Spontaneous and induced summer hibernation in 13-lined ground squirrels

Among several mammalian hibernators, an endogenous circannual sequence of physiological events is believed to mediate the timing of torpor. Dawe and Spurrier (3) reported that a bloodborne substance (hibernation induction trigger) is important in initiating the torpor phase of those events in the 13-lined ground squirrel. We have reported the induction of summer hibernation among 13-lined ground squirrels using dialysates of serum from hibernating golden hamsters (a nonseasonal hibernator). While those animals receiving saline injections hibernated in 36.3 ± 2.9 days, an earlier induction (22 ± 8.8 days) occurred among those receiving the hibernation serum dialysate (P = 0.05). It was also observed that naive animals departed from a strict circannual rhythm and displayed a high incidence of hibernation, although not significant when compared to the experimental saline controls. The spontaneity of torpor in summer among the naive sample may in part be a characteristic of wild-caught animals employed in the bioassay. Nevertheless, the induction of hibernation among those animals receiving the hibernation serum preparations is supportive of the studies of Dawe and Spurrier (2, 3). That a “trigger” material apparently is present in the hamster, a phylogenetically distinct nonseasonal hibernator, suggests that a characteristic of rodent hibernators is the presence of a material which is associated with the initiation of torpor.     

Effect of hibernation on liver and kidney metabolism in 13-lined ground squirrels

• 1.1. Metabolic rates and adenine nucleotide content of liver and kidney from hibernating ground squirrels were measured and compared to rats to study the biochemical adaptation to hibernation.
• 2.2. High rates of renal and hepatic gluconeogenesis were observed in squirrels, particularly from propionate and glycerol compared to rat.
• 3.3. During hibernation and starvation soluble phosphoenolpyruvate carboxykinase activity was increased in both liver and kidney.
• 4.4. Although metabolic rates are decreased during hibernation the results suggest that the enzymic complement is maintained at high activity even during torpor.

     

Capture, care, and captive breeding of 13-lined ground squirrels, Spermophilus tridecemlineatus

Researchers use the 13-lined ground squirrel for studies of hibernation biochemistry and physiology, as well as for modeling a variety of potential biomedical applications of hibernation physiology. It is currently necessary to capture research specimens from the wild; this presents a host of unknown variables, not least of which is the stress of captivity. Moreover, many investigators are unfamiliar with the husbandry of this species. The authors describe practical methods for their capture, year-round care (including hibernation), captive mating, and rearing of the young. These practices will allow the researcher to better standardize his or her population of research animals, optimizing the use of this interesting model organism.     

Circannual cycles in golden-mantled ground squirrels: fall and spring cold pulses

Summary Endogenous circannual rhythms of male golden-mantled ground squirrels (Spermophilus lateralis) exposed to low ambient temperature (6 °C) at various times were compared to the rhythms of a control group kept in a warm room (21 °C) throughout. A 20-week pulse of cold in the spring delayed the subsequent peak body weight and molt offset. However, in the second year after the spring cold pulse, a delay in the rhythm was evident only using peak weight as a phase marker. A 20-week cold pulse in the fall resulted in an earlier peak body weight and earlier onset of pigmented scrotum and descended testes, but there was no evidence from subsequent years that rhythms had been phase advanced. A third of the animals kept continuously at 6 °C remained with pigmented scrotum, descended testes and low body weight for more than a year. Circannual periodicities of animals that continued to display rhythms in the cold room were not significantly longer than those of controls in the warm room. The results suggest that low temperature in the fall can alter the expression of circannual rhythms without much affecting their phasing, while low temperature in the spring produces phase delays which last longer but have not been proved to be persistent.Abbreviation PRC phase response curve     

Summer hibernation of infant (six week old) 13-lined ground squirrels, Citellus tridecemlineatus

In an endeavor to more clearly define the physiological action of hibernation trigger (previously identified by the authors) an experiment was devised using infant ground squirrels. In one case, pregnant ground squirrel mothers were transfused intravenously with cold dialysate of serum of hibernating woodchuck, and in another case, infant (1 week to 3 week) squirrels were transfused intraperitoneally with the same material. When isolated and placed in a cold room (15 °C) at 6 weeks post partum, both the infant born of the transfused mothers, as well as the infant which has been transfused directly, hibernated in the summer. This was in contrast to controls of several kind, i.e., animals not injected, or receiving active or arousing woodchuck serum or heat-treated hibernation serum. Some details for a theory, which had been advanced in a previous article (4), were essentially substantiated by this experiment. It is our belief that this represents the first time mammalian hibernation has been observed in infant hibernators.     

