International Journal of Molecular Zoology 2024, Vol.14, No.2, 111-127 http://animalscipublisher.com/index.php/ijmz 112 interplay of genetic, physiological, and environmental factors in shaping these remarkable behaviors, contribute to a broader understanding of migration biology, and inform conservation strategies for migratory species facing environmental changes. 2 Biological Rhythms and Migration 2.1 Types of biological rhythms Biological rhythms are intrinsic cycles that regulate various physiological and behavioral processes within organisms. These rhythms can be classified into several types based on their periodicity, including circadian rhythms, circannual rhythms, ultradian rhythms, and infradian rhythms. Circadian rhythms have a period of approximately 24 hours, synchronized with the Earth's rotation, and influence daily activities such as the sleep-wake cycle, feeding, and hormone release. Circadian rhythms are driven by internal biological clocks and are synchronized with external cues like light and temperature (Shochat and Tauber, 2021). Circannual rhythms follow an annual cycle, regulating seasonal behaviors such as migration, reproduction, and hibernation. Circannual rhythms help organisms adapt to the changing environmental conditions throughout the year (Clercq et al., 2023). Ultradian rhythms have periods shorter than 24 hours, such as the 90-minute sleep cycle in humans or certain feeding patterns in animals (Mazzoccoli, 2022). Infradian rhythms have periods longer than 24 hours but shorter than a year, such as the menstrual cycle in humans (Bellastella et al., 2021). 2.2 Mechanisms of biological clocks Biological clocks are complex molecular mechanisms that generate and regulate these rhythms. The core components of biological clocks are highly conserved across different species, involving feedback loops of gene expression and protein interactions. At the cellular level, biological clocks are regulated by a set of core clock genes and proteins that produce rhythmic oscillations. In the circadian rhythm system, genes such as CLOCK, BMAL1, PER, and CRY form feedback loops, resulting in rhythmic gene expression and protein activity (Yuan et al., 2018; Clercq et al., 2023). In mammals, the suprachiasmatic nucleus (SCN) located in the hypothalamus acts as the master circadian regulator. The SCN receives light information from the retina and synchronizes peripheral clocks throughout the body, maintaining the coordination of circadian rhythms (Bellastella et al., 2021; Shochat and Tauber, 2021). These clocks exist in various tissues and organs, and while they can operate independently, they are usually synchronized by the SCN. Peripheral clocks regulate specific functions of tissues, such as metabolism and hormone secretion (Yeung and Naef, 2018; Costa, 2021). Moreover, light is the primary environmental cue influencing biological rhythms. The photoperiod, or the length of daylight, is particularly important for circannual rhythms, as changes in daylight duration provide appropriate signals for migration, reproduction, or other seasonal behaviors. Hormones, such as melatonin and cortisol, play crucial roles in transmitting circadian and seasonal information to various physiological systems, thereby influencing the preparation and execution of migration. 2.3 Interaction between biological rhythms and migration Migration is a complex behavior influenced by both circadian and circannual rhythms. These rhythms help migratory animals anticipate and prepare for seasonal changes, ensuring that they migrate at the optimal time to enhance their chances of survival and reproduction. Studies have shown that circadian rhythms can affect the timing of migratory activities, such as the initiation of flight in birds or the timing of feeding and resting during migration. Circadian rhythms help animals maintain energy balance and navigation during migration (Shochat and Tauber, 2021; Clercq et al., 2023). Circannual rhythms are crucial for the timing of long-distance migrations. These rhythms ensure that animals' migrations are synchronized with seasonal changes in food availability, breeding conditions, and climate. For example, birds use circannual rhythms to align their migration timing with favorable conditions at their breeding and wintering grounds (Clercq et al., 2023). Research indicates that variations in clock genes can also influence migratory behavior. Polymorphisms in genes such as Clock and Adcyap1 have been associated with differences in migration timing and distance in birds (Clercq et al., 2023).
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