Indeed, a literature review on temporal niche switching found that most temporal niche switches occurred in typically nocturnal mammals under conditions of increased energetic requirements (e.g. The energetic benefits of diurnality proposed by the CTE hypothesis would be most relevant for animals challenged to maintain energy balance. As night temperatures are usually lower than day temperatures and the rest phase is associated with behavioral strategies to reduce heat loss, resting during the coldest part of the day allows animals to optimize the energetic benefits of insulation and minimize daily energy expenditure (DEE). The circadian thermo-energetics (CTE) hypothesis predicts that diurnal activity patterns allow endothermic animals to reduce energy expenditure ( Hut et al., 2012 van der Vinne et al., 2014a). Extrinsic benefits of circadian organization can therefore derive from optimizing the timing of activity and rest with daily environmental T a rhythms. Thus, endothermic energy expenditure varies with time of day as a result of daily changes in T a and solar radiation. The energetic savings benefit of these strategies increases when ambient temperature ( T a) is lower. Examples of behavioral energy-saving strategies typically occur during the rest phase and include daily torpor ( Heldmaier et al., 2004), insulating body posture, huddling and resting in insulated nests ( Gilbert et al., 2010). Reducing thermoregulatory costs allows animals to divert energy to different tasks and so optimize growth and reproduction. Circadian rhythms are believed to provide adaptive benefits by distributing conflicting metabolic and behavioral processes to different times of day (intrinsic benefits Pittendrigh, 1993), and allow organisms to predict daily environmental fluctuations in thermal conditions, predation risk and food availability (extrinsic benefits Daan, 1981).įor all endotherms living in temperate climates, maintaining body temperature consumes energy. These circadian clocks allow animals to optimize physiology and behavior to specific times of day, resulting in species being mainly active during the day (diurnal), night (nocturnal) or around dawn and dusk (crepuscular).
Circadian rhythms in behavior and physiology are ubiquitous in a wide range of species and molecular circadian clocks have been described in virtually all taxa ( Edgar et al., 2012 Panda et al., 2002). Anticipation of these daily environmental changes is enabled by the presence of a circadian system, involving circadian molecular oscillators (circadian clocks). This results in large daily environmental changes to which organisms have adapted during evolution. The rotation of the Earth around its axis causes predictable daily changes in light and temperature. The optimal circadian organization of an animal ultimately depends on the balance between energetic consequences and other fitness consequences of the selected temporal niche. Diurnality allows mammals to compensate for reductions in food availability and temperature as it reduces energetic needs. The reduction of DEE by diurnality provides an ultimate explanation for temporal niche switching observed in typically nocturnal small mammals under energetically challenging conditions. Furthermore, diurnality provides energetic benefits at all investigated geographical locations on European longitudinal and latitudinal transects. Analysis of weather station data shows that diurnality is energetically beneficial on almost all days of the year in a temperate climate region. Compared with nocturnal animals, diurnal animals encounter higher ambient temperatures during both day and night, leading to reduced thermogenesis costs in temperate climates. The dominant factor determining the energetic benefit of diurnality is thermal buffering provided by a sheltered resting location.
Combined with night-time torpor, as observed in mice under prolonged food scarcity, DEE can be reduced by ∼20%. Under natural conditions, diurnality reduces DEE by 6–10% in energetically challenged mice. Here, we tested the CTE hypothesis by quantifying the energetic consequences of relevant environmental factors in mice. The circadian thermo-energetics (CTE) hypothesis predicts that diurnal activity patterns reduce daily energy expenditure (DEE) compared with nocturnal activity patterns. However, typically nocturnal mammals become diurnal when energetically challenged by cold or hunger. Most mammals are specialized to be active during the night (nocturnal) or day (diurnal). Endogenous daily (circadian) rhythms allow organisms to anticipate daily changes in the environment.
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