REU: Leah Card
 
Physiological and behavioral adaptations of the Golden-mantled ground squirrel
to the arid alpine environment of the White Mountains

Abstract
Introduction
Methods
Discussion
Acknowledgements
References
Appendix

Powerpoint

ABSTRACT

Using temperature-sensitive radio transmitters on collars, the skin temperatures of fourteen golden-mantled ground squirrels (Spermophilus lateralis) were measured for the non-active time periods before dawn and after dusk to determine whether periods of torpor, when Tb (body temperature) < 30°C were used during the summer days preceding winter hibernation. Four of the fourteen squirrels showed at least one state of torpor during this time period. The reasons for the squirrels’ use of this daily torpor during the summer are not completely clear. Radio telemetry was also used to find the burrow locations for each squirrel. Using a geographical positioning system (GPS), the burrows were mapped. Then, nightly tracking determined if the squirrels moved from their original burrow. Seven of the fourteen squirrels moved from their original burrows. Two of these seven moved from their original burrows to a new burrow at which they remained. One squirrel moved from its original burrow to a second burrow, and then it moved from this burrow to a third burrow at which it remained. The remaining four squirrels moved frequently between two to three burrows. The reasons for the squirrels’ use of more than one burrow are not clear and the distances traveled are not consistent among the seven that move. All four of the squirrels that used daily torpor lived in more than burrow. Statistical analysis of the torpor use and burrow movement was used to determine whether there was a substantial difference between the male and female squirrels, however due to the small number of squirrels, no conclusions could be made.

INTRODUCTION

Many mammals use hibernation as a way to survive in extreme environments where the temperatures are very high or low and where the food and/or water are scarce. At an elevation of 3,810 m, the temperatures in the White Mountain range of California can be as low as -23.7 °C and the winds can be 23 m/s in mid-winter months (McCurdy, G. 2004.) The golden-mantled ground squirrel (Spermophilus lateralis) that lives near Mt. Barcroft, south of White Mt. Peak, uses hibernation for about 8 months of the year, from late August to April, to survive the harsh winters. Like many hibernators, the ground squirrels reproduce immediately following spring emergence giving birth to 1 litter per year of 4–6 young born in early summer, after gestation of 26–33 days (Kunz et al. 1998; Michener 1998). During the brief summer months, the ground squirrels spend most daylight hours foraging in order to fatten for hibernation.

During this hibernation period, the squirrels undergo bouts of torpor, which is characterized by the rapid decline in metabolism and body temperature (Tb). Rates of metabolism during deep torpor are typically less than 3% of euthermic, or non-torpid, rates at the same ambiant temperature (Ta), or environmental temperature (Geiser 1988; Heldmaier and Ruf 1992.) Torpor is defined for the ground squirrel to be when the squirrel’s Tb < 30°C (Barclay et al. 2001.) While many studies have focused on the physiological changes in the squirrels during the hibernation period, little is known about the initial onset of torpor.

One of the goals of this study was to determine how the golden-mantled ground squirrels immerge, or enter, the torpid state in the first bout of their hibernation. One of the hypotheses tested in this study was that the squirrels enter hibernation gradually by using daily torpor (when Tb <30°C) during the summer nights in preparation for hibernation. In this study Tb was determined by measuring the squirrels’ skin temperature (Tsk.) Measuring Tsk is a less-invasive way to measure Tb of an animal because it only needs to touch the skin of the animal rather than be surgically implanted.

The second goal of this study was gain a better understanding of the burrowing behavior of the squirrels. The specific questions that were asked were if the squirrels live in one or more burrows and if the squirrels have different burrows for the summer and winter. It was hypothesized that the squirrels move from a summer burrow to a more sheltered winter burrow prior to hibernation.

METHODS

Fourteen AVM Instruments Co. model G3-1V radiocollars were set to send signals with frequencies ranging from 160.073-161.651 MHz. These collars were placed on eight female and six male golden-mantled ground squirrels. Each collar was individually calibrated to measure the squirrels’ Tsk to the nearest 0.5°C. After the squirrels were released with their collars, the signals were located using the AVM Instruments Co. model LA 12-Q radiotelemetry receiver and an antenna. The time for the collars to send 50 temperature-dependent signals was recorded for each squirrel. The shorter the time period was for 50 signals, the higher the Tsk. The recorded time was used to calculate the Tsk for each squirrel. The exact time of day when each tracking occurred was recorded in order to know Ta at that time.

The majority of tracking and recording of the signals occurred before sunrise (5:30am-6:00am) and after sunset (7:45pm-8:15pm), when the squirrels would be in a resting state. By recording the temperatures at these times, the normothermic, or non-torpid, Tsk for each squirrel was determined (Barclay et al. 2001.) Therefore a period of torpor, when Tb < 30°C, would be noticed as comparably different. The squirrels’ Tsk was also recorded for various times throughout the day and night in order to determine the normothermic temperatures during activities such as foraging and sleeping.

