Biological Rhythms & Chronobiology – Comprehensive Bullet-Point Notes

Key Vocabulary and Core Concepts

• Circadian rhythm - Your body's natural 24-hour cycle that controls sleep and other functions, typically running for $<br>20 ext{–}28<br>\ \text{h}$ (about 1 day).

  • This governs sleep, how you eat, hormone release, and more.

• Circannual rhythm - A yearly cycle in biology; like for hibernation, migration, or reproduction.

• Zeitgeber ( “time-giver” ) - Any external signal (like light) that helps set your body's internal clock.

  • The classic example is light-dark (LD) cycles; others include temperature, changes in tides, food availability, predator warnings, and moonlight.

• Entrainment - How external signals (Zeitgebers) reset an organism's internal clock to match the environment.

• Free-running rhythm - A rhythm that continues inside an organism even without external cues (like light or dark).

  • This proves the rhythm is generated internally; its timing often changes slightly from the original external cycle.

• Seasonality - Predictable yearly changes in the environment that affect things like breeding, migration, or insect dormancy.

• Actogram - A graph that shows activity patterns over many days, helping to spot rhythms and entrainment.

• Circadian Time (CT) - An internal time system used when there are no outside time cues (e.g., CT0 = the start of activity in animals active at night).

General Properties of Biological Rhythms

• They come from inside an organism (endogenous) and can be adjusted by the environment (modulated). - Both are needed to prove it's a true biological rhythm.

  • Being adjusted by the environment is entrainability; coming from inside is self-sustainability.

• These rhythms can match daily, tidal, lunar, monthly, or annual cycles.

• These rhythms exist at all levels of life, from tiny molecules → organelles → cells → organs → organ systems → individuals → populations → communities → ecosystems.

Molecular Architecture: The Canonical Negative-Feedback Loop

1. A starting signal (positive element) turns on clock genes.

2. These genes make clock proteins (e.g., PER in fruit flies & vertebrates).

3. When enough proteins are made, they move back into the cell's nucleus and stop their own gene's activity (negative element).

4. The proteins break down, which lifts the stop, and the cycle starts again.

5. This whole cycle takes about $24<br>\ \text{hours}$.

Example: PER Feedback Cycle

• Daytime: During the day, the protein PER is made more.

• Evening: In the evening, PER builds up faster than it breaks down, then moves into the cell's control center (nucleus).

• PER then stops the *
  per*
  gene from making more PER.

• Overnight degradation lowers PER → its stopping effect is removed.

• This repeats around every $<br>\sim24<br>\ \text{h}$.

Master Pacemaker – Suprachiasmatic Nucleus (SCN)

• Two small groups of cells in the front part of the brain called the hypothalamus.

• Each SCN cell can keep time on its own; and they all work together by sending chemical signals (neuropeptides/neurotransmitters).

• It gets direct light signals from the eyes (via the retino-hypothalamic tract).

• It sends chemical and nerve signals to other body clocks.

• A key thing the SCN controls is the release of melatonin (a sleep hormone) from the pineal gland (in vertebrates).

Melatonin: A Key Modulator

• Made in the pineal gland mostly at night; light stops its production.

• Functions:

  • It signals
    “biological night.”

  • It helps control sleep, reproduction, body temperature, and movement.

• In many feedback cycles, melatonin often acts as the negative element (feeding back onto the SCN and other body parts).

Evolutionary & Comparative Insights

• These rhythms are found in all kinds of living things (from bacteria to humans).

  • The basic way they work (feedback loop) is similar, but the exact genes involved have changed over time or developed similarly.

• Convergent evolution: Interestingly, clock genes in bees/ants (Hymenoptera) are more like human genes than other insect genes, making them good for studying human rhythms.

Fitness Consequences & Behavioral Ecology

• Being in sync helps animals reproduce better, find food more easily, and avoid predators.

  • For instance: If mating times are off, fewer chances to mate occur.