Sarcocystis campestris from naturally infected 13-lined ground squirrels, Spermophilus tridecemlineatus tridecemlineatus, from Nebraska

Grossly visible sarcocysts were seen in the skeletal muscles of 1 of 12 13-lined ground squirrels, Spermophilus tridecemlineatus tridecemlineatus, collected in Nebraska. The tissue cyst wall was up to 5.0 microm thick and contained spikelike projections. Transmission electron microscopy of tissue cysts revealed they were similar to Sarcocystis campestris Cawthorn, Wobeser, and Gajadhar, 1983, previously known only from experimental infections in Richardson's ground squirrel Spermophilus richardsonii. Prominent electron-dense bodies were observed lining the microfilaments present in the spikelike projections of the sarcocyst wall. This is the first report of S. campestris in a natural intermediate host and the first report of this parasite outside of Saskatoon, Canada.     

The influence of cholecystokinin octapeptide and angiotensin II on in vitro duodenal motility in hibernating and aroused 13-lined ground squirrels

Because cholecystokinin octapeptide and angiotensin II are directly involved in intestinal food and water absorption, the effect of these two compounds on intestinal motor responses of hibernating and alert 13-lined ground squirrels was investigated. Both cholecystokinin octapeptide and angiotensin II caused a greater increase in the composite motility of intestinal segments of normothermic in contrast to hypothermic hibernating ground squirrels. Additionally, cholecystokinin caused an increase in the contraction frequency of the intestine from normothermic as compared to hypothermic squirrels. This differential response may provide an adaptive advantage by decreasing food and water consumption; depressing motility and inhibiting digestive enzyme release; reducing the release of bile which also may have an irritant effect on the intestinal mucosa under prolonged exposure. As a result, the GI tract is devoid of food and digestive enzyme irritants during torporous periods.     

The hibernating 13-lined ground squirrel as a model organism for potential cold storage of platelets

Hibernating mammals have developed many physiological adaptations to extreme environments. During hibernation, 13-lined ground squirrels (Ictidomys tridecemlineatus) must suppress hemostasis to survive prolonged body temperatures of 4-8°C and 3-5 heartbeats per minute without forming lethal clots. Upon arousal in the spring, these ground squirrels must be able to quickly restore normal clotting activity to avoid bleeding. Here we show that ground squirrel platelets stored in vivo at 4-8°C were released back into the blood within 2 h of arousal in the spring with a body temperature of 37°C but were not rapidly cleared from circulation. These released platelets were capable of forming stable clots and remained in circulation for at least 2 days before newly synthesized platelets were detected. Transfusion of autologous platelets stored at 4°C or 37°C showed the same clearance rates in ground squirrels, whereas rat platelets stored in the cold had a 140-fold increase in clearance rate. Our results demonstrate that ground squirrel platelets appear to be resistant to the platelet cold storage lesions observed in other mammals, allowing prolonged storage in cold stasis and preventing rapid clearance upon spring arousal. Elucidating these adaptations could lead to the development of methods to store human platelets in the cold, extending their shelf life.     

Sexual differences in over-winter torpor patterns of Richardson's ground squirrels in natural hibernacula

Summary Over-winter torpor patterns of Richardson's ground squirrels hibernating in southern Alberta were monitored with temperature-sensitive radiocollars to determine if these patterns differed between males and females in a manner related to the greater costs of mating effort by males than females. The hibernation season (from immergence to emergence) was composed of three periods: post-immergence euthermy, heterothermy, and pre-emergence euthermy. The hibernation season was shorter for juveniles than adults both among males (< 150 versus 234 days) and females (185 versus 231 days), a reflection of the later immergence into hibernation by juveniles. However, regardless of the absolute duration of hibernation, heterothermy accounted for a smaller proportion of the hibernation season of males (93±5%) than females (98±1%) and, within the heterothermal period, males had shorter torpor bouts and longer inter-torpor arousals. Overall, males spent a smaller proportion of the hibernation season in torpor (85±6%) than females (92±1%). This sexual difference was largely attributable to the longer duration of preemergence euthermy for males than females. Males terminated torpor in January and February, when hibernacula were at their coldest, then remained euthermic for 8.8 days (range 0.5–25.0 days) before emergence. In contrast, females terminated torpor in March, when hibernaculum temperatures were increasing, then remained euthermic for only 1.1 days (range 0.5–2.0 days) before emergence. Males lost less mass per euthermic day during hibernation than females (7.0 versus 9.3 g/day). Males and females hibernated at similar depths (56 cm), but males had larger chambers than females (18 versus 16 cm³/g). Many males, but no females, cached seeds in the hibernaculum. Males met the costs of thermogenesis and euthermy from a combination of fat reserves and food caches, whereas females relied solely on fat. Access to food caches permitted males to terminate torpor several weeks in advance of emergence, during which time they recouped mass and developed sperm in preparation for the forthcoming mating season.     

