation chamber than females (28 cm versus 22 cm in diameter). The size difference is due in part to the larger size of the males compared with females and in part to the storage space used by males to cache seeds. The typical depth of the hibernaculum chamber is 50-60 cm.
Although the hibernaculum is prepared weeks in advance of its use, this chamber is not used as a sleeping site during the active season. Advantages of not sleeping in the future hibernaculum include minimizing build up of odor, which could attract predators, and minimizing contamination of bedding with ectoparasites, such as fleas, ticks, or mites.
When a Richardson's ground squirrel enters hibernation, it blocks all entries into the system with soil plugs, thereby sequestering itself in a closed system and reducing vulnerability to adverse microclimate changes and limiting ingress by predators. The only predators able to gain access to the hibernaculum are badgers, which can rapidly excavate through soil and capture a ground squirrel before it has time to arouse from torpor. In years with heavy badger predation, as many as 60% of hibernating ground squirrels are captured. In the absence of badger predation, 85-95% of ground squirrels survive hibernation.
Sources: PDFs of many of these articles can be downloaded from the Michener Publications page
- Michener, G. R. 2002. Seasonal use of subterranean sleep and hibernation sites by adult female Richardson's ground squirrels. Journal of Mammalogy, 83: 999-1012.
- Michener, G. R. 1993. Sexual differences in hibernaculum contents of Richardson's ground squirrels: males store food. pp. 109-118 in Life in the Cold: Ecological, physiological, and molecular mechanisms. (eds. C. Carey, G. L. Florant, B. A. Wunder, and B. Horwitz.) Westview Press, Boulder.
By other authors:
- Charge, T. D. 2001. Hibernation biology of Richardson's ground squirrels: Energy utilization and hibernaculum systems. M.Sc. thesis, University of Lethbridge, Lethbridge, Canada. http://hdl.handle.net/10133/151
- Ovens, C. 2011. Kinship and use of underground space by adult female Richardson's ground squirrels (Urocitellus richardsonii). M.Sc. thesis, University of Lethbridge, Lethbridge, Canada. http://hdl.handle.net/10133/2619