Two of the most important biological rhythms of mammals are the daily rhythms (circadian, approximating 24h) and annual rhythms (circa-annual, approximating 365 days). We must first focus on the important circa-annual rhythms of reproduction for seasonal breeding mammals. Everyone naturally associates the springtime with frolicking lambs, nesting birds and the appearance of foals in the fields.
Throughout evolution, the young of prey animals, such as sheep and horses, had a better start in life and a greater likelihood of surviving to adulthood if born during the longer and milder days of spring.
Why is this? Well, let’s consider the needs of a new mother caring for her young. The lengthening and increasingly warm days of spring provide new grass growth, increased plant photosynthesis and therefore greater nutrition for the high energy demands of a lactating herbivore. What’s more, the longer hours of daylight provide added protection for her young by increasing the visibility of predators and providing a less harsh environment for young animals to get to grips with their limbs, to keep up with their mums when a rapid flight from danger is required.
Annual Breeding Rhythms – A Timer for Birthing, not Breeding
What is interesting is that the circa-annual seasonality of breeding behaviour for many mammalian species is a timer for birthing rather than breeding. The reproductively active period of seasonally breeding animals occurs one gestation length before the optimal time for offspring to be born. Thus, in the case of the sheep, with a 6-month gestation length, the breeding season occurs in the autumn so that lambs are born in the spring. For birds, a gestation length of one month means that breeding occurs quite close to the emergence of chicks. And finally, in the case of the mare, an approximate 11-month gestation (335 days) means that mares are long-day breeders with peak fertility during the summer months so that their offspring are again born under favourable spring conditions.
How Light Signals the Breeding Season for the Mare
So, we now know that reproduction in seasonal breeding animals is regulated by light. But how is the changing day length communicated to the mare’s reproductive system? The answer is via a very important hormone called melatonin. Melatonin is a protein hormone produced in the brain by the pineal gland and it has been given the ominous title of ‘Hormone of Darkness’. In the horse, melatonin is produced primarily during the hours of darkness and is inhibited by light. From research carried out, we identified that, melatonin levels are low during daylight hours and high at night. This provides the horse with a means of translating the seasonal changes in day length into a hormonal signal that can regulate the reproductive system.
Melatonin has a very important role in the regulation of reproduction in the mare. When there is a long duration of melatonin, such as during the long winter nights, melatonin prevents the release of a hormone called gonadotropin releasing hormone (GnRH). GnRH is the hormone at the top of the hormonal cascade controlling equine reproduction. As its name suggests, it is responsible for the release of the gonadotropins, another subset of hormones. ‘Gonadotropin’ literally means to stimulate the growth of reproductive organs (gonads). Hence, in winter there is little GnRH around as it is turned off by melatonin and the ovaries of the mare are usually small, hard and inactive.
As the seasons change from winter to spring, the days get longer and light prevents the production of melatonin by the pineal gland. GnRH levels now start to increase and this ultimately stimulates the growth of the mare’s ovaries. This occurs gradually over an 8-10 week period so that the ovaries slowly start to wake up and start developing follicles that will eventually ovulate as the season progresses.
The Universal Birth Date for Horses
As is our nature, man has interfered with Nature’s approach to seasonality in the mare. At some time in history, it was decided that all horses should have the same birthday to make competitions fairer by restricting races to certain age groups. The date that was arbitrarily chosen at the time was January 1st. To this day, horses within most breed registries officially turn a year older on January 1st (or August 1st in the Southern Hemisphere). Within some equestrian disciplines, particularly that of Thoroughbred racing, this has led to a demand for foals born early in the year, or, as soon after Jan 1st as possible (or August 1st in Southern Hemisphere). Now, if we look closely at the normal annual reproductive cycle in the mare we can see that we have a problem.
The natural breeding season of the mare extend from April to November, with a transitional period of sub-optimum fertility at either end. Normally, most mares experience a winter period of ovary shutdown, when ovulations do not occur, during December, January and February in the Northern Hemisphere. This clearly conflicts with an industry desire to produce foals as early as January such that mares need to be reproductively active in February.
How have we overcome this? Using artificial light of course! For decades, the practice of exposing mares to an artificially lengthened day, beginning around Dec 1st, has resulted in successful advancement of reproductive activity to facilitate early breeding. By stabling our mares for 8 weeks and providing sufficient light for approximately 16 hours per day to inhibit melatonin, we can manipulate the early release of GnRH and activation of the ovaries.
Equilume Performance Lighting has been designed to influence a mare’s reproductive system through smart Stable Lights and mobile Light Masks.