Radio telemetry was also used to locate the squirrels in their burrows. Using the attenuator of the receiver, which decreased the frequency/volume of the signal, the exact location of each squirrel in its burrow could be found. Once the burrow site had been located for each specific squirrel, it was marked with a visible flag. Then, using the Garmin GPS 12XL, Version 2.01 N. America City Data mapping unit, the latitude, longitude, and altitude was recorded for each burrow site. The burrows were located each night. This information about each burrow was used to detect if a squirrel stayed in the same burrow or moved to a different burrow.

The characteristics of each burrow were recorded: altitude, incline, direction that the burrow hole was facing, and surrounding environmental elements. These characteristics allowed for the comparison between two different burrows that a certain squirrel lived at.

RESULTS

Daily Torpor:

Of the fourteen squirrels that skin temperatures were recorded for, four decreased their Tsk to below 30°C at least once in the period of days between July 14th and August 9th, 2004. On the mornings of July 17th and 18th, Squirrel no. 2 decreased its Tsk to 29.6°C and 28.28°C with a long photoperiod (14.2:9.8-h light-dark cycle) and with the air temperature at 6.9 and 7.7°C. In between these two separate bouts of daily torpor, the squirrel raised its Tsk to 35.5°C. The temperature changes of Squirrel no. 2 are demonstrated in Figure 1 in Appendix A.

During the entire day of July 15th, squirrel no. 4 was in a torpid state. This squirrel lowered its temperature to 28.78°C in the morning and 27.98°C in the evening. In a separate instance on August 5th, this squirrel decreased its Tsk to 29.58°C. The changes in the Tsk of Squirrel no. 4 are demonstrated in Figure 1.

Squirrel no. 5 decreased its Tsk to 27.89°C on the night of July 18th. This decrease in temperature is demonstrated in Figure 1.

Squirrel no. 13 also decreased its Tsk< 30°C to 29.7°C during the morning of July 16th. This decrease in Tsk is demonstrated in Figure 1.

The majority of the squirrels’ lower Tsk’s, including when Tsk < 30°C, occurred during the early hours around 5:00-6:00 AM when the Ta ~7-8. The skin temperatures for all 14 squirrels are presented over a typical 48-hr period in Figure 2.

Burrowing Behavior:

The burrowing behavior of the squirrels was researched by locating the squirrel each night over a two-week period. During this time period, seven of the fourteen squirrels moved from their original burrow. Two squirrels moved from their original burrows to a new burrow at which they remained. One squirrel moved from its original burrow to a second burrow. Then the squirrel moved from this burrow to a third burrow at which it remained. Three squirrels moved frequently between 2 different burrows. Lastly, one squirrel moved frequently between 4 burrows. Two of the female squirrels shared burrow locations for two of their burrows.

Squirrel no. 2 was originally located at N 37°34.848’, W 118°14.339’ on July 20th. Later, on July 28th this squirrel was found at a different burrow located 16m from the first burrow. This squirrel remained at the second burrow during the remainder of the study. Squirrel no. 13 was originally located at N 37°34.995’, W 118°14.250’ on July 27th. This squirrel remained in this burrow until August 2nd when it moved 150m to a new burrow at N 37°35.067, W 118°14.201’ where it remained.

Squirrel no. 5 was originally found at N 37°35.079’, W 118° 14.104’ during the night of July 21st. It remained in this burrow until July 30th, when it moved 80m to a new burrow located at N 3735.120, W 118°14.092’. The squirrel remained in this second burrow until August 1st when it moved 200m to a third burrow located at N 37°35.135’, W 118°14.201’. This squirrel remained in its third burrow for the remainder of the study.

Squirrel no. 4 was originally found on July 21st under the garage of the station at N 37°35.022’, W 118°14.162. It remained at this burrow until July 31st when it moved 20m to N 37°35.032’, W 118°14.170. Then the squirrel made two more moves to and from the first and the second burrow.

Squirrel no. 1 was originally located at N 37°34.746’, W 118°13.919’, south of a large boulder hill on July 20th. It remained at this burrow until July 29th when it moved 250m to the north side of the boulder hill at N 37°34.873’, W 118°13.964. Then on August 3rd, this squirrel moved back to the first burrow only to return to the second burrow on August 4th to remain for the rest of the study.

Squirrel no. 7 was originally located at N 37°34.986’, W 118°14.166’ within the rock formation of the east side of the bridge over a ditch along the road to Barcroft on July 22nd. The squirrel remained there until the night of July 30th when it moved to a burrow located at N 37°34.938’, W 118°14.202’under a boulder of a tundra field. On August 4th, this squirrel returned to its original burrow in the bridge only to return to its second burrow on August 5th. Once again, on August 7th, this squirrel returned to its original burrow, returning to its second burrow on August 8th.

