The majority of animals restrict their behavioural activity to defined times during the 24-hr day. This temporal niche is specified by an intricate interplay between the animal’s circadian clock and external factors like preferred temperature, light intensity, predators and food availability. Being active at the ‘wrong time’ can have severe impact on an organism’s fitness. But can animals actively choose their preferred temporal niche, and do individuals of the same species and population differ in their choice?
Circadian clocks are self-sustained molecular oscillators and keep ticking in the absence of any environmental cues. But in order to be a useful tool for advantageous timing of biological processes in a natural day/night setting, these environmental changes must be able to influence (or reset) the molecular oscillations.
Most clock genes are expressed in a rhythmic fashion, meaning that like the biological processes they control, their expression reaches peak and trough values once during a 24-hr day (Stanewsky et al., 2003). In order to adjust these molecular oscillations to changing environmental conditions (for example the seasonal changes of day length), clock gene expression must somehow respond to changes in illumination.
In nature, both the daily changes of light and dark and the concomitant changes in ambient temperature serve as signals to synchronize circadian clocks. In order to understand how these two pathways are integrated on a neuronal and molecular level we initiated studies using only temperature cycles as synchronizing factor (Glaser & Stanewsky, 2005).
Even though daily temperature fluctuations serve as a synchronizing signal for circadian clocks (see above), clocks are also temperature compensated. This means that when kept under constant conditions (i.e., no daily fluctuations of either light/dark or temperature), circadian clocks tick at the same speed at low or high constant temperatures.
Wanyama wengi huamua tabia zao kwa muda fulani katika siku ya saa 24. Wakati huu ni maalum na mwingiliano tata kati ya saa ya circadian ya mnyama na mambo ya nje kama vile joto la kupendwa, nguvu ya mwanga, wawindaji na upatikanaji wa chakula. Kuwa hai katika "wakati usiofaa" kunaweza kuwa na athari kubwa kwa fitness ya kiumbe. Lakini je, wanyama wanaweza kuchagua kwa bidii nafasi yao ya wakati, na je, watu wa spishi na idadi ileile ya watu hutofautiana katika uchaguzi wao? Saa za circadian ni oscillators za molekuli zenye kujitegemeza na huendelea kupiga wakati hakuna ishara zozote za mazingira. Lakini ili kuwa chombo muhimu kwa ajili ya wakati mzuri wa mchakato wa kibiolojia katika mazingira ya asili ya mchana na usiku, mabadiliko haya ya mazingira lazima iweze kuathiri (au kuanzisha tena) oscillations molekuli. "Kama ilivyo kwa ""kiwango cha juu"" cha maonyesho ya chembe za urithi, ""kiwango cha juu"" cha maonyesho ya chembe za urithi ni kiwango cha juu cha maonyesho ya chembe za urithi katika kipindi cha saa 24 kwa siku." Ili kurekebisha oscillations hizi za molekuli kwa mabadiliko ya hali ya mazingira (kwa mfano mabadiliko ya msimu ya urefu wa siku), usemi wa jeni la saa lazima kwa njia fulani uitize mabadiliko ya taa. Katika asili, mabadiliko ya kila siku ya mwanga na giza na mabadiliko yanayoambatana na joto la mazingira hutumika kama ishara za kusawazisha saa za circadian. Ili kuelewa jinsi njia hizi mbili zinavyounganishwa katika ngazi ya neuronal na molekuli, tumeanzisha masomo kwa kutumia mzunguko wa joto tu kama sababu ya kusawazisha (Glaser & Stanowski, 2005). Hata ingawa mabadiliko ya joto ya kila siku hutumika kama ishara ya kusawazisha kwa saa za circadian (tazama hapo juu), saa pia hulipwa joto. Hii inamaanisha kwamba wakati unahifadhiwa chini ya hali ya kawaida (yaani, hakuna mabadiliko ya kila siku ya mwanga au giza au joto), saa za circadian zinasonga kwa kasi sawa kwa joto la chini au la juu.