The Moon’s Hidden Hand: Lunar Cycles Shape Reproduction Across the Vertebrate Family Tree

For as long as humans have looked up at the night sky, we have suspected that the Moon has something to do with fertility. Ancient cultures tied planting, harvest and childbirth to the lunar phases, and the idea never quite went away. What has changed is the quality of the evidence. A new review published in Frontiers in Mammal Science by Daniela Alberghina of the University of Messina synthesizes decades of chronobiological research and finds that, far from being pure folklore, lunar rhythms leave measurable fingerprints on vertebrate reproduction — from the spawning of reef fish to the timing of calving in dairy cows and, perhaps, the menstrual cycles of women. The review spans fish, amphibians, reptiles, birds and mammals, and its central message is that the Moon may be a far more important biological timekeeper than mainstream science has been willing to admit.

The physical basis is easy to state and hard to study. As the Moon orbits the Earth, its gravitational field drags tidal bulges across the oceans, imposing dramatic environmental swings on coastal organisms. Biological responses come in a hierarchy of periods: circatidal rhythms repeat roughly every 12 hours, circasemilunar rhythms every 14 or so days, and circalunar rhythms about every 29.5 days, matching the synodic month. Crucially, these are not simply reactions to external cues; organisms inherit clock genes that endow them with the capacity for rhythm, and local factors such as light, temperature and tides then entrain, or set, the phase of the internal oscillator. The molecular machinery behind the short-period tidal and circadian clocks appears distinct from whatever drives the longer lunar clock. And while circadian clocks across species are built on similar foundational principles, circalunar clocks seem to have evolved multiple times using diverse mechanisms — a puzzle that researchers say the next decade of molecular work could finally untangle.

Fish offer the clearest and most widespread examples. Rabbitfish spawning across the tropical Pacific is consistently tied to a specific lunar phase, with so little geographical variation that the Moon must function as a genuine reproductive cue rather than an incidental correlate. The forktail rabbitfish, which spawns around the last quarter moon, shows lunar-driven correlations between testicular hormone levels and sperm motility. Depending on the species, spawning runs on a monthly cycle of roughly 29 days, a semi-lunar cycle of about 14 days timed to spring tides, or a daily tidal cycle. In all cases, the maturation of egg cells and the production of sex steroid hormones are precisely aligned with the lunar event, a regulation thought to involve the hypothalamus-pituitary-gonad axis, the central control system of vertebrate reproduction.

How fish translate an external lunar signal into an internal rhythm remains unresolved, but several candidate pathways have emerged. Melatonin levels fluctuate with nighttime brightness, magnetic fields and tidal cycles. In one tropical species, moonlight inversely regulates the expression of melatonin receptor genes in the pineal organ, with mRNA levels highest around the new moon and lowest near the full moon. Cyclic changes in hydrostatic pressure associated with tides alter monoamine neurotransmitter content in the brain, and light-sensitive clock genes respond to moonlight in ways that may phase-shift the animal’s biological clock to match the lunar month. In the grass puffer, expression of key reproductive genes — kisspeptin, gonadotropin-inhibitory hormone and their receptors — oscillates with lunar and semilunar cycles, peaking during spring tides. Together, these findings sketch a sensory-to-molecular pipeline that runs from moonlight and pressure to the hypothalamic genes that switch reproduction on and off.

Amphibians add an ecological twist: different species appear to use different lunar cues for different purposes. Mass spawning events and amplexus, the mating embrace of frogs and toads, are significantly more frequent around the full moon. In Java, where climate varies little across the year, both the cane toad and the Javanese frog show peak female reproductive activity around the full moon, and the synodic cycle also shapes the spring migration and breeding of the common toad in Europe. These unimodal full-moon peaks are thought to be triggered by the brightest moonlight of the month. But some species do the opposite. The threatened Kloof Frog breeds less as lunar brightness increases, apparently avoiding exposed, moonlit nights to dodge predators. Meanwhile, newts of the genera Lissotriton and Triturus show first sightings and peak arrivals near both the full and new moons — a bimodal pattern that points to the gravitational cycle rather than moonlight as the driver. The coexistence of moonlight-driven and gravity-driven events suggests amphibians exploit multiple lunar cues to time distinct phases of their reproductive lives.

Reptiles are less thoroughly documented, but the evidence is growing. Female taricaya turtles nest around the full moon, plausibly relying on bright moonlight as a visual cue that enables social facilitation, where the presence of nesting neighbors stimulates further nesting. A three-year study of green sea turtles in Veracruz, Mexico, found that both nesting and hatchling emergence peaked at new and full moons, while hatchling survival was highest during the first and last quarter phases; the incubation period itself varied with lunar phase. The authors argue that incorporating lunar cycles into conservation planning could improve habitat protection and boost hatchling success. Hawksbill turtles add a note of caution: although oviposition was more frequent during the first and last quarters, remigrant females did not consistently choose the same lunar phase in successive years, and overall nesting activity showed no relationship to whether the Moon was in the sky.

