Disturbances in the annual weight and hibernation cycles of thirteen‐lined ground squirrels kept in constant conditions and the effects of temperature changes

Avian circannual clocks: adaptive significance and possible involvement of energy turnover in their proximate control

Endogenous circannual clocks are found in many long-lived organisms, but are best studied in mammal and bird species. Circannual clocks are synchronized with the environment by changes in photoperiod, light intensity and possibly temperature and seasonal rainfall patterns. Annual timing mechanisms are presumed to have important ultimate functions in seasonally regulating reproduction, moult, hibernation, migration, body weight and fat deposition/stores. Birds that live in habitats where environmental cues such as photoperiod are poor predictors of seasons (e.g. equatorial residents, migrants to equatorial/tropical latitudes) rely more on their endogenous clocks than birds living in environments that show a tight correlation between photoperiod and seasonal events. Such population-specific/interspecific variation in reliance on endogenous clocks may indicate that annual timing mechanisms are adaptive. However, despite the apparent adaptive importance of circannual clocks, (i) what specific adaptive value they have in the wild and (ii) how they function are still largely untested. Whereas circadian clocks are hypothesized to be generated by molecular feedback loops, it has been suggested that circannual clocks are either based upon (i) a de-multiplication ('counting') of circadian days, (ii) a sequence of interdependent physiological states, or (iii) one or more endogenous oscillators, similar to circadian rhythms. We tested the de-multiplication of days (i) versus endogenous regulation hypotheses (ii) and (iii) in captive male and female house sparrows (Passer domesticus). We assessed the period of reproductive (testicular and follicular) cycles in four groups of birds kept either under photoperiods of LD 12L:12D (period length: 24h), 13.5L:13.5D (27 h), 10.5L:10.5D (23 h) or 12D:8L:3D:1L (24-h skeleton photoperiod), respectively, for 15 months. Contrary to predictions from the de-multiplication hypothesis, individuals experiencing 27-h days did not differ (i.e. did not have longer) annual reproductive rhythms than individuals from the 21- or 24-h day groups. However, in line with predictions from endogenous regulation, birds in the skeleton group had significantly longer circannual period lengths than all other groups. Birds exposed to skeleton photoperiods experienced fewer light hours per year than all other groups (3285 versus 4380) and had a lower daily energy expenditure, as tested during one point of the annual cycle using respirometry. Although our results are tantalizing, they are still preliminary as birds were only studied over a period of 15 months. Nevertheless, the present data fail to support a 'counting of circadian days' and instead support hypotheses proposing whole-organism processes as the mechanistic basis for circannual rhythms. We propose a novel energy turnover hypothesis which predicts a dependence of the speed of the circannual clock on the overall energy expenditure of an organism.

Effects of manipulations of glucoregulation on feeding in the ground squirrel

Insulin and 2-deoxy-D-glucose (2-DG) stimulate feeding in rats, while glucagon inhibits feeding. We report here their effects on food intake in the 13-lined ground squirrel (Spermophilus tridecumlinectus). These hibernating animals are an interesting model for studying appetite regulation because of the marked seasonal variations in food consumption. Food intake reached a peak of 286 +/- 7 g/kg/day in mid July, decreasing to 16.1 +/- 3 g/kg/day in September. Studies during the hyperphagic period showed that glucagon (0.5 to 1 mg/kg) decreased feeding 30 minutes post injection (p less than 0.01) whereas at 4 hours glucagon produced a 37% increase in feeding (p less than 0.05). During the hypophagic period, glucagon failed to alter food consumption. Insulin (5-100 Units/kg) produced no effect on feeding in hyperphagic animals despite a decrease in glucose from 193 +/- 10 mg/dl to 55 +/- 4 mg/dl after 100 Units/kg. However, insulin (50 and 100 Units/kg) resulted in significant increases in food consumption at 2 and 4 hours (p less than 0.01 and 0.05, respectively) when administered while the animals were hypophagic. 2-DG (250-750 mg/kg) increased food consumption (hyperphagic phase) by 76% at 20 hours (p less than 0.01) with significant increases being present as early as 4 hours, although when 2-DG was given to hypophagic animals it resulted in decreased food intake at 4 hours (p less than 0.05). We conclude that the effects of glucoregulatory manipulations on food consumption are markedly influenced by the circannual rhythm of feeding in the 13-lined ground squirrel.

Interrelationships between body weight, food consumption and plasma thyroid hormone concentration cycles in the woodchuck, Marmota monax

The interrelationships between body weight, food consumption and plasma thyroid hormone concentrations were studied during the spring, summer and fall in two groups of woodchucks: those allowed to eat ad libitum and those kept at their March weight by food restriction. The food consumption of the ad libitum fed control animals was greatest in April and May and then declined. Body weight in these animals peaked in July and then slowly declined during the rest of the study. When the restricted animals were allowed to eat ad libitum in August they rapidly increased their food consumption and brought their body weight up to that of the controls in less than six weeks. Plasma thyroxine (T4) and triiodothyronine (T3) concentrations decreased during the summer in the control animals and then rose again in the fall. The T4 changes were blunted in the restricted animals. T3 concentrations decreased during the summer in the restricted animals and remained low until one month after they were allowed to eat ad libitum. The low plasma concentrations of T3 during the summer in the woodchuck may facilitate weight gain in preparation for hibernation.

The amplitude and period of circannual cycles of body weight in golden-mantled ground squirrels with medial hypothalamic lesions

Golden-mantled ground squirrels, Citellus lateralis, were kept for several years in conditions of constant temperature and photoperiod. Control animals showed the expected circannual cycles of body weight, becoming fat in the autumn and thin in the spring. Animals with lesions in the ventromedial region of the hypothalamus also had circannual cycles of weight but in most animals the amplitude was increased. At one time of year the lesioned ground squirrels were grossly obese while often at other times their weight was close to that of control animals or their own preoperative weights. There were unusually early peaks in weight after the lesion in some animals and it is probable that a phase shift occurred. Observations on hibernation, food intake, transient weight gains and delayed weight gains following the lesions are also reported. Evidently during the winter phase of the annual cycle in these ground squirrels there exist mechanisms that lead not only to a loss of weight gained naturally before the time for hibernation but also to a temporary attenuation of additional obesity produced by ventromedial hypothalamic lesions.