Animal Behavior Module 8

Which group of organisms has been universally shown to possess internal biological clocks?

All eukaryotic organisms studied to date.

What primary functions does a biological clock allow an animal to perform?

It allows an animal to keep time and synchronize itself to cyclical environmental variables critical for its existence.

Name three specific cyclical environmental variables that an animal synchronizes itself to.

Time, seasons, and tides

What is the first defining property of a biological clock?

Persistence in Constant Conditions.

What does the term "endogenous" mean in chronobiology?

The ability of a biological clock to control metabolic cycles even in the complete absence of environmental cues.

What structural discrepancy occurs to a biological rhythm in the absence of environmental cues?

The rhythm becomes slightly longer or slightly shorter than it naturally is in nature

What does the prefix "circa" mean, and why is it used?

It means "about". It is used to reflect the slight length discrepancies that occur in biological rhythms when environmental cues are absent

Define a "circadian rhythm."

A biological cycle that is about 24 hours in length.

What does it mean for a biological rhythm to be "free running"?

It means the cycle is operating in the absence of environmental cues and is no longer being manipulated by the environment.

Is the free-running discrepancy evidence that biological rhythms are not truly endogenous?

No, it is considered an artificial result of laboratory manipulation.

While circadian rhythms are endogenously regulated, how are other broad categories of biological cycles regulated?

By external signals

In some multi-cyclic systems, what can drive the rhythms of the other member cycles?

The circadian clock of one of the member cycles.

What is the second defining property of a biological clock?

It is entrainable

Define "entrainment" within the study of chronobiology.

It occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation

What is the main functional purpose of entrainment for an organism?

It keeps the biological clock on schedule and avoids the distorting effects of free running.

What is the most powerful entrainment factor for circadian rhythms?

The light-dark cycle.

How do scientists in laboratories easily manipulate the timing of a subject's circadian rhythm?

By altering the subject’s light-dark cycle over time.

Mechanistically, why does entrainment occur when an exogenous cue is introduced?

Because an exogenous (external) cue affects a clock differently depending on exactly when in the clock's cycle the cue takes place

How does entrainment structurally protect an animal's activity period during seasonal change?

It allows the biological clock to adjust an animal's activity periods as days get longer and shorter

What is the third defining property of a biological clock?

Temperature Compensation.

Define "temperature compensation" in regard to circadian systems.

The ability of an organism's circadian clock to maintain roughly a 24-hour periodicity over a range of physiological temperatures, despite changing cell kinetics caused by differences in thermal energy

What mathematical metric is used to measure the temperature compensating effect?

The Q10 Temperature Coefficient.

Under what condition is a biological rhythm officially considered to be temperature-compensated as temperature increases?

If its Q10 coefficient remains approximately 1

Which biological cycle is the most important for most animals, and how many criteria of biological clocks does it show?

Circadian rhythms; they show all three criteria.

With what environmental feature do most animals coordinate their periods of rest and activity?

The light-dark cycle of their environment.

What are the three behavioral subdivisions of circadian rhythms based on activity timing?

1) Diurnal (daytime active),

2) Nocturnal (nighttime active)

3) Crepuscular (active at dawn and dusk)

Besides behavioral activity periods, what else do circadian rhythms regulate?

Physiological processes (such as daily alterations in mammalian body temperature) and oscillations in gene expression

How can circadial variations in gene expression cause changes in an animal's behavior?

Because genes play a direct role in driving behavior.

Define "ultradian rhythms" and list four examples.

Rhythms that have a period shorter than 24 hours.

Examples include eye blinks, respiration, heartbeat, and sleep patterns.

Are ultradian rhythms endogenous and entrainable?

Yes, they are both endogenous and entrainable.

What is the baseline frequency of human eye blinking as an ultradian rhythm?

24 eye blinks per minute.

Define "infradian rhythms" and list four examples.

A rhythm with a period longer than a circadian rhythm (a frequency greater than one cycle in 24 hours).

Examples include menstruation, breeding, tidal rhythms, and seasonal rhythms.

What is a "circalunal rhythm" and what is its specific cycle duration?

