How photoperiod affects reproduction in seasonal breeders?
Seasonal breeders are known to breed (reproduce) only during certain seasons of the year. Photoperiod plays a key role in determining that time of the year for their successful breeding to occur.
Photoperiod affects reproduction in seasonal breeders by altering the hormonal production in their bodies. Based on photoperiodism which is the availability of light during the time of the year determines the amount of melatonin hormone and GnRH production in their body which directly or indirectly causes the initiation of their reproduction cycle at the right time (season) of the year through the required hormonal positive feedback loop.
Melatonin is a very crucial hormone that is mainly released by the pineal gland at night (during the darkness). The external stimulus which is due to the non-availability and availability of light stimulates melatonin secretion.
The rapid increase in melatonin production results in the initiation of ovarian activity in females and testicular activity in males leading to the commencement of the breeding season in the seasonal breeders.
Also that the increase in GnRH (Gonadotropin-Releasing Hormone) is initiated from the hypothalamus that eventually causes the release of LSH (Luteinizing Hormone) and FSH (Follicle Stimulating Hormone) from the Pituitary Gland respectively through a positive feedback loop which in turn also stimulates the initiation of ovarian activity in females and testicular activity in males.
Now, it is to be noted that the production of melatonin signals the release of GnRH which will then only signal the release of LH and FSH from the pituitary gland.
So, this melatonin production is affected by photoperiodism and it is the key hormone in the reproduction of seasonal breeders.
These all happens generally through the stimulation of darkness received by the organism from the eyes via the optic nerve primarily, skin surface contact with the daylight, and other indirect responses of the environment like the availability of proper food during the suitable season.
It is due to the effect of photoperiod that animals who are the long-day seasonal breeders generally reproduce during spring and summer to take advantage of the warmer temperatures.
Some seasonal breeders are also there who are called short-day seasonal breeders that will reproduce during the autumn and winter time of the year to take advantage of the colder temperatures.
Now, how these two kinds of seasonal breeders breed during two different times of the year readily depends on the availability of light that will secrete their hormonal production required for the onset of their reproductive cycles.
So, it can be said that the long-day seasonal breeders only breed when they are exposed to more amount of light during the day.
While, the short-day seasonal breeders can only breed when they are exposed to less amount of light during the day.
Well-known examples of long-day seasonal breeders include ring-tailed lemurs, horses, hamsters, groundhogs, mink, etc. and those of the short-day seasonal breeders include sheep, goats, etc.
We will learn more about this in detail. So, just keep reading…
Who are the seasonal breeders?
Seasonal breeders are the those animals that breed only during certain season (time) of the year. The time they breed is determined by the biochemical hormonal action going on in their bodies.
Seasonal breeders are those animal species that only show sexual interest or urge in their sexual behavior during a certain time of the year.
These times of the year when these seasonal breeders breed are best suitable for them in terms of the physical environmental factors such as surrounding temperature, food and water availability, and changes in the predation behaviors of other species.
Seasonal breeders are those that do strictly express their sexual interest towards the other sex and accept and react in accordance to those only during certain times of the year.
They will also show the initiation of their reproductivity with the onset of hormonal flow for the initiation of possible breeding only during certain times of the year.
In seasonal breeders, male individuals are known to not show any keen interest towards the opposite sex, while the female seasonal breeders will enter the anestrous phase when the right season for breeding has not approached.
As soon as the right season for breeding has approached these female seasonal breeders will have one or more estrus cycles only and becomes willing to mate and have offsprings.
Male seasonal breeders during the right breeding season will show a rapidly increasing surge in their testosterone levels. Moreover, their testes weight also seems to increase as a result.
Now it is to be highly noted that the biochemical factors of their body’s hormonal influence is highly responsible in determining the season and time when actually these seasonal breeders will breed respectively.
The hormonal influence will cause the production of hypothalamic secretions like the GnRH, by also influencing the LH and FSH increased-production reproductive responses in both male and female seasonal breeders.
A few of the famous examples of seasonal breeders include goats, pigs, cows, ring-tailed lemurs, horses, hamsters, groundhogs, mink, etc.
What actually is the photoperiod effect?
The photoperiod effect is also termed photoperiodism, which is actually the physiological reaction of the biological body of an organism to respond in accordance to the length of day (light) and night (darkness) periods.
Photo meaning light and period meaning time. So, photoperiod can also be defined as the relative amount of light to dark (day to night) period available in a 24-hour daily period.
