Certain biological activities are triggered by specific day lengths (e.g., insects breaking diapause, birds breeding).
Critical day lengths are around 11-13 hours of light for temperate region animals.
Seasonal cues vary for tropical and polar animals.
Spring and Autumn Breeders
Animals can be categorized as spring or autumn breeders.
Endocrine System Response:
Hormones, such as FSH, LH, progesterone, estrogen, and testosterone, mediate reproductive cycles.
These hormones have stimulatory effects influenced by day lengths.
Hormonal Feedback Loops
Negative Feedback Loop:
When animals are exposed to prolonged stimulatory light, breeding activity can initially increase but will eventually decrease without the necessary cues (e.g., short days).
Animals may cycle through reproductive phases even in constant light conditions.
Circadian Clocks and Experiments
Key Experimental Findings:
Brian Follett's work with quail exemplifies hormonal changes in response to day length shifts.
LH (Luteinizing Hormone) levels were measured after transitioning quail from short to long light periods.
LH surged within a single long day, indicating responsiveness to light.
Subsequent long days led to cumulative effects, providing insights into how day length influences reproductive cycles.
Photo-Inducible Periods
The concept of a photo-inducible effect indicates that if light is detected during a specific timeframe, it affects the animal's biological processes.
A study involving white crowned sparrow examined the response to night breaks with light pulses to measure the LH levels, demonstrating the oscillatory nature of biological clocks.
Understanding Biological Mechanisms
Biologists theorize the existence of molecular clocks controlling these mechanisms at the genetic level.
Gene Transcription and Proteins:
Genes are expressed to produce proteins, which help in maintaining the circadian rhythms.
Key genes include "period" (per) and "timeless" (tim), which oscillate to regulate various physiological activities.
Cellular Oscillators and Light Reception
Cryptochrome is essential for light detection in the molecular clock process.
Photoreceptive proteins such as CRY interact with light to regulate protein stability and modify molecular clock outputs.
Mechanisms of Entraining Biological Clocks
The transcription-translation loop composed of positive elements (e.g., clock and cycle genes) and negative elements (e.g., per and tim) enables the intricate working of biological clocks.
Protein Interactions:
Positive elements stimulate the transcription of target genes while negative elements inhibit this transcription, creating a cycle of production and degradation.
Effects of Environmental Cues
Phase Response Curves provide insights into how light exposure can advance or delay biological clocks.
Varying the timing and duration of light exposure affects the clock's alignment with environmental cycles.
Specific gene interactions and hormonal responses create feedback loops influencing reproductive timing and other behaviors in animals.
Conclusion and Further Readings
Ongoing research continues to uncover the complexity of circadian rhythms, with various species demonstrating different adaptations and mechanisms.
Suggested reading includes studies on the circadian clocks of migratory species like monarch butterflies for a deeper understanding of biological timing systems and their ecological roles.