Squirrel no. 12 was originally located on July 27th at N 37°34.981’, W 118°14.161’ underneath a row of old, wooden telephone poles. The squirrel remained at this burrow until July 30th when it moved 20m to a burrow located at N 37°34.981’, W 118°14.160’. On August 2nd, this squirrel once again moved back to its original burrow. Then on August 4th, this squirrel moved 100 m to N 37°34.938’, W 118°14.200’ for its third burrow. On August 5th, the squirrel moved back to its second burrow. On August 6th, the squirrel moved to a fourth burrow near the second burrow, on the western edge of the bridge at N 37°34.986’, W 118°14.166’. On August 7th, this squirrel once again returned to its original burrow location under the telephone poles. Lastly, on the night of August 9th, this squirrel moved to its fourth burrow on the western edge of the bridge.

The distances traveled by the seven squirrels that moved from their original burrow are presented in Figure 3.

Figure 3: Comparison of distances travelled by moving squirrels

The remaining seven squirrels: Squirrel no.’s 14, 6, 9, 8, 10, 3, and 11 did not move from their original burrow.

The seven squirrels that moved included five of the eight female squirrels and three of the six male squirrels. All four squirrels which decreased their Tsk < 30° (squirrels 2, 4, 5, and 13), moved from their original burrows.

DISCUSSION

Daily Torpor:

Because four of the fourteen squirrels have decreased their Tsk < 30°C, analysis concludes that these squirrels were in a period of torpor during these low temperatures (Barclay et al. 2001). One of the squirrels that used daily torpor, Squirrel no. 2, was in a torpid state during the mornings of July 17th and 18th. Because this squirrel had a normothermic period during the early evening readings of July 17th raising its Tsk to 35.5°C, it can be concluded that this squirrel had two periods of torpor. This data and other data of the other squirrels show that squirrels have the ability to decrease and increase their Tb rapidly, demonstrating their control over their body temperature.

Of the squirrels that showed daily torpor, one was male and three were female.

Statistical analysis was used to determine if there was a clear relationship between the sex of the squirrel and the use of torpor. However, because the number of squirrels that used daily torpor was small, no clear connection was found.

Although the ambiant temperature (Ta) was recorded, there was not a clear relationship between it and the use of torpor. The lowest Tsk’s were recorded at the morning readings possibly because the squirrels begin decreasing their temperature through the previous night and raise their temperature through the day, which would explain the higher temperatures during the nightly readings ~8:00PM.

Because only four of the fourteen squirrels used torpor, it may not be a requirement to use daily torpor in order to prepare for the upcoming winter hibernation. Due to the lack of time, this study ended before the squirrels immerged into hibernation. Thus, it is not known whether the daily torpor is directly related to the winter hibernation, such as a preparation mechanism, or if it is used independently during the summer months. It is very interesting that these squirrels use a form of daily torpor infrequently. The data indicates that some squirrels keep their Tb at a relatively consistent temperature while others increase and decrease their temperatures to a large degree from day to day.

The golden-mantled ground squirrel has the ability to control its body temperature ranging from its normothermic temperature at ~ 37°C - 4°C. When compared to endotherms, which regulate their body temperatures within a few degrees, the temperature control and range of the squirrels are impressive.

Burrowing Behavior:

The data shows that seven of the fourteen squirrels have moved at least once from their original burrows. Three of the moving squirrels have moved permanently, never returning to previous burrows. The remaining four moving squirrels move frequently between the 2-3 burrows. Because the other seven squirrels have not moved from their original burrows, it must not be a requirement for hibernation. Also, because four of the squirrels move back and forth from one burrow to another, the squirrels can live in more than one burrow. Two of the squirrels live under buildings and two others live within the built bridge along the road over the ditch. Because many of the squirrels live within areas that have high human activity, this suggests that the squirrels are not greatly disturbed by humans and may use these locations for the greater protection offered by these structures from the cold conditions experienced throughout the year.

The characteristics of each burrow were compared to see whether the squirrels were choosing a second burrow for a specific reason. However, the data did not explain why the squirrels chose the sites for their burrows. The differences between the two or three burrows that the seven squirrels lived in, varied with each squirrel. There was not a consistent change in burrow characteristics.

Statistical analysis was used to determine if there was a clear relationship between the sex of the squirrel and having more than one burrow. However, because the number of squirrels that moved was small, no clear connection was found.

ACKNOWLEDGEMENTS

Frank, Craig – Fordham University

The National Science Foundation for the grant, IBN-036330, which covered all costs in this study

APPENDIX A

Figure 1 – Squirrels no. 2, 4, 5, and 13 decreasing Tsk < 30°C over time:

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