Hibernation is a topic that has attracted attention from physiologists, ecologists, pharmacologists and biochemists. Most of these studies have utilized animals in laboratory settings, and none have addressed the physical environment of the hibernacula and how it relates to the physiology and behavior of its hibernating occupant. In a field study at the Toolik Lake Research Station on the North Slope of Alaska, I set out to determine whether over-winter temperature profiles differ among arctic ground squirrel (Spermophilus parryii) hibernacula sites and how hibernacula site quality as measured by winter soil temperature influences the occupant's over-winter survivorship, change in body condition and spring reproductive condition.
Over-winter soil temperature profiles revealed that there is considerable variability in hibernacula quality with respect to minimum temperature reached, degree days below zero and the date soil temperatures first dropped below 00 C. Variation can be attributed to differences in depth, physical aspect, soil conductivity, and particularly snow cover (depth and density) that accumulates above the different hibernacula sites. Minimum winter soil tempera ture at hibernacula depth of the 26 instrumented sites ranged from -11.2 to -27.30 C. These temperatures are considerably lower than any recorded from other ground squirrel species. The number of degree days below zero for the sites ranged between 1238 and 2572. The maximum depth of the hibernacula is limited by the top of the permafrost layer, and it is just above the thawed-frozen soil interface that arctic ground squirrels make their nests. Although the depth of the active layer in the Toolik Lake area averages 40-50 cm, the hibernacula of arctic ground squirrels that I have investigated are located in soils with active layers averaging 1 meter in depth and often in the lee of hills and ridges where snow tends to drift, implying that squirrels may preferentially select sites for burrows in well with deep active layers.
In this study, I have attempted to relate hibernaculum quality to the ability of the hibernating occupant to survive winter and emerge in sufficiently good body condition to take on the energetically expensive behaviors associated with breeding. Since arctic ground squirrels are limited in their ability to decrease their body temperatures during hibernation to -30 C, any decrease in soil temperatures below this level will require active thermoregulation by the animal. The lower the soil temperature drops, the greater will be the thermal gradient between the body and the soil, and the more energy in the form of stored fat the ground squirrel must expend. Significant differences in the metabolic rate necessary to sustain constant body temperatures while hibernating should result in widely varying costs of hibernation and variation in the body condition of ground squirrels at emergence from hibernation in spring. In other ground squirrel species it has been reported that without substantial excess fat left over after hibernation has ended, males will forgo puberty and all breeding opportunities for that year.
Although hibernacula quality varied among sites, there was no detectable correlation between this environmental parameter and the occupant's over winter survivorship and spring reproductive condition and status. All squirrels inhabiting the instrumented hibernacula were trapped upon first emergence in spring indicating little, if any, winter mortality in this population. About 15% of the males emerged lacking behavioral, morphological and endocrinological signs of being reproductive. Unfortunately, all of these animals emerged from hibernacula that were not instrumented with data loggers so I was unable to determine whether hibernacula temperature was causal in this physiological phenomenon. All of the females emerged in spring fully reproductive and were impregnated within 2 -5 days of emergence. All animals lost body mass and fat during the hibernation season, however, this decrease appears to be not strongly correlated to hibernacula quality.
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