Seasonal energetics and torpor use in North American flying squirrels

Seasonal cold temperatures require mammals to use morphological, behavioural, or physiological traits to survive periods of extreme cold and food shortage. Torpor is a physiological state that minimizes energy requirements by decreasing resting metabolic rate (MR) and body temperature (T_b). Many rodent species are capable of torpor, however, evidence in northern and southern flying squirrels (Glaucomys sabrinus and Glaucomys volans, respectively) has remained anecdotal. We experimentally attempted to induce torpor in wild-caught flying squirrels by lowering ambient temperature (T_a) and measuring MR using open-flow respirometry. We also studied seasonal differences in MR and T_b at various T_a. Both MR and T_b provided evidence for torpor in flying squirrels, but only infrequent, shallow torpor. MR decreased infrequently and any decreases were rarely sustained for longer than one hour. We found a significant positive relationship between T_a and T_b only in G. volans, which suggests that G. volans is more susceptible to low T_a compared with G. sabrinus, possibly due to their small body size. We observed no substantive seasonal or interspecific differences in the relation between MR and T_a, with the exception that northern flying squirrels expended more energy at cold T_a during warm season trials than other species-season combinations. The infrequency of torpor use in our experiments suggests that other energy-saving strategies, such as social thermoregulation, may limit the reliance on torpor in this lineage.     

Differential response of pineal melatonin levels to light at night in laboratory-raised and wild-captured 13-lined ground squirrels (Spermophilus Tridecemlineatus)

Pineal melatonin levels were compared in laboratory-raised or wild-captured 13-lined ground squirrels (Spermophilus tridecemlineatus) that were either exposed to 10 h of darkness at night or to light which had an irradiance of 400 μW/cm². In laboratory-born squirrels the period of darkness was associated with a gradual rise in pineal melatonin levels with peak values being reached at 0200 h, 6 h after darkness onset. Thereafter, melatonin levels decreased and were back to low daytime levels by 0800 h, 2 h after light onset. The exposure of laboratory-raised animals to an irradiance of 400 μW/cm² during the night totally prevented the nocturnal rise in pineal melatonin levels in these animals. In wild-captured ground squirrels the period of darkness at night was associated with a rapid rise in pineal melatonin such that by 2200 h, 2 h after lights out, peak melatonin values were already attained; additionally, melatonin levels remained high throughout the period of darkness but returned to daytime values by 0800 h. Exposure of wild-captured squirrels to a light irradiance of 400 μW/cm² during the normal dark period was completely incapable of suppressing pineal melatonin levels. The difference in the sensitivity of the pineal gland of laboratory-raised and wild-captured ground squirrels may relate to their previous lighting history.     

Induction of summer hibernation in the 13-lined ground squirrel, Citellus tridecemlineatus

The induction of summer hibernation in the 13-lined ground squirrel (Citellus tridecemlineatus) by intravenous injection of plasma obtained from winter hibernating ground squirrels was confirmed. Hibernation was also induced by injection of urine from arousing winter ground squirrels. Results support the “trigger” theory of hibernation proposed by Dawe and Spurrier (3) and also suggest that tissues are set free from “trigger” influence during winter arousal by the excretion of “trigger.”     

Diabetes mellitus in the 13-lined ground squirrel (Citellus tridecemlineatus).

Spontaneous diabetes mellitus was diagnosed in six of 126, 13-lined ground squirrels, Citellus tridecemlineatus. Serum glucose values were significantly higher in the diabetic ground squirrels than in the non-diabetic ground squirrels, while serum insulin values of fasted diabetic squirrels were significantly lower than fasted nondiabetic ground squirrels. In addition, the classic diabetic signs of poly-dipsia, polyuria, glycosuria, ketonuria, polyphasia, and weight loss were present. The proportion of islet tissue to total pancreatic area in diabetic ground squirrels was less than 25% of that in the nondiabetic ground squirrels. Both the number and size of the islets of Langerhans in diabetic ground squirrels were less than those in nondiabetic ground squirrels.     

Hemoglobin alterations of the 13-lined ground squirrel while in various activity states.

1. Characterization of fetal, winter-hibernating, winter-active, summer-active and summer-induced hibernating hemoglobins of 13-lined ground squirrels (Citellus tridecemlineatus) by isoelectric focusing (IEF) pH 7.0-9.0 indicated that this molecule is extremely responsive to the various activity states of this hibernator. 2. Major alterations of ground squirrel hemoglobin occur with the varying activity states as evidenced by the distinctive changes in the isoelectric points (pIs) of these protein components. 3. Hemoglobin from winter-hibernating or summer-induced hibernating ground squirrels does not revert to a fetal type of hemoglobin. 4. The presence of an additional hemoglobin peak pI 6.55 in the summer-induced hibernator may serve as a possible assay for hibernation inducing trigger(s) (HIT) molecules under study in our laboratory.     

Carbohydrate and torpor duration in hibernating golden-mantled ground squirrels (Citellus lateralis)

Summary Plasma glucose concentrations were increased in torpidCitellus lateralis to test the hypothesis that plasma glucose depletion stimulates periodic arousals from torpor during hibernation.It was found that plasma glucose depletion is not the stimulus for arousal inC. lateralis (Figs. 1 and 2). Plasma glucose and hepatic and skeletal muscle glycogen remain stable during torpor in this species (Figs. 2–4), in marked contrast to the carbohydrate depletion reported forCitellus undulatus. The significance of the differences in carbohydrate status during torpor betweenC. lateralis andC. undulatus is discussed.