International Journal of Molecular Zoology 2024, Vol.14, No.3, 166-181 http://animalscipublisher.com/index.php/ijmz 174 cycles. For instance, animals in polar regions, where the day-night cycle can be absent for extended periods, often exhibit arrhythmic or ultradian activity patterns to cope with the constant environmental conditions (Bloch et al., 2013; Bertolini et al., 2019). High-latitude species, such as certain Drosophila species, have evolved specific clock adaptations that allow them to remain active under constant darkness, demonstrating the flexibility of circadian systems in response to ecological demands (Bertolini et al., 2019). Similarly, herbivores and social animals may exhibit around-the-clock activity patterns during specific life-history stages, such as migration or reproduction, to maximize their fitness in their respective environments (Bloch et al., 2013). The molecular mechanisms underlying these adaptations involve complex interactions between genetic and environmental factors. For example, the circadian clock network in high-latitude Drosophila species has evolved to support arrhythmic behavior under constant darkness, a trait that is advantageous for survival in polar regions5. This plasticity in circadian rhythms allows animals to optimize their physiological and behavioral processes to the unique challenges of their habitats, thereby enhancing their overall fitness and survival (Harmer et al., 2001; Pilorz et al., 2018). In summary, the adaptive functions of circadian rhythms are multifaceted, encompassing survival advantages, predator-prey interactions, and ecological niche adaptations. These rhythms enable animals to synchronize their internal processes with external environmental cues, optimize their energy use, and adjust their behavior to minimize predation risk and maximize resource acquisition. The evolutionary flexibility of circadian systems underscores their critical role in the survival and fitness of organisms across diverse ecological contexts. 7 Circadian Rhythms in Different Animal Taxa 7.1 Mammals Circadian rhythms in mammals are governed by a complex interplay of genetic and biochemical mechanisms. The core of the mammalian circadian clock consists of a set of genes, including Clock, Bmal1, Period, and Cryptochrome, which form a transcription-translation feedback loop. This loop is responsible for generating and maintaining the near-24-hour cycles of physiological and behavioral processes (King and Takahashi, 2000; Van Gelder, 2003). The mammalian circadian system is highly adaptive, allowing organisms to synchronize their internal clocks with external environmental cues, such as light and temperature (Harmer et al., 2001; Pilorz et al., 2018). In mammals, circadian rhythms regulate a wide array of physiological functions, including sleep-wake cycles, hormone secretion, metabolism, and immune responses (Panda, 2016; Pilorz et al., 2018). Disruption of these rhythms, often due to modern lifestyle factors like shift work and exposure to artificial light, has been linked to various health issues, including metabolic disorders, cardiovascular diseases, and mental health conditions (Panda, 2016; Cao, 2023). Experimental studies using animal models, such as mice and hamsters, have been instrumental in elucidating the molecular mechanisms underlying circadian rhythms and their impact on health (Cao, 2023). 7.2Birds Birds exhibit circadian rhythms that are crucial for regulating their daily activities, such as feeding, singing, and migration. These rhythms are primarily influenced by the light-dark cycle, which acts as a powerful zeitgeber (time-giver) to synchronize their internal clocks with the external environment (Singh and Kumar, 2018). The avian circadian system shares similarities with that of mammals, including the presence of clock genes and feedback loops that generate rhythmic patterns of gene expression (Harmer et al., 2001). One of the most fascinating aspects of avian circadian rhythms is their role in seasonal behaviors, such as migration and reproduction. Changes in photoperiod (day length) trigger physiological and behavioral adaptations that prepare birds for long-distance migration or breeding (Singh and Kumar, 2018). For instance, the timing of migration is tightly regulated by circadian clocks, ensuring that birds embark on their journeys at the most favorable times of the year10. Additionally, circadian rhythms in birds are known to influence their reproductive cycles, coordinating ovulation and mating behaviors to optimize reproductive success (Singh and Kumar, 2018).
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