Melatonin Around Foaling: Maternal–Foetal Endocrine Adaptation and Its Clinical Significance in Horses

Foaling represents one of the most complex physiological transitions in the life of the horse. Beyond mechanical parturition and passive immune transfer, subtle endocrine adaptations occur in both mare and foal to ensure neonatal survival. Among these, melatonin has emerged as a biologically significant but understudied hormone in the periparturient period. Recent findings provide valuable insights into how melatonin behaves immediately after foaling and how management practices may influence early neonatal physiology¹. 

Melatonin: More Than a Circadian Hormone 

Melatonin is a peptide hormone secreted primarily by the pineal gland during darkness and acts as a messenger translating environmental light–dark signals into endocrine responses. While classically associated with circadian rhythm regulation, melatonin also plays a key role in reproductive timing, antioxidant defense, immune modulation, and neurodevelopment^2,3. 

In long-day breeders such as horses, melatonin exerts an inhibitory influence on reproductive function by suppressing hypothalamic GnRH secretion and pituitary LH and FSH release⁴. However, its role during late pregnancy and the immediate postpartum period appears to extend beyond reproductive suppression. 

Periparturient Melatonin in the Mare: A Physiological Peak 

During gestation, melatonin concentration in mammals increases progressively, reaching a peak close to parturition. Although this phenomenon has been well documented in humans, comparable data in horses were historically lacking. Recent investigations have now demonstrated that mares exhibit elevated plasma melatonin concentrations at the moment of foaling, with an adjusted mean value of approximately 34.6 pg/mL. 

This elevation is consistent with the hypothesis that melatonin contributes to preparation for parturition by supporting placental function, reducing oxidative stress, and modulating inflammatory pathways. In humans, melatonin intensifies oxytocin-induced uterine contractions and sensitizes the myometrium to oxytocin, acting as an endogenous trigger of labour. While direct mechanistic evidence in mares remains limited, similar endocrine principles likely apply¹. 

Placenta as a Key Source of Melatonin 

One of the most clinically relevant observations is the rapid decline in maternal melatonin concentration during the first 12 hours postpartum. This sharp decrease is consistent with the loss of the placenta, which is recognized as a major extrapineal source of melatonin in humans^5,6 . 

Although placental melatonin synthesis has not been definitively confirmed in horses, the observed postpartum decline strongly suggests a comparable role. This finding emphasizes that periparturient melatonin concentrations should not be interpreted solely as pineal output but rather as a combined pineal–placental endocrine signal¹. 

Melatonin in the Newborn Foal: Partial Independence from the Mare 

Newborn foals demonstrate lower plasma melatonin concentrations at birth compared with their dams, with adjusted values around 27.6 pg/mL. Unlike mares, foals show only moderate changes in melatonin levels during the first 12 hours of life¹. 

This relative stability is particularly notable because mares and foals share the same stall environment and light exposure immediately after foaling. The finding suggests that neonatal melatonin secretion may be less responsive to external light stimuli or that the neural pathways regulating pineal activity are not yet fully mature¹. 

Previous studies have similarly reported minimal postnatal variation in melatonin concentrations in young foals under natural light conditions, while mares exhibited greater temporal variability¹. 

Clinical Implications for Neonatal Health Assessment 

Melatonin exerts antioxidant, anti-inflammatory, anxiolytic, and neuroprotective effects. In human medicine, it plays a critical role in foetal central nervous system development and placental homeostasis¹. These properties raise important questions regarding its potential contribution to neonatal adaptation in foals. 

Given the increasing emphasis on early neonatal assessment, it has been proposed that melatonin concentration could complement traditional indicators such as colostral immunoglobulin transfer when evaluating neonatal health status¹. While this concept remains investigational, it opens a new perspective on biochemical profiling in equine neonatology. 

Effect of Supplemental Lighting During Late Pregnancy 

Artificial light supplementation is commonly used to manipulate reproductive timing, particularly in performance breeding programs. However, recent data indicate that extended daytime lighting during late gestation significantly reduces nocturnal melatonin concentrations in mares at foaling. Foals born to light-supplemented mares also exhibit significantly lower melatonin levels at birth. 