• Pollinator–plant mutualisms: Plants release nectar and scent when pollinators are around; pollinators visit when conditions are right.

• Predator–prey interactions: Moonlight (lunar rhythm) affects predator-prey actions:

  • Prey hide more on bright moonlit nights.

  • Predators change their hunting times.

Signal vs Cue Question (Lecture Poll)

• Signals = features that animals developed to communicate.

• Cues = just bits of information animals pick up that weren't meant for communication.

• Biological rhythms are set by cues (e.g., light, temperature) rather than by intentional signals.

Temporal Scales We Focus On

• Circadian ($ \approx1 \ \text{day}$ ) – controls daily things like sleep, feeding, or midshipman fish humming.

• Circannual ($ \approx1 \ \text{year}$ ) – like bird migration or insect dormancy.

  • Both are controlled by similar systems of 'clock genes'.

Detecting a Rhythm – General Protocol

1. See if a behavior or body function changes with an external factor.

2. Check if the rhythm continues even without external cues (free-running conditions) to prove it's internal (endogenous).

3. Use activity charts (actograms) or similar graphs to see the pattern over time.

4. Change external cues (light, temperature, moonlight) to see how the rhythm adjusts (entrainment properties).

Case Study 1 – Golden Spiny Mouse (Acomys russatus)

• Observation: They are active during the day in nature but often at night in the lab.

• Methods: They had 3 test setups:

  • LD1: Normal light/dark cycle ($12<br>\ \text{h}$ light/$12<br>\ \text{h}$ dark) as a starting point.

  • LD2: 24 days of constant light, then back to light/dark.

  • LD3: 24 days of constant darkness, then back to light/dark.

• Prediction: Scientists expected their activity to vary a lot and not respond much to light.

• Result: Their activity starting times were all over the place; it was hard to tell if they were day or night active.

• Poll outcome: This study didn't show if their rhythm could be set by light (entrainment) or if it was truly internal (self-sustaining).

Case Study 2 – Circannual Rhythm in Garden Warbler (Sylvia borin)

• Variables: They measured things like how big their reproductive organs were, feather shedding (molt), and how restless they were (like preparing for migration).

• Treatments:

  • Constant light/dark (LD $11{:}11$).

  • Changing light periods that mimicked 12- or 6-month cycles.

• Outcome: The birds' body changes and behaviors matched the changing light schedules, showing that their yearly clocks were set by light changes.

Abiotic Cues & Hibernation (Vole Poll)

• These animals live underground, so they probably use temperature more than light to know when to wake up from hibernation.

Case Study 3 – Lunar Rhythm & Predation (Deermice vs Short-Eared Owls)

• Environment: The study was done in controlled rooms (specified temperature of $20^{\circ}\text{C}$, 30% relative humidity, and light/dark cycle of $11<br>\ \text{L}{:}13<br>\ \text{D}$).

• Three trial phases: getting used to the room, measuring activity, and then adding a predator.

• Prediction: Scientists thought: more moonlight + more prey activity = higher risk of being caught.

• Results:

  • Before predators: Deermice were less active on bright moonlit nights.

  • With predators: Owls hunted more in the dark; during a quarter moon, mice balanced risk and activity.

• Conclusion: Moonlight acts as an external cue (Zeitgeber) that affects how predators and prey interact.

Case Study 4 – Midshipman Fish Courtship Vocalization

• Observation: Male fish
  “hum”
  at night during their mating season.

• Hypotheses:

  1. The humming follows a daily (circadian) rhythm and continues even without light cues.

  2. Melatonin encourages humming.

• Treatments:

  • Normal light/dark cycle (LD $15{:}9$), then constant dark (DD) or constant light (LL).

  • Constant light plus a melatonin implant (or a dummy implant as control).

• Results:

  • DD: Humming continued but shifted slightly each day, proving it's an internal rhythm.

  • LL: Humming stopped, but a melatonin implant made them hum again.