The biological response to photoperiod can be termed as photoperiodism or the photoperiod effect. In seasonal breeders, this photoperiod effect can be well seen and studied extensively.
It is also to be noted that both the photoperiod and the photoperiod effect don’t remain constant for a particular place all throughout the year.
However, it is also to be noted that the pattern of change of both the photoperiod and the photoperiod effect is constant over the year.
So, the photoperiod is the total period of daily illumination received by an organism, and the photoperiod effect is the body’s physiological response to that illuminated period.
You should always note that the photoperiod effect remains constant between years at any given geographic location.
Now let’s know how the natural photoperiod gets influenced?
It is to be noted that the natural photoperiod and its effects are greatly influenced by the change in latitude.
Near the equator, you will find that there is a little variation in photoperiod and so its effects.
However, at latitudes greater than 30° North and 30° South the range in photoperiod is very much noticeable.
What actually causes seasonal breeding?
The change in photoperiod causes seasonal breeding in the seasonal breeders. This change in photoperiod can be well detected by the body’s sense organs like eyes, etc. which triggers the release of hormones names melatonin, GnRH, LH, and FSH.
Also in addition to those hormones, there is also a rapid surge in the hormonal concentration of estrogen, neuropeptides, Kisspeptin, GnIH (Gonadotropin-inhibitory hormone), and Thyroid glands produced hormones.
All of the hormones maintain a positive feedback loop for stimulating the initiation of ovarian activity in females and testicular activity in males.
And when the threshold is reached a negative feedback loop is produced for maintaining the balance of testicular and ovarian stimulation for the onset of breeding.
In addition to the hormones, many other environmental factors like availability of food, water, housing, space, and climate also influence the onset of breeding indirectly.
So, it can be well stated that the rapid hormonal surge in the blood prepares the body and the various reproductive organs for the initiation of a possible pregnancy in the appropriate season of the year.
Regulation of Hormonal activity due to Photoperiod affects causing reproduction in Seasonal Breeders
As the animal body gets the external cue of photoperiod by visual mode, meaning that when the animal gets the amount or time pattern of light seen by its eyes each day, then this external cue is transmitted via the retina and the optic nerve of the eyes to the brain and then eventually towards the Anterior Cervical Sympathetic Fibers and then finally reaching the Pineal Gland.
Here the changes in light-dark cycles are sensitized by the photopigments of the mammalian eye thereby sending information to the SCN.
The central dogma is that the pineal gland is controlled via a polysynaptic pathway from the SCN.
So, as the pineal gland receives the signal it starts its job of secretion of melatonin hormone in the blood stream of the animal respectively through the activity of N-acetyl Transferase (NAT) regulatory enzyme.
This pineal gland also termed the epiphysis cerebri, is a small endocrine gland present in the brain (deep in the center of the brain) of most vertebrates.
This pineal gland produces melatonin, a serotonin-derived hormone that controls the sleep cycles and also the seasonal cycles.
As melatonin gets secreted during the period of short days with more than 13-14 hours of darkness, then this through a positive feedback loop signals the Hypothalamus part of the brain to release GnRH.
As soon as there is an increasing surge of GnRH in the blood, then the small blood vessels carry the hormone to the pituitary gland.
As a consequence, the Anterior Pituitary Gland starts producing Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) into the bloodstream.
FSH in males stimulates testicular growth and enhances the production of an androgen-binding protein by the Sertoli cells thus aiding in the making of mature sperm cells.
While FSH in females helps control the menstrual cycle and stimulates the growth of eggs in the ovaries.
LH in males causes the testicles to make testosterone, thus aiding in sperm production.
LH in females signals the ovaries to make estrogen leading to ovulation and implantation of an egg in the uterus.
Both the FSH and LH are highly responsible for the onset of breeding and also for the successful breeding of the animals.
Later, as there is a seasonal shift and so the day-light shift is seen then the decreasing surge in melatonin production can also be seen.
This surge appears to act directly on GnIH (Gonadotropin Inhibitory Hormone) neurons through its receptor to induce expression and release of GnIH.
GnIH in turn through a negative feedback loop decreases the production of LH and FSH from the Anterior Pituitary respectively. This results in the animal entering its non-breeding (anestrous) phase during the off-season.
So, it can also be stated that the neurons in the hypothalamus that results in the production of GnIH are very much crucial for causing a seasonal shift in the reproductive activity of the seasonal breeders.