The circadian rhythm of body temperature.

The activity patterns of animals, whether diurnal, crepuscular or nocturnal, are usually fixed endogenous rhythms, entrained by environmental Zeitgebers. Here we demonstrate that juvenile Atlantic salmon, Salmo salar, switch between diurnal and nocturnal foraging solely in response to environmental temperature, and independently of photoperiod and season. Above 10 degrees C juvenile Atlantic salmon fed predominantly during daylight, spending the night exposed in the water column but relatively quiescent. As temperature dropped below 10 degrees C they became increasingly nocturnal, hiding in refuges by day but emerging to feed at night. It has previously been shown that parallel physiological changes take place in the retinae of several species of salmonids: the quantity and composition of the visual pigments change so as to make the fish more dark adapted at low temperatures. As the fish were found to be far less aggressive by night than by day at all temperatures, the switch to nocturnal activity was also accompanied by a change in social structure. We suggest that this temperature-dependent strategy maximizes feeding efficiency in summer but reduces predation risk in winter.

Temperature-Dependent Switch between Diurnal and Nocturnal Foraging in Salmon

This article reviews the literature on the circadian rhythm of body temperature. It starts with a description of the typical pattern of oscillation under standard laboratory conditions, with consideration being given to intra- and interspecies differences. It then addresses the influence of environmental factors (principally ambient temperature and food availability) and biological factors (including locomotor activity, maturation and aging, body size, and reproductive state). A discussion of the interplay of rhythmicity and homeostasis (including both regulatory and heat-exchange processes) is followed by concluding remarks.

The influence of climate (mean annual rainfall, rainfall variability, ambient temperature, T
 a) on the basal metabolic rate (BMR) of 267 small mammals (<1 kg) from six zoogeographical zones was investigated using conventional and phylogenetically independent data (linear contrasts). All climate variables varied between zones, as did BMR and body temperature (T
 b) , but not thermal conductance. Holarctic zones were more seasonal and colder, but rainfall was less variable, than non-Holarctic zones. In general, the BMR was most strongly influenced by body mass, followed by T
 a and the rainfall variables. However, there was significant variation in the strength of these relationships between zones. BMR and T
 b increased with latitude, and mass-independent BMR and T
 b were positively correlated. The latter relationship offers evidence of a slow-fast metabolic continuum in small mammals. The fast end of the continuum (high BMR) is associated with the highest latitudes where BMR is most strongly influenced by T
 a and mean annual rainfall (i.e. mean productivity). The slow end of the continuum (low BMR) is associated with the semi-tropics, low productivity zones, and climatically unpredictable zones, such as deserts. Here rainfall variability has the strongest influence on BMR after body size. The implications of a slow–fast metabolic continuum are discussed in terms of various models associated with the evolution of BMR, such as the aerobic capacity models and the "energetic definition of fitness" models.

The influence of climate on the basal metabolic rate of small mammals: a slow-fast metabolic continuum

Endocrines and Osmoregulation

Daily circadian rhythms of body temperature (Tb) and oxygen consumption (VO2) were measured in two murid species, which occupy extremely different habitats in Israel. The golden spiny mouse (Acomys mssalus) is a diurnal murid distributed in arid and hot parts of the great Syrio-African Rift Valley, while the broad-toothed field mouse (Apodeinns mystacinus) is a nocturnal species that inhabits the Mediterranean woodlands. In both species, the daily rhythms of Tb and VO2 are entrained by the photoperiod. Under laboratory experimental conditions (ambient temperature Ta = 33oC and photoperiod regime of 12L: 12D), Acomys russatus exhibits a tendency towards a nocturnal activity pattern, compared to the diurnal activity displayed by this species under natural conditions. Under the same photoperiod regime and at Ta = 28oC, Apodemus mystacinus displays nocturnal activity, as observed under natural conditions. The maximal values of Tb were recorded in Acomys russatus at midnight (23:50 h), while the maximal values...

Daily rhythms of metabolic rate and body temperature of two murids from extremely different habitats.

Thermoregulatory abilities, which may play a role in physiological adaptations, were compared between two field mouse species (Apodemus mystacinus and A. hermonensis) from Mount Hermon. While A. hermonensis is common at altitudes above 2100 m, A. mystacinus is common at 1650 m. The following variables were compared in mice acclimated to an ambient temperature of 24°C with a photoperiod of 12L:12D, body temperature during exposure to 4°C for 6 h, O2 consumption and body temperature at various ambient temperature, non-shivering thermogenesis measured as a response to a noradrenaline injection, and the daily rhythm of body temperature. Both species could regulate their body temperature at ambient temperatures between 6 and 34°C. The thermoneutral zone for A. mystacinus lies between 28 and 32°C, while for A. hermonensis a thermoneutral point is noted at 28°C. Both species increased O2 consumption and body temperature as a response to noradrenalin. However, maximal VO
2 consumption as an response to noradrenaline and non-shivering thermogenesis capacity were higher in A. mystacinus, even though A. hermonensis is half the size of A. mystacinus. The body temperature rhythm in A. hermonensis has a clear daily pattern, while A. mystacinus can be considered arhythmic. The results suggest that A. hermonensis is adapted to its environment by an increase in resting metabolic rate but also depends on behavioural thermoregulation. A. mystacinus depends more on an increased non-shivering thermogenesis capacity.

Comparative thermoregulatory adaptations of field mice of the genus Apodemus to habitat challenges.

Resting metabolic rates and daily energy intake in desert and non-desert murid rodents

Food and Energy Consumption in Rodents from Different Environments: The Role of Photoperiod in Seasonal Acclimatization

Oxygen consumption (VOz) and body temperature (Tb) were measured in the Asian Garden Dormouse, Eliomys melanurus, from a population inhabiting the semi-arid steppe habitats of the Negev Highlands in Israel Prior to measurements, the animals were acclimated for at least three weeks to an ambient temperature of 24°C with a photoperiod regime of 12L: 12D The results of this study reveal that the resting metabolic rates of this species are relatively low and that even at Ta=2S0C they may enter daily periods of torpor which can save up to 65% of the average daily energy expenditure of a normothermic individual under the same conditions The emergence from torpor is accompanied by an overshoot in VOZ Both norinothermic dormice and those in torpor show a daily rhythm of VO? and Tb with a clear pattern of a nocturnal species

http://www.italian-journal-of-mammalogy.it/pdf-77773-13966?filename=Thermoregulation and.pdf

Ami Rubal

Papers

Daily rhythms of metabolic rate and body temperature of two murids from extremely different habitats.

Comparative thermoregulatory adaptations of field mice of the genus Apodemus to habitat challenges.

Resting metabolic rates and daily energy intake in desert and non-desert murid rodents

Food and Energy Consumption in Rodents from Different Environments: The Role of Photoperiod in Seasonal Acclimatization

Thermoregulation and rhythmicity in Eliomys melanurus from the Negev Highlands, Israel