Although no immediate association was found between reduced melatonin and colostrum IgG content, concerns remain regarding subtle effects on neonatal physiology. Supporting this caution, blue-light exposure during gestation has been associated with reduced foal height at birth, even in the absence of weight differences. Longitudinal follow-up studies did not identify developmental differences over the first year of life, but the long-term performance implications remain unknown^7,8. 

Practical Considerations for Veterinarians and Breeders 

From a clinical standpoint, these findings highlight the need for careful evaluation of lighting strategies during late pregnancy. While photoperiod manipulation remains an effective reproductive management tool, its endocrine consequences around foaling should not be overlooked¹. 

Importantly, the administration of exogenous melatonin is not permissible in Thoroughbred breeding programs, further emphasizing the need for optimized environmental management rather than pharmacological intervention. 

Future Directions 

The availability of a large postpartum dataset measuring melatonin concentrations in mares and foals represents a significant advancement in equine reproductive endocrinology. However, further studies are needed to track melatonin dynamics throughout gestation and compare pregnant and non-pregnant mares under identical environmental conditions¹. 

Such data will be essential to determine whether melatonin can be reliably integrated into neonatal health assessment and whether periparturient endocrine modulation has lasting implications for foal welfare and performance. 

References 

1. Gáspárdy A, Gallagher G, Bartha B, Cseh S, Fekete SG, Somoskői B. Plasma melatonin concentration during the early post-partum period in Thoroughbred mares and their foals. Acta Veterinaria Hungarica. 2023 Oct 17;71(2):119-27. https://akjournals.com/downloadpdf/view/journals/004/71/2/article-p119.pdf 

2. Carcangiu, V., Giannetto, C., Luridiana, S. and Piccione, G. (2018): Features of the daily rhythms of blood melatonin and glucose in goats during different natural photoperiod, Chronobiol. Int. 35, 329–335. https://doi.org/10.1080/07420528.2017.1405968 

3. Giannetto, C., Carcangiu, V., Luridiana, S., Parmeggiani, A. and Piccione, G. (2020): Twenty-four-hour rhythm patterns of plasma melatonin in short-day and long-day breeders maintained under natural environmental conditions, Chronobiol. Int. 37, 974–979.  https://doi.org/10.1080/07420528.2020. 1772808 

4. Talpur, H. S., Chandio, I. B., Brohi, R. D., Worku, T., Rehman, Z., Bhattarai, D., Ullah, F., JiaJia, L. and Yang, L. (2018): Research progress on the role of melatonin and its receptors in animal reproduction: a comprehensive review. Reprod. Domest. Anim. 53, 831–849. https://doi.org/10.1111/rda.13188 

5. Hardeland, R. (2017): Melatonin – more than just a pineal hormone. Biomed. J. Sci. Tech. Res. 1(4). https://doi.org/10.26717/ BJSTR.2017.01.000351 

6. Ejaz, H., Figaro, J. K., Woolner, A. M. F., Thottakam, B. M. V. and Galley, H. F. (2021): Maternal serum melatonin increases during pregnancy and falls immediately after delivery implicating the placenta as a major source of melatonin. Front. Endocrinol. 11: 623038. https://doi.org/10.3389/fendo.2020.623038 

7. Lutzer, A., Nagel, C., Aurich, J., Murphy, B. A. and Aurich, C. (2022a): Development of foals until one year of age when the dam was exposed to blue monochromatic light directed at one eye during late pregnancy. J. Equine Vet. Sci. 112: 103922. https://doi.org/10.1016/j.jevs.2022.103922 

8. Lutzer, A., Nagel, C., Murphy, B. A., Aurich, J., Wulf, M., Gautier, C. and Aurich, C. (2022b): Effects of blue monochromatic light directed at one eye of pregnant horse mares on gestation, parturition and foal maturity. Domest. Anim. Endocrinol. 78: 106675. https://doi.org/10.1016/j.domaniend. 2021.106675