Reptile Egg Incubation: Temperature, Humidity & the Sex Mystery

For many keepers, breeding is the most rewarding stop on the reptile journey: from mating and egg-laying to the first tiny head breaking out of the shell two months later. But incubation is also the stage where "going by feel" fails hardest — a 1°C difference can shift hatching time by a week, and wrong humidity means mouldy or shrivelled eggs. This guide walks through the complete incubation process.

1. Pre-lay preparation

Two to three weeks before laying, a gravid female shows classic "nesting behaviour": restlessness, digging, and repeatedly checking corners. Set up a lay box — a container of moist vermiculite or coco fibre placed in a quiet corner of the enclosure. When the eggs are laid, do not flip or shake them. Pick them up gently, mark the top (the side that was facing up) with a soft pencil, and keep that side up for the entire incubation. Turning reptile eggs dislodges the embryo and kills it — the most common beginner breeding mistake there is.

2. Incubation medium: vermiculite vs perlite

The two standard media are vermiculite and perlite, and they work well mixed. Vermiculite holds water better and suits eggs that need high humidity; perlite is more open and airy. Moisten the medium with pure or dechlorinated water before use — the classic test is "squeezes into a ball, breaks apart when touched," roughly a 1:1 to 1:1.2 water-to-medium ratio by volume. Avoid tap water: chlorine and minerals disturb the egg's osmotic balance.

Half-bury each egg (top mark exposed) and space them apart so mould cannot spread between eggs. Leave a few ventilation holes in the box lid, aim for 85–95% relative humidity, and check the medium weekly, topping up moisture in small amounts when dry.

3. Temperature: stability above everything

Incubation temperature decides hatch timing and hatchling quality. For leopard geckos as an example: about 75–85 days at 27°C, 65–75 days at 29°C, and 55–65 days at 31°C. Warmer hatches faster, but sustained temperatures above 33°C raise deformity rates; below 25°C the embryo stalls or stops.

More important than "what temperature" is how stable. Keep day–night swings under 1–2°C; the bigger the swing, the higher the deformity rate. Strongly consider an incubator or a thermostatically controlled setup instead of "a corner of the room at room temperature." Log the temperature at the same time every day and correct any drift immediately.

4. Balancing humidity and ventilation

Too dry and the eggs dent and shrivel; too wet with no airflow and mould takes over. The practical check is the egg itself: a healthy egg is plump and faintly translucent. Dented means the medium needs water (add to the medium, never spray the egg directly); white mould patches mean humidity is too high — reduce medium moisture and increase ventilation.

Another danger is shell adhesion: when the medium is too dry, the shell sticks to the vermiculite and the hatchling cannot escape. Keep the medium slightly on the wet side rather than the dry side, and gently loosen the medium around the eggs during the weekly check.

5. Temperature-dependent sex determination (TSD)

In many reptiles — leopard geckos and some bearded dragon lines among them — sex is not set by chromosomes but by incubation temperature. This is temperature-dependent sex determination (TSD). For leopard geckos: cooler incubation (around 27°C) skews female, hotter (around 33°C) skews male, and middle temperatures produce a mix. You can target a sex by controlling temperature precisely — but extreme temperatures, while skewing sex, also carry higher deformity and health risks. Beginners should not gamble on sex this way.

6. The first 72 hours after hatching

One to two days before hatching, a fine slit appears in the shell. The hatchling then opens it with its egg tooth — a process that can take hours to a full day. Never tear the shell open "to help": let it come out on its own, because yolk absorption in those first hours is critical to survival. The newborn will carry a yolk sac on its belly; do not feed it yet, and place it in a small tub on damp paper towel to rest.

Between days three and five, once the yolk sac is absorbed, offer the first meal — the smallest possible prey, in small quantity. From there, follow the species' juvenile feeding rhythm, and start the Growth Tracker with the very first weigh-in. You will watch a satisfying growth curve appear.

Incubation is really "simulating nature while removing risk": stable temperature, humidity that is just right, eggs that are never flipped, and enough patience.