Biological Rhythms slides 1

Students can….
Explain why there are environmental rhythms
Explain why plant and animal behaviours are
linked to environmental rhythms.
Earth’s Rhythms
• Yearly orbit of Earth about the sun, together
with the tilt of its axis, cause the seasons
• Seasonal contrast increases with increasing
latitude (ie. least contrast is near the equator)
• Seasons have characteristic abiotic conditions
(temp, day length, rainfall etc)
Daily Rhythms, 24 hr
• Caused by daily rotation of Earth on its axis
• Produces alternating light and dark periods
• Length of day depends on latitude and season
• Sun also affects daily rhythms in temp,
humidity, wind
Effects of the moon
• Moon orbits the earth once per 27.3 days
• Affects the tides: moon’s gravitational pull
combined with Earth’s rotation
• Twice-daily ocean tides
• Tidal levels vary during moon’s phases (spring
and neap tides)
• Changes in illumination (light) at night
Environmental Rhythms and
Animal Behaviour
• Most daily and annual animal rhythms are
linked to cyclical environmental changes
caused by Earth’s rhythms
• Includes periods of activity & sleep, feeding &
drinking, breeding, migrations, fluctuations in
body temp, sensitivity, hormone levels
What are the biological advantages to
an animal species of synchronising its
activities to Earth’s rhythms??
• Better food supply
• Avoiding predators
• Avoiding competitors
• Finding more favourable environmental cond’ns
• Avoiding harsh conditions
• Finding mates or a breeding place
• Raising young successfully
Biological Rhythms
• Animals and plants exhibit all kinds of
regularly repeated behaviours
• eg. activity patterns, reproduction, migration,
• EXOGENOUS rhythm: A direct response to
environmental cues (external)
• ENDOGENOUS rhythm: Internally controlled,
will continue in absence of environmental
cues (involves a biological clock)
• BIOZONE Pg 187 and 188 questions
• Explain what BIOLOGICAL RHYTHMS are
• Explain the ADVANTAGE of these rhythms
• Explain what a BIOLOGICAL CLOCK is
What is the advantage of having
a biological clock?
Activity remains in sync with environment
Keeps sleep/wake cycles constant
Able to predict events (eg. winter, spring)
Synchronise breeding stages with others of
your species and the seasons
• Plants: flowers when the bees are out, lose
leaves in winter when cold
• Animals and plants exhibit all kinds of
regularly repeated behaviours
• eg. activity patterns, reproduction, migration,
• EXOGENOUS rhythm: A direct response to
environmental cues (external)
• ENDOGENOUS rhythm: Internally controlled,
will continue in absence of environmental
cues (involves a biological clock)
Remaining in sync with environment
• Biological clocks must be reset at regular
• This is the purpose of the ZEITGEBER or
external timekeeper
• This is the environmental cue that resets the
biol clock
• ENTRAINMENT is the process of resetting the
biol clock
BIOZONE activities
• Pg 189-190
The Human Biological Clock
• Runs at about a 25.5 hr day
• But we need it to be synchronised with our
environment (a 24hr cycle)
• Mammals including humans have a master
circadian clock in our brain
• Two tiny groups of cells called the
suprachiasmatic nuclei (SCN) in hypothalamus
• Located behind our eyes, 1/3mm in size
What’s involved?
• Eyes
• Hypothalamus
• Pineal gland
• Melatonin
Light stimulus
Darkness stimulus
Shuts off
In Hypothalamus
Zeitgeber = external timekeeper
• The light/dark cycle acts as the ZEITGEBER to
reset our internal clock to a 24hr cycle
• Resetting of the clock is called ENTRAINMENT
• By following the day/night cycle, animals can
better exploit the seasonal changes in the
The Importance of Melatonin
• Is the hormone controlling our sleep/wake cycle
• When produced at night it promotes sleep
• May help in the timing of onset of reproductive
cycles in many species of animals
• Enhances the immune system
• More is produced over 24 hrs in the short days
of winter than in summer (sleepier in winter?)
Today: Circa- rhythms &
• Learn how to read and interpret Actograms
• Weta as an example
Lots of human daily rhythms
BIOZONE “Human Biorhythms”
Susceptibility to alcohol
Body temperature
Blood pressure
Cell divisions in skin
Time of labour onset
What are Actograms?
• An actogram is a special graph
• Shows the activity of an animal or plant during
the day/night and over many successive days.
• Gives a good picture of when the organism is
active at any time.
• Can use them to monitor changes over time
under constant conditions
Actograms show us:
• Light/dark regime in lab conditions
• Period = length of one full cycle eg. From start
of activity to the next start of activity.
