Seasonal Breeding

Question 1

Ultradian rhythm

Answer 1

those that occur at intervals of less than a day

heartbeat, respiration

Question 2

Circadian rhythm

Answer 2

those that occur around a day in length

body temperature, adrenal activity, sleep/wake cycles

Question 3

Intradian rhythm

Answer 3

those that occur at periods of longer than a day

estrous cycles, migration, pelage changes, hibernation, seasonal reproduction

Question 4

Circannual rhythm

Answer 4

those with an annual occurrence

Question 5

Cyclical histogenesis or histogenesis hypothesis

Answer 5

• cells/tissues have an approximately annual cycle of synchronized regeneration (growth, remodeling, degeneration, and replacement)
• changes in the local environment of the stem cells are considered vital for generation of these circannual rhythms

Question 6

Why develop a seasonal reproduction?

Answer 6

to give birth at the time of year most favorable for the survival of offspring

Question 7

Explain change in seasonality and closeness to the equator

Answer 7

species living on the equator may breed continuously while those at higher latitudes will exhibit strict seasonality

Question 8

Explain change in seasonality between males and females

Answer 8

• females usually exhibit stricter seasonality in their reproductive capacities than males
• males of most species can often breed year round

Question 9

Name some ultimate factors in seasonality (7)

Answer 9

food availability
expected milder environmental temperatures in certain seasons
food competition
predator pressure
lifespan of individuals
foraging strategy

Question 10

Name some proximate factors in seasonality (1)

Answer 10

photoperiod (light = photophase, dark = scotophase)

Question 11

What is the predictor option?

Answer 11

planner species that use a reliable predictor of a future seasonal change as a timing device for maximal reproductive competence

Question 12

What must a species take into account when considering triggering reproductive competence?

Answer 12

gestation length

how long it will take them to transition from anestrus to fully functional cyclicity

Question 13

Critical photoperiod

Answer 13

length of time that melatonin must be elevated each day to achieve the desired effect (melatonin elevated at night)

Question 14

Photoperiod history

Answer 14

animal detects changes in day length based on light exposure (8L:16D to 12L:12D = increase in day length)

Question 15

Photorefractory

Answer 15

when an animal is kept under stimulatory photoperiods and they eventually stop responding to them

Question 16

Photoinducible

Answer 16

animals that need exposure to non-stimulatory photoperiods before they become responsive to stimulatory ones

Question 17

What is the critical event for breeding season vs. non-breeding season?

Answer 17

alteration in the frequency of GnRH secretion from the pulse generator

Question 18

Describe GnRH secretion in the breeding season vs. non-breeding season of females

Answer 18

Breeding season - high frequency GnRH secretion gives gonadotropin release and active ovaries

Non-breeding season - little synthesis or secretion of GnRH and ovaries are inactive

Question 19

What stimulates GnRH neurones?

Answer 19

a peptide called kisspeptin

Question 20

Steps in photoperiod control of reproduction

Answer 20

1. ambient daylength perceived by retina, entrains endogenous circadian clock in SCN
2. SCN controls melatonin production by pineal gland (occurs at night)
3. circulating melatonin levels control activity of target cells
4. thyrotropes of pituitary respond to short melatonin signals (long days) by secreting TSH
5. TSH activates cells in hypothalamus to convert T4 to T3
6. T3 controls tissue histogenesis causing increased GnRH circulation in long-day breeders and decreased GnRH circulation in short-day breeders

Question 21

Explain the steps in measuring day length

Answer 21

retina projects to suprachiasmatic nucleus (SCN) - SCN projects to superior cervical ganglion (SCG) - SCG innervates pineal gland - pineal gland secretes melatonin

Question 22

What can stop melatonin secretion?

Answer 22

during hours of darkness, light exposure of sufficient intensity and duration will cause the cessation of melatonin secretion

Question 23

How does light exposure stop melatonin secretion?

Answer 23

light exposure on retina causes cessation of adrenergic stimulation on N-acetyl transferase, the rate-limiting enzyme in melatonin synthesis

Question 24

How do melatonin signals act on the pituitary gland to cause TSH release?

Answer 24

thyrotrophys of the pars tuberalis have melatonin receptors - melatonin receptors release TSH in response to short daily melatonin signals

Question 25

How does TSH act within the hypothalamus?

Answer 25

TSH binds to receptors in hypothalamus - hypothalamus converts T4 to T3 - T3 important in histogenesis

Question 26

How is the frequency of GnRH pulses inhibited during the non-breeding season? (ewe is example)

Answer 26

• increased sensitivity of the GnRH pulse generator to inhibitory gonadal steroid (estrogen) feedback
• also a steroid independent mechanism at work

Question 27

Explain the inhibition of GnRH using mechanism 1 (altered sensitivity to steroid feedback)

Answer 27

• GnRH stimulatory kisspeptin system is down-regulated by estrogen feedback
• GnRH inhibitory dopamine system up-regulated by estrogen-sensitive glutaminergic and GABAergic inputs to it

Question 28

Explain the inhibition of GnRH using mechanism 2 (steroid-independent inhibition of GnRH release)

Answer 28

• isolation of GnRH neurons from capillaries

- Gonadotropin Inhibiting Hormone (GnIH)

Question 29

What are opportunistic breeders?

Answer 29

small, short-lived animals that alter their reproductive activity based on food availability

(have short gestation periods)