In birds, the Moon acts mainly through light, shaping the signals animals use to attract mates. Neotropical nightjars and the ferruginous pygmy-owl vocalize six to eight times more on full-moon nights than on new-moon nights, suggesting that moonlight facilitates nocturnal communication. Male African houbara bustards perform their nocturnal courtship displays exclusively during the full moon, increasing booming vocalizations on moonlit nights while limiting visual displays to a flash of white neck feathers. Eagle owls go further, exploiting moonlight to make a white throat patch more visible, calling more often and from higher perches when the night is bright. Hormonal echoes exist too: daily variations in melatonin and corticosterone appear to flatten during full-moon periods in some species. Most strikingly, a five-year study of breeding hens held under a controlled photoperiod found that the lunar cycle correlated strongly with egg production, hatch rates and hen mortality — with the effect concentrated at fertilization rather than laying — implying a long-term lunar influence on conception itself. In roosters, semen quality declined during the new moon under natural light.

Among mammals, the statistical case is broad if circumstantial. A comparative analysis of gestation lengths across 213 species of terrestrial placental mammals found that the number 30, approximating the 29.53-day synodic month, was consistently the best-fit multiple to the data, outperforming random control sets. Individual studies add texture: spontaneous deliveries in Holstein cows peaked just before the full moon and bottomed out at the new moon, while tropical Brahman cows showed a bimodal calving and estrus rhythm linked to both full and new moons. Serengeti wildebeest concentrate conceptions into two consecutive full-moon periods, a strategy thought to reduce predation risk. Lunar effects on sex ratio and delivery timing appear in pigs, goats, horses, guinea pigs and dogs — first-litter bitches deliver more often during the waxing moon, multiparous ones during the waning moon — though artificial lighting and human management clearly weaken these patterns in domesticated animals. Intriguingly, mammals harbor a recurring 12-hour rhythm in gene expression and metabolism that resembles the circatidal clocks of marine ancestors, hinting at an evolutionary vestige of tidal timekeeping embedded in terrestrial physiology.

Humans are where the science gets contentious. Two decades ago, the consensus was that lunar cycles do not regulate human biology, a conclusion drawn largely from aggregate studies that may have canceled out individual variation. More recent longitudinal work challenges that view: menstrual, sleep-wake and even manic-depressive cycles can synchronize with lunar rhythms. Studies of menstrual timing have produced conflicting directions — some find more menstruations near the full moon, others near the new moon — but a very recent analysis found that cycles recorded before 2010, before LEDs and smartphones became ubiquitous, synchronized significantly with the Moon, whereas post-2010 cycles coupled only in January, when the combined gravitational pull of Moon, Sun and Earth is strongest. Artificial light at night is the leading suspect for erasing the synchrony. Conception data from Vienna show a peak around the full moon, and one hypothesis holds that lunar gravity differentially selects lighter Y-bearing or heavier X-bearing sperm, though the crucial question of whether cervical mucus density varies with lunar phase remains untested. Large-scale studies have repeatedly failed to confirm the popular claim that births cluster at the full moon, yet a Japanese analysis of 1,507 full-term spontaneous births found that nocturnal deliveries peaked at or around the full moon while daytime births peaked at the new moon — a pattern consistent with an evolutionary advantage of moonlit-night delivery.

The review’s author is careful about mechanisms. Full moonlight is far too dim to suppress melatonin directly, so non-photic forces — gravity or geomagnetic fields — are the leading candidates for how lunar signals enter mammalian physiology, and cryptochrome, a light- and magnet-sensitive circadian protein, is one proposed conduit. Whether these moon-dependent clocks are inherited traits, driven by external cues, or emergent properties of internal metabolic cycles is still unknown. What is clear is that the correlations are too widespread to ignore. Researchers argue that the study of marine relatives may illuminate our own evolutionary past, and that expanding this work to more organisms could reveal universal mechanisms of biological timekeeping. From spawning rabbitfish to women’s menstrual cycles, the Moon’s rhythmic dance may run far deeper in the vertebrate lineage than folklore ever imagined.

Subject of Research: How lunar rhythms influence reproductive behavior and timing across vertebrate species

Article Title: A rhythmic dance: how lunar cycles influence reproductive behavior in vertebrates

Article References: Alberghina, D. (2025). A rhythmic dance: how lunar cycles influence reproductive behavior in vertebrates. Frontiers in Mammal Science, 4, Article 1690167. https://doi.org/10.3389/fmamm.2025.1690167

Image Credits: AI Generated

DOI: 10.3389/fmamm.2025.1690167

Keywords: lunar cycles, circalunar rhythms, vertebrate reproduction, chronobiology, melatonin, spawning, courtship, menstrual cycle, biological clocks, tides, animal behavior, evolution