• The Free-running Period = how long the cycle
runs in the absence of all environmental cues
• Phase-shift = New start time for the activity
(eg. 8am start changes to 10am) How much
has it shifted? 2 hours later
• Daily night/day cycle of about 24 hours
• Three types of activity are seen in animals:
1. Diurnal – mostly active during the day
Eg. Humans, honey bees, tui
2. Nocturnal – mostly active at night
Eg. Kiwi, owls, bats, moths, kakapo
3. Crepuscular – most active at dawn and dusk
eg. Rabbits, mosquitos, wombats
Today: Types of Biorhythms
• Explain what is meant by circannual,
circalunar, semi-lunar, circatidal and
compound rhythms
• Give examples of each of the above rhythms
Environmental rhythms
Circa- rhythms
• Environmental rhythms are daily, annual, tidal,
lunar etc
• Animal rhythms can only be called circadian,
circannual etc if they persist under constant
conditions in the lab.
Cycle length
• Circadian (about 24 hours)
• Light/dark cycle
• circatidal (about 12.5 hours)
• Incoming/outgoing
• Moon phases
• circalunar (about 29.5 days)
• circasemilunar (about 15 days) • Moon phases /
spring high tides
• circannual (about 365 days)
• Day/night length (not
temperature, too
NZ Example of a Circatidal Rhythm
• Chione stutchburyi cockle
• exhibits clear endogenous
circatidal rhythmicity in shell
gaping and siphon extension
and also in adductor activity
• occurred at the times of
expected high tides
Advantage of the cockle’s
endogenous rhythm being in time
with high tides?
* avoid dessication at low tide
* increased feeding time
* increased survival
Circatidal example
• Fiddler crabs forage
during low tide
• If taken away from the
shore, the circatidal
activity can persist for
5 weeks
Tidal/lunar cycle animation
Remember: the position of the
sun and moon generate our tidal
patterns each month
Circalunar rhythm example
• Young salmon migrate downstream
• Soon they will migrate to the sea
• At new moon, surge in output of hormone
• Brings about changes in physiology to cope
with being in salty water
Circalunar example
• Marine worms called Palolo worm
• Spawning (egg laying and sperm
releasing) is synchronised by the
• Advantage? Males and females
will spawn at the same time
= increased reproduction
Example of Semi-lunar rhythm
• Californian grunion
• Leaves the sea to deposit its eggs (spawning)
in the sand of several beaches, Feb-Sept
• Spawning only takes place on 3-4 nights after
full or new moon (when tides are highest).
• Eggs buried in sand to develop, will be washed
out again during next spring tide in 12 days
Advantages of spawning at spring tide?
• Eggs deposited high on the sand by high tide
so won’t be washed away
• spawning bed is not disturbed by the tides
until the next spring tide cycle
• The eggs have a safe refuge buried in moist
sand in which to develop.
• Increased survival and reproduction
NZ Inanga: Galaxias maculatus
• Adult life is spent in
lowland streams and
• During spring high
tides, ripe adults move
up on to the grassy flats
of river estuaries, go to
areas covered by water
only at the spring tides
• The eggs develop and usually hatch at the
next spring tides that cover the grasses
• Advantage = reproduction and survival
• The manner in which the fish are able to
perceive and respond to the lunar cycle
remains a complete mystery.
Annual Rhythms
eg. Reproduction, migration, hibernation
Eg. Long-tailed Bat in NZ
Hibernates 4-5 months over autumn/winter
Reproduces annually in summer months
Responding to day/night length
Advantages for the bats?
• Breeds at the warmest time of year
• More food around during breeding season
• Avoids the cold months
Compound Rhythms
• Animal responds to more than one
environmental rhythm
• Eg. Sandhopper – uses lunar orientation at
night, solar navigation during the day.
Palola marine worm
• Highly synchronised breeding
• Research shows this is related to
not only the moon’s phases, but
also solar, daily and tidal
• Some plants exhibit rhythmic activity
• DAILY RHYTHMS: eg. Opening/closing of some
flowers such as night-scented jasmine
• Eg. Leaf movements – the prayer plant leaves
move to an erect position at night, horizontal
during the day.
Plant rhythms….
• Eg. Production of flowers, loss of leaves in
autumn, germination of seeds
• Usually in response to change in light/dark hours
• Plants synchronise with animal activity
– Eg. Open flowers and secrete nectar when animal
pollinators are active in daytime
– Production of flowers coincides with the life cycles
and activity of animal pollinators
• PHOTOPERIODISM: the regulation of activity
by the length of daylight (the photoperiod)
• Photoperiod detected by leaves
• Flowering is induced by the length of the
Plants are in 1 of 3 groups:
• Short Day Plants need a period of long nights
before they can flower (ie. They need a
photoperiod of less than a certain critical day
• Long day plants need a period of short nights
before they can flower
• Day Neutral Plants flower independently of
the length of photoperiod
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