• Key negative element exploited = LL cycles (using constant light removed the natural light cue, helping to show how the internal clock works).

Developmental Arrest & Diapause

• Diapause: A planned stop in growth or reproduction, often triggered by changes in light period (photoperiod) or temperature.

• It can be obligatory (required) or facultative (optional); and sometimes it just ends on its own.

Voltinism & Bet-Hedging

• Voltinism: Means how many generations an animal has in a year.

• Univoltine: One generation per year.

• Semivoltine: More than one year per generation, with long larval sleeping periods that span seasons.

• Bet-hedging: A strategy where an organism produces different types of offspring (phenotypes) to survive unpredictable environments.

• Semivoltinism is like
  “bet-hedging”
  : it might mean fewer offspring on average, but it makes sure some survive if conditions are bad.

Case Study 5 – Osmia iridis (High-Elevation Mason Bee)

• Observation: Normally these bees have one generation per year. But high up in the mountains, some nests show parsivoltinism (a mix of one-year and two-year life cycles).

• Hypothesis: They thought the time it takes for a generation to grow depends on how long and warm the summer is.

• Methods:

  • They took bee cocoons from the wild and put them in dark incubators.

  • They then created fake summers with different lengths and temperatures (e.g., 5 weeks cool vs. hot).

• Predictions:

  1. Longer summers would mean more bees with a one-year life cycle.

  2. Warmer summers would mean more bees with a one-year life cycle.

• Results (Table 1 summarized):

  • Warmer larval temperatures greatly increased the chance of bees emerging after just one winter.

  • Short and cool summers led to more bees emerging after two years.

• Conclusion: This means warmer climates could lead to more bees having a one-year life cycle.

• Limitation: A limitation was that they didn't change the amount of pollen the bees had; food availability could also affect how fast they grow.

Gradescope Reflection Prompts (Study Skills)

• The voltinism idea relates to Ultimate Fitness (UF), which asks why a strategy developed.

• To show an internal clock, you need to test in constant conditions (e.g., 5 weeks cool/hot without outside cues).

• To prove bet-hedging, you need to show a trade-off in survival and different types of offspring; this study showed some, but not full, evidence.

• For why costly semivoltinism evolved: In places with unpredictable summers (high altitude/latitude), splitting emergence over two years helps prevent total failure.

• It's important to mention pollen because food affects growth; pointing out this wasn't tested helps understand the study's limits.

Practical & Philosophical Implications

• Knowing about biological rhythms helps in medicine (like for jet lag or shift-work disorders), farming (when to pollinate), and conservation (how climate change affects natural cycles).

• The fact that these rhythms are so old and widespread shows how important it is for living things to be organized by time.

Quick Reference Numbers & Equations

• A daily (circadian) cycle is about $<br>\tau<br>\ \approx<br>\ 24<br>\ \text{h}$ long (when free-running, it's often $<br>\tau<br>\ =<br>\ 23 ext{–}25<br>\ \text{h}$).

• The SCN brain area controls other body clocks using nerve signals and hormones (e.g., melatonin).

• A common light/dark cycle for experiments is $12<br>\ \text{hours}<br>\ \text{light}<br>\ \text{and}<br>\ 12<br>\ \text{hours}<br>\ \text{dark}$ ($12<br>\ \text{L}{:}12<br>\ \text{D}$).

• In labs, moonlight is mimicked to show different moon phases (new, quarter, full).

Study Tips

• First, learn the main definitions (circadian, Zeitgeber, free-running) well, as they appear in every example.

• Practice understanding actograms (activity charts)—learn to find shifts in timing, patterns without external cues, and when rhythms are set by the environment.

• See how internal workings (like clock genes and SCN) lead to big impacts on survival (like fitness, bet-hedging, avoiding predators).

• Use the case studies (mice, fish, bees) as examples for how to design experiments: focus on one thing at a time, use groups with constant conditions, and try to guess the results.