Seasonal Breeding Flashcards
seasonal breeding
animals mate at certain times of the year (timed HPG axis activation)
timed mating (birth which gives the best chances of survival)
sheep/hamster
continuous breeding
all year reproductively active
constant
independent of time of year/environment
photoperiod
seasonal changes in physiology (weight,metabolism, fat content, colour, fur)
summer fur=brown winter fur = white
important in industry
true seasonality
birth rate switched on/off
(HPG axis on/off)
human birth rate and exceptions
human birth rate is constant throughout year, except for
northern finland higher during Jan-May, lower in summer-winter (extreme seasonality)
inuits (northern canada) 1970s-higher birth rates in spring 1980s-moved away from traditional substinence)
aim of seasonal breeders
give birth during spring (Mar-Jun)
different animals have different lengths of gestation (horse>mink)
seasonal breeders short day
fertile when daylight shortens (autumn) oct-feb
anestrus in spring/summer
sheep/goat/deer/elk
gestational period of 5 months
spring: increased temperature/food (new grass)
seasonal breeders long day
fertile when daylight gets longer (spring)
anestrus in autumn/winter
hamster/mice/horse/hare/rabbit
small = breed in spring/early summer, short gestational period (~20days), offspring can be reproductively competent
horse = 11 month gestational period, breed in spring/summer to give birth in spring
what does the photoperiod influence
prolactin (skin)
gonads
signals the pineal gland (behind brainstem, contact with 3rd ventricle via pineal recess)
neural pathway
light (RHT) signals to SCN (circadian clock)
SCN nerve fibres synapse in PVN
preganglionic fibres run into superior cervical ganglia
pineal gland releases pineal cells (release melatonin and synthesis)
melatonin
produced and released by only the pineal gland
increased synthesis and release during night in biological fluids (blood/CSF/urine) short t1/2
daytime has low levels of synthesis and fast breakdown
released from SCN no matter what time of year
hamster testes size
remove testes in hamsters = no change to testes size
hamster - testes size changes (large in spring/summerdecrease in winter)
3 aspects of melatonin signals
1) duration
2) coincidence hypothesis (sensitivity window to melatonin)
3) amplitude (little change in peak during darkness)
large change in melatonin with photoperiod
melatonin release in hamsters
long days causes increased melatonin release = loss of reproductive function (only when administration overlaps with endogenous melatonin)
short melatonin = active reproduction
duration of nightly melatonin in plasma = mediates changing daylength through seasons
GnRH
activation by melatonin
GnRH neurons contain melatonin receptors: MT1/2
no evidence of direct effects causing seasonal changes (only on CR)
melatonin signalling
kisspeptin neurons contain MT receptors
kisspeptin stimulates GnRH and HPG axis
kisspeptin in short and long day breeders
short day breeders (sheep): sheep Kiss-1 mRNA decreased in long days and increases in short days
long day breeders (hamsters): decreases kiss-1 mrna in ARC increased in long days
pinealectomy = stops reduction in kiss-1 and testes weight during short days
kisspeptin administration = stops decrease in testes weight and testosterone during short days
GnIH
located in dorsomedial nucleus (DMN) projects to preoptic area (where GnRH neurons located) and form connections
inhibit GnRH/LH release
sheep = high GnIH in long days low GnIH in short days (kisspeptin off GnIh on)
hamster = low GnIH in short days high GnIH in long days (stimulates reproduction/direct response to melatonin)
examples of opportunistic breeders
only mate when environmental conditions are suitable (gestation and birth)
e.g. golden spiny mouse/amphibians
breed: 3-4 times a year
gestation: 5-6 weeks
habitat = hot/dry desserts of Egypt (survive with less water)
AVP signalling in opportunistic breeders
secreted from posterior pituitary
primary hormone responsible for water retention
increased water permeability of kidney collecting duct = increased water absorption
secreted in reduced plasma volume and increased plasma osmolality (salinity) - when water is scarce
female vs male golden spiny mouse
females = smaller ovaries and uterus/failure of follicle development
males = decreased sperm motility/mating/fertilisation
not well understood
amphibian reproduction determined by water availability
melatonin in summer/winter
summer = (low) kisspeptin and GnIH stimulated and activates HPG
winter = (high melatonin) inhibits kisspeptin and GnRH
day length changes
each daylength occurs twice a year (mar/oct same daylight:darkness ratio) = same nocturnal signal
oct/mar have different environmental period = need to track melatonin duration)
hamsters = increased testes weight during increased daylight (same photoperiod has different effects)
