Biology
The Neurobiology of Circadian Rhythms and Seasonal Breeding in Mammals
Quick fact
Mammals detect the changing season by the length of the night, not the day. The pineal gland releases the hormone melatonin only during darkness, and the duration of that nightly melatonin pulse is the chemical message that tells the brain whether to turn reproductive systems on or off—even though the seasons are caused by changes in day length, it is the night that carries the critical information.
Why this is interesting
Why do most mammals give birth in spring, even though they mated months earlier? The answer lies not in a calendar, but in a brain region that reads the changing length of daylight.
Read the full explanation
Understanding The Neurobiology of Circadian Rhythms and Seasonal Breeding in Mammals
Think of your body as a clock shop: every cell has its own ticking clock, but there is one master clock that keeps them all in sync. In mammals, that master clock is a pair of tiny clusters of neurons deep in the brain, called the suprachiasmatic nucleus (SCN). It runs on a roughly 24-hour cycle and is reset every day by light entering through the eyes—not for seeing, but for timing. This daily reset is the 'circadian rhythm'—the biological day. Now imagine you need to know the season. The SCN doesn't just keep time; it also measures the length of the day. When light fades in the evening, the SCN sends a signal to the pineal gland, a small endocrine gland at the base of the brain. The pineal gland responds by secreting melatonin, a hormone that makes you feel sleepy. But crucially, melatonin is only produced at night. In summer, nights are short, so melatonin is released for only a short time. In winter, nights are long, so the melatonin pulse is prolonged. The brain reads the duration of this nightly melatonin signal as a measure of the season: short nights mean summer, long nights mean winter. This is the interface between circadian timekeeping and seasonal reproduction. The SCN is like a technician that reads the light signal, and melatonin is the messenger it dispatches. When the melatonin signal indicates that days are lengthening (that is, nights are getting shorter), the brain responds by releasing hormones that kick-start the reproductive system. In long-day breeders like hamsters and many rodents, increasing day length stimulates breeding. In short-day breeders like sheep and deer, it's the opposite—shrinking day length triggers mating. Either way, the result is that offspring are born at the optimal time of year, usually when food is plentiful and temperatures are mild. So, the mechanism is: light → eyes → SCN → pineal gland → melatonin → reproductive centers → hormones → breeding. Each step is a carefully tuned link in a chain that connects the environment to the reproductive system.
A deeper explanation
The core of this process is a neuroendocrine pathway that converts the daily rhythm of light into a seasonal reproductive switch. The pathway begins with specialized photoreceptors in the retina (melanopsin-containing retinal ganglion cells) that project via the retinohypothalamic tract directly to the SCN. The SCN, a pair of nuclei above the optic chiasm, is the master circadian clock. It generates a near-24-hour rhythm in electrical activity, which drives a rhythm in sympathetic output to the pineal gland. The critical step is the night-time secretion of melatonin. Melatonin is produced from serotonin by the enzymes arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT). The activity of AANAT is regulated by the SCN via a multi-synaptic pathway from the SCN to the paraventricular nucleus (PVN), then to the intermediolateral cell column of the spinal cord, and then to the superior cervical ganglion, which sends postganglionic sympathetic fibers to the pineal gland. This ensures that melatonin is only released during darkness. The melatonin signal that matters for seasonality is the duration of the night-time elevation. Melatonin binds to receptors in the pars tuberalis of the pituitary gland, a region that acts as a photoperiodic sensor. There, the duration of melatonin exposure changes the expression of the transcription factor EYA3 and the hormone thyrotropin (TSH). Secreted TSH acts on tanycytes in the mediobasal hypothalamus, stimulating the expression of the enzyme deiodinase type 2 (DIO2), which converts thyroxine (T4) into triiodothyronine (T3). The local increase in T3 then modulates neurons that control the release of gonadotropin-releasing hormone (GnRH). GnRH acts on the anterior pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which in turn stimulate the gonads to produce sex steroids and gametes. Thus, the pathway is: light → SCN → pineal melatonin → pars tuberalis → TSH → DIO2 in hypothalamus → T3 → GnRH → pituitary gonadotropins → gonadal activity. This elegant cascade allows mammals to predict the changing season and time their breeding cycles precisely. The system is robust but vulnerable: artificial light at night can disrupt the melatonin signal, and climate change can alter the predictive value of photoperiod, leading to misalignment with food availability—a concern for many species.