Lesson 5: Timing Responses in Animals

Vocabulary

• Circadian Rhythm - the natural cycle of physical, mental, and behavior changes that the body goes through in a 24-hour cycle

• Diurnal - active during day

• Nocturnal - active during night

• Crepuscular - active during dawn and dusk

• Cicatidal Rhythms - a biological rhythms that follow a cycle of about 12.4 hours, syncing with the rise and fall of tides

  - caused by the gravitational pull of the moon, coupled with the rotation of the Earth on its axis

• Circalunar Rhythms - a biological rhythm linked to the lunar cycle, which lasts about 29.5 days.

• Semilunar Rhythms - a biological rhythm that happens in two cycles every lunar month, each lasting about 15 days, linked to the moon's rotation around Earth.

• Spring tides - tides that occur every 15 days during a new or full moon, causing very high and low tides

• Neap tides - a moderate tide that occurs when the sun and moon are at right angles to each other

• Circannual Rhythms - a biological cycle or behavior that repeats once a year, often influenced by seasonal changes like light and temperature.

• Compound Rhythms - it’s when an organism follows more than one rhythm

  
  

Circadian Rhythms

Circadian rhythms are linked to day-night cycle.

Circadian comes from the Latin “circa”, meaning “approximately”, and “diem” meaning “day”.

Types of Circadian Rhythms:

• Diurnal - animals active during the day (eg. bees, hawks, cicadas)

• Nocturnal - animals active during the night active (eg. snails, kiwis, crickets)

• Crepuscular - animals active in the twilight of dawn and dusk active (eg. bobcats, rabbits, jaguar)

Circatidal Rhythms

Circatidal Rhythms are biological rhythms that follow a cycle of ~12.4 hours, aligning with the ebb and flow of the tides.

Tides are caused by the pull of the moon.

The daily rise and fall of the tides is due to the rotation of the Earth relative to the moon.

Circatidal rhythms is caused by the gravitational pull of the moon (and to lesser extent, the Sun), coupled with the rotation of the Earth on its axis (12h 15min). 

Any organism living near the middle of the tidal zone alternately inhabits two environments - land and sea. Immersion by the sea is the time for feeding and reproduction.

The intertidal zones consists of an environment that shifts drastically between submerged and exposed conditions due to tidal cycles.

Some examples are:

1. Crustaceans (e.g., Fiddler Crabs, Eurydice pulchra) show burrowing and foraging activity in sync with tidal movements.

2. Molluscs (e.g., Limpets, Periwinkles, Mussels) regulate attachment and feeding behaviors based on tidal cycles. Mussels open their shells for filter-feeding during high tide and close them to avoid desiccation during low tide.

Adaptive Advantages of Circatidal Rhythms

• Avoiding Desiccation - Organisms time their activity to minimize water loss during low tide.

• Predator Avoidance - Many species synchronize their movement and feeding patterns with high tide to avoid predation.

• Efficient Foraging - Feeding behaviors align with tidal influxes that bring nutrients and food particles.

• Reproductive Timing - Many marine species use tides for gamete release and larval dispersal to enhance survival rates.
  

Circalunar Rhythms

Circalunar rhythms are linked to the rotation of the moon around the Earth. 

A lunar cycle is 29.5 days.

Eg. Young salmon, prior to their seaward migration, migrate downstream to estuaries. At the new moon, there is a surge in the output of thyroxine (hormone) which brings about physiological changes that enable fish to regulate their internal salt concentration in seawater.

Semilunar Rhythms

Semi-lunar rhythms also associated with the rotation of the moon around the Earth, but there are two cycles every lunar month. 

→ Spring tides occur every 15 days (highest tides), a few days after the new and full moon. 

→ Neap tides (smaller tides) occur when the pull of the Sun and moon are at right angles to each other. 

As a consequence of spring and neap tides, organisms on the lower shore are only uncovered about every 15 days and those on the upper shore are only covered every 15 days.

Circannual Rhythms

Circannual rhythms are associated with the combined effects of the Earth orbiting the Sun and the Earth’s axial tilt causing seasonal cycles over a period of 365.25 days. 

These rhythms are often associated with photoperiod (day length) as the year progresses. 

Some examples of these rhythms are:

1. Hibernation - period of inactiveness in animals, over winter (e.g. bears)

2. Migration - a cyclic trip to a different geographical location for a period of time

3. Aestivation - period of inactiveness in animals during dry and arid conditions (e.g. many land crabs spend the dry season at the bottom of the burrow)

Compound Rhythms

Compound Rhythms is when an organism follows more than one rhythm.