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11-rhythms s - Biological rhythms and seasonal reproduction Many species breed only at certain times of year in order to give birth when ample food

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Biological rhythms and seasonal reproduction.
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Many species breed only at certain times of year – in order to give birth when ample food is available Breed during the summer Stop breeding when days get short in fall Spontaneous recovery in the spring the summer Hamsters:
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Quite a few seasonal rhythms are cued by photoperiod (day length), because photoperiod is the single most reliable environmental index of seasonal change. Days are longer in the summer than in the winter, and animals can detect these differences in photoperiod. Maine NC Peru The further you live from the equator, the greater the difference between summer and winter.
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Reproduction Before artificial lighting Energy balance Some species fatten in preparation for winter, others lose weight Depression – seasonal affective disorder. Some seasonal rhythms cued by photoperiod (day length): – maybe humans, too.
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Animals use circadian rhythms to keep track of day length. (circa = around, dies = day) Level of some physiological variable τ ( The two important parts of any rhythm are the amplitude (how much it changes) and the period (time required for a full cycle, τ ). For circadian rhythms, τ ≈ 24 hours.
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They are approximately ( not exactly ) 24 hours in length, τ ≈ 24 hr. Properties of circadian rhythms: They are endogenous – generated from within the animal not by external events.
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DAY NIGHT Measuring daily activity patterns in hamsters. Hamsters are nocturnal active at night. Animals that are active during the day are diurnal .
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If running wheel activity reflects a circadian rhythm, then it should persist in constant conditions. If running wheel activity reflects a circadian rhythm, then the period should be roughly, but not exactly , 24 hours. constant darkness And it does. And it is. τ > 24 τ = 24 constant conditions day & night
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Another example: Resetting the light-dark cycle phase-shifts the running rhythm Reentrainment Activity free runs in constant conditions, and B is always close to 24 hr, e.g ., 23.9 < B >24.1 In constant conditions, activity free-runs and τ is always close to 24 hr, e.g ., 23.9 < τ >24.1 τ >24.1
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Humans have daily rhythms, too. And some of them are circadian.
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So where is it? It turns out that we have little clocks all over our bodies, but the biggie (the master clock) is in the suprachiasmatic nucleus (SCN) of the hypo- thalamus. OK we have this internal endogenous clock . . .
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What’s the evidence that the SCN is the master clock?
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This note was uploaded on 03/04/2009 for the course PSYCH 335 taught by Professor Georgewade during the Fall '08 term at UMass (Amherst).

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11-rhythms s - Biological rhythms and seasonal reproduction Many species breed only at certain times of year in order to give birth when ample food

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