A rhythm synchronized to the waxing and waning of the moon that forms a lunar month, which lasts 29.5 days

Give an example of an organism that breeds on a circalunal basis, and state exactly when it occurs.

Atlantic fireworms (Odontosyllis spp.); they breed just before the fourth quarter of the moon throughout most of the year

What is a "circatidal rhythm"?

A biological cycle synchronized by the flood and ebb of the tides.

Which group of animals is primarily affected by circatidal rhythms?

Littoral (shoreline) animals.

Why are circatidal cycles considered infradian rhythms even though they are only 12.4 hours in length?

Because tides are physically affected by the phases of the moon, making circatidal cycles partly circalunal cycles

Give an example of an organism demonstrating a circatidal rhythm, and describe its behavior.

The marine diatom Hantzschia; it descends into the sand at high tides but rises to the surface at low tide

What is a "circannual rhythm," and what is its precise cycle duration?

A rhythm synchronized with a 365-day year

List six physiological or behavioral processes driven by circannual rhythms.

Body weight, gonad size, nest-building activity, antler shedding and replacement, molting, and reproduction.

Are all infradian rhythms endogenous and entrainable?

yes

Why is the ability to respond to environmental changes via biological clocks uniquely vital for temperate species?

Because they experience dramatic seasonal changes, giving a distinct survival advantage to animals that can prepare for winter in autumn (e.g., via increased food consumption or nest building).

What is the evolutionary advantage of clock-mediated synchronization for migratory species?

It allows them to synchronize their behavior with distant events they cannot directly sense, such as migrating to a location in time for a seasonal abundance of food.

How do biological clocks help honeybees forage effectively at far-away flowers that only open briefly?

The clocks tell them exactly when to visit those flowers, even though the flowers are too far from the hive for the bee to directly see or smell that they have opened.

Why must an animal possess a biological clock to perform "time-compensated orientation"?

To determine exactly how much time has elapsed, which is essential when using moving or changing celestial cues for navigation

Name four distinct navigational cues that require an animal to use time-compensated orientation.

The sun, stars, polarized light, and constellations.

How do honeybees apply time-compensated orientation when communicating with conspecifics?

They continuously alter the angle of their waggle dance to reflect the shifting angle of the sun as time passes.

Do animals rely on a single, isolated biological clock? Explain.

No, they have multiple clocks distributed throughout the body; different organ systems possess their own internal clocks to coordinate specific local functions.

What specific region of the brain serves as the master biological clock in rats?

The hypothalamus (the main visceral control center of the brain).

What specific clusters of cell bodies within the hypothalamus comprise the master biological clock?

The Suprachiasmatic Nuclei (SCN).

What happened when rat SCN cells were extracted and grown in a petri dish?

They continued to fire rhythmically on their own, completely independent of exogenous stimuli

Through what two pathways does the SCN master clock exert control over peripheral clocks?

Hormonal and neuronal pathways

Give an example of a hormone controlled by the SCN, and state its purpose regarding peripheral clocks

Cortisol (released from the adrenal glands); it is believed to be highly important for the entrainment of peripheral clocks.

In what broader groups of animals is the SCN believed to perform this same master clock role?

All mammals, and possibly all amniotes

Describe how the circadian system of Drosophila contrasts sharply with that of a rat.

Drosophila have no master control exerted by the head over their various distributed biological clocks.

What biological role do sensory organs play regarding internal biological clocks?

They play a direct role in the entrainment of those clocks.

Which specialized sensory structures mediate clock entrainment to the light-dark cycle in many animals?

Photoreceptors.

Where are the rudimentary photoreceptors located in mammals, and how do they receive light information?

In the pineal gland; they respond to light signals routed from the optic nerve through the thalamus.

What hormone is produced in higher amounts when a mammal's pineal gland is exposed to light, and what is its role?

Melatonin; it serves a functional role in clock entrainment.

Where are the specialized entrainment photoreceptors located in many vertebrate retinas, and how do they differ from rods and cones?

They are located in the ganglion cell layer of the retina; unlike rods and cones, they serve no role in vision and instead project signals directly to the SCN for entrainment.