How do tortoises hatch their eggs?

Answer 1

Hatching tortoise eggs in a home-made incubator

Copyright 2006 by Peter Kacalanos

Our largest western Hermann's tortoise (Testudo hermanni hermanni) laid another clutch of seven eggs on May 18, and yet another clutch of seven eggs on June 16, 2006. Each clutch was immediately moved to our incubator to assure the safest hatching environment. The tortoise is Josephine, aged 16, and her chelonian consort (naturally yclept Napoleon, aged 50) was the male involved. He's been courting her since last year in the style typical of this species: He follows her all over the garden, nips at her toes to get her attention, then keeps butting his shell repeatedly against hers. (It sounds like a carpenter enthusiastically hammering on a nail.) Whether this sexually stimulates her or simply annoys her, she finally permits him to mount her to consummate the relationship. This occurred frequently last summer and this spring, resulting in these latest clutches of eggs.

For several days before laying the eggs, Josephine showed the traditional signs that she was ready to dig a nest by pacing all over the 200 square meters of our fenced-in garden (excluding the goldfish ponds) to find a suitable spot. Typically, she dug several tentative holes in various areas, and abandoned them all as unsuitable. Our garden's soil is mostly hard-packed clay, so I had made a large mound of a loose organic soil mix in the sunny center of the property. Finding that mound the easiest place to dig, she proceeded to dig a hole with her hind legs about six inches (15 cm) deep. We watched in fascination as she deposited the seven large eggs in the hole. Then she used her hind legs to meticulously cover the eggs with the excavated soil so that the nest site wouldn't be discovered by predators. The process took a full three hours, and she was working in a light rain all the while. Turtles and tortoises usually lay their eggs during or after a rain, when the soil is easier to dig.

In nature the vagaries of weather produce very variable results in the hatching of tortoise eggs left in situ. A controlled environment creates a much better rate of successful hatching, so I carefully dug up the eggs and placed them in our home-made incubator. It consists of a shallow 30cm by 50cm glass box, covered with a sloping glass roof for easy visibility of the developing eggs.

I placed a dot on the top of each egg to indicate which side is up. Bird eggs are regularly rotated by the parent birds to

warm them uniformly. If bird eggs are kept in an incubator, they should be turned over at least once a day to assure proper development. But in nature, turtle and tortoise eggs buried in an underground nest of course would never be moved. They shouldn't be turned over in an incubator either, lest the delicate blood vessels developing in the embryo get damaged.

The Hermann's tortoise eggs in our incubator rest on dampened vermiculite in a small tray, placed on a wire-mesh shelf positioned above a large tub of water. Aquarium heaters in glass tubes with automatic thermostats are in the water to generate the necessary warmth and humidity in the incubator. Several battery-operated digital thermometer-hygrometers placed next to the eggs register the ambient temperature and humidity within the incubator. These devices aren't always accurate, so we average the readings of several to determine the actual temperature and humidity.

This arrangement created a humidity of 95%, but the ideal humidity for most terrestrial tortoise eggs is 70%. (For aquatic turtle eggs, whose shells are softer and thinner, humidity should be about 90%.) I decreased the humidity to 70% by covering most of the vessel holding the water with a sheet of acrylic plastic to lessen the water's surface area, and thus the rate of evaporation in the incubator. (Having much less water in the vessel would have accomplished the same result, but that would create a different problem: The smaller volume of water would be heated much more quickly by the heaters, so the built-in thermostats would shut the heaters off almost immediately, resulting in insufficient heat in the incubator.) The 70% humidity is enough to condense on the inside of all the glass surfaces, including the incubator's ceiling. If the ceiling were horizontal, some of the water condensing there would drip directly onto the eggs. The ceiling is slanted like a gable roof so that the water condensing there will run down to the inside front wall of the incubator instead.

Incubating eggs at the proper temperature is even more important than the relative humidity. If the temperature is a bit too low or too high, the hatchlings could be born with deformed shells or other abnormalities; in extreme cases, turtles have been born with two heads, and soon perished. If the temperature is much too low or too high, the eggs won't hatch at all. Within the appropriate temperature range, warmer temperatures tend to speed up the eggs' development, and cooler temperatures retard it.

Interestingly, incubation temperature also determines the ratio of males and females developing in the eggs. Herpetologists call this effect Environmental Sex Determination (ESD), or Temperature-dependent Sex Determination (TSD). Cooler temperatures tend to produce more males, while warmer temperatures tend to produce more females. In other reptiles whose sex is determined by temperature, such as alligators and some lizards, the reverse is true.

The precise temperature and hatching time varies for each species of turtle and tortoise. For our Hermann's tortoises, a temperature of less than 26°C is insufficient to hatch the eggs. From 26°C to 29.5°C, mostly males will hatch within 75 to 140 days. From 30°C to 32°C, a roughly equal number of males and females will hatch within 65 to 80 days. From 32.5°C to 34°C, mostly females will hatch within 60 to 75 days. And above 34°C, any hatchlings that develop will be deformed or die within the shell.

With this latest clutch of seven eggs laid by our Josephine, we'd like a nice mix of males and females, with perhaps one or two extra females to add to our breeding colony. With that in mind, I've regulated the temperature to range between 31°C and 32°C, which should produce the results we'd like. We expect the eggs to hatch in from nine to twelve weeks. But we won't know for at least ten years the sex of each hatched tortoise, because they don't show any clear external sexual characteristics until they're sexually mature. Stay tuned to this web site for the next decade to get the results.

In the meantime, members of the Geneva Area Turtle and Tortoise Society and their guests can view the developing eggs and the design of our incubator when attending the Society's monthly meetings. Also on view in their indoor and outdoor playpens are the growing Hermann's tortoise babies that were born in September 2005. Details of each monthly meeting can be found by going to the Society's web site at http://ponds.meetup.com/about and clicking on the link m

Answer 2

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