"Natural" Hatchings of Captive Chelonians

by Michael J. Connor, PhD

For many turtle and tortoise keepers, the bench-mark for success in the long term maintenance of healthy turtles and tortoises is the production of viable hatchlings from their animals. Generally, for successful reproduction to occur in captive chelonians careful attention needs to be paid to a host of requisite conditions. Although the available published knowledge of the natural history of many species is scanty, one key element of most successful breeding programs is that the animals are housed outdoors in an environs that mimics the species origins as much as possible. A great advantage of living in southern California is that, provided an artificial heat source is made available in the winter months, even tropical species can be maintained outdoors year round. Consequently, here in the Los Angeles area a relatively large variety of turtles and tortoises are successfully kept and bred.

With experience and careful observation most dedicated keepers learn to judge when their female chelonians are getting ready to nest. An animal may become unusually belligerent, pace erratically, attempt to climb out of it's enclosure, or spend hours sniffing at the ground and even dig trial nests here and there. Some animals will lay predictably at the same time every year or lay multiple clutches at definite intervals. Some keepers regularly palpate their female turtles for eggs and weigh them to check for unusual weight changes during the nesting season. Fortuitously too for their keepers, turtles and tortoises tend to take a considerable time to locate a nest site, excavate, lay their eggs and cover them. Thus, the likelihood that they will be observed in the nesting process is quite high. However, despite all this, some turtles (especially box turtles) are very secretive about their egg-laying activities and given the fact that most of us are not in our yards 24 hours a day, successfully manage to evade their keeper's vigilance. The result of this may be that the first time an owner is aware that a certain turtle or tortoise has nested is when a hatchling makes a sudden appearance in their yard.

Over the last few years I have had the delightful experience of discovering unexpected hatchlings on several occasions. Some of the species that have hatched in my yard, such as the Cistoclemmys flavomarginata, were real surprises since I hadn't realized that the animals were mature enough to breed. But for several species (particularly the hard to place sliders and cooters) I now deliberately leave the eggs in the nests to incubate naturally. This is not without its dangers of course. Predators may take eggs or hatchlings; nesting turtles may dig up an existing nest and destroy the eggs in the process; and given the enclosures that I provide for my animals sometimes it can be very hard to find emergent hatchlings. Many of these problems can be circumvented simply by covering the nest site with a mesh screen, and for some keepers this is actually their preferred method of incubating the eggs of hardier species.

The list of species of turtle and tortoise that have hatched either accidentally or intentionally following "natural" incubation here in the Los Angeles area is extensive. Table 1 below lists some examples that I am aware of, obtained by polling a small number of CTTC members. I have no doubt that many more species belong on the list. Since their natural range includes regions of southern California, it's hardly surprising that the list includes Gopherus agassizii (desert tortoise) and Clemmys marmorata (Western pond turtle). Nor, for that matter does it seem too unusual to see that "natural" incubation has worked for hardy North American, European and Asian species. However, the appearance of the tropical African spurred tortoise, Geochelone sulcata, and leopard tortoise, Geochelone pardalis on the list is probably something of an eye opener for many.

In the wild it is known that hatchlings of temperate turtles often over-winter in their nest, not emerging until the following spring. My own experience has been that over-wintering in the nest can also occur with captives. For example, I frequently get telephone calls from excited desert tortoise owners who have found hatchlings in their yards, and these almost always occur in the spring or fall. But over the years my impression has also been that even if a clutch of eggs has been laid in the spring it takes much longer for hatchlings to emerge from the nest when the eggs are left in situ than it takes for eggs to hatch when incubated artificially. This is despite the Valley's summer heat, with temperatures routinely 20°F warmer in the shade than in my incubator.

A little experiment in "natural" incubation

I recently had the opportunity to perform a little experiment to compare "natural" and artificial incubation. Six years ago I gave two of my newly hatched second generation captive-bred red-eared sliders to a colleague at work. Two years later he returned them--they were now 6 inches long and too big for the aquarium in which they had been raised. I kept these two young females in a small, east facing side-pond well away from my other sliders for two years. I was amazed one hot summer afternoon when, as I drained their pond to clean it, both turtles eschewed the shade I had temporarily placed them in and began to dig furiously in the sunny lawn just a few feet apart from each other and ultimately lay clutches of 10 eggs each. Later that year, I moved my adult male from his solitary confinement (isolated in my attempt at slider population control, although 3 years later his ex-mate is still producing hatchlings) to the young females' pen. In April 1996, I noticed both the females digging trial nests. I'm not sure what causes this synchronized behavior (it could be genetic since they are sisters or environmental since they've always lived together). I carried them back to the same dry, sunny lawn area where they had nested the previous year. Both promptly excavated nests and laid their eggs. When this little arribada was concluded I removed one of the clutches to be incubated indoors (clutch a) and left the other in situ (clutch b).

Clutch (a) containing 12 eggs was incubated artificially in a substrate of damp vermiculite in a plastic shoe box kept at 27°C, 81°F. One damaged egg spoiled after a few weeks and was discarded. The remaining 11 eggs hatched after 61 days incubation. The hatchlings all showed distinct umbilical scars and/or traces of yolk sac and caruncles (egg teeth).

Clutch (b) containing 12 eggs was left to develop in the nest. Hatchlings emerged January 27, 1997 following heavy rain and 2 consecutive warm nights (on which the minimum temperature was 10°C, 50°F). Time to emergence from the nest was 276 days. None of the 12 hatchlings had conspicuous umbilical scars or caruncles.

Obviously, 276 days is considerably longer than 61 days and certainly bears out my general impression. Since in the past I've often found slider hatchlings with both pronounced umbilical scars and egg teeth, some clutches must emerge from their nest considerably sooner after hatching than did clutch (b). The most likely time to find hatchling box and water turtles is following rain, or accidental flooding of their enclosure through injudicious use of sprinklers. Perhaps the moisture activates the hatchlings or simply makes it easier for them to escape from the nest. The summer of 1996 was particularly hot here in the San Fernando Valley, and this winter's rainfall has been somewhat relentless and well above average. The combination of a brief warm spell and moist conditions evidently triggered the emergence of clutch (b). Clearly, the seemingly longer incubation period for "natural" compared to artificial incubation is not simply because of a lower incubation temperature in the nest but because after hatching the emergence of hatchlings from the nest is dependent on other climatic factors.

A little experiment by mother Nature

Sandy Chase has had four separate clutches of the African leopard tortoise, Geochelone pardalis, hatch in her north San Fernando Valley yard in the last few years. Certain events of 1994 allowed Sandy to determine a minimum time to emergence for one of these clutches. She was forced to evacuate her home with her animals following the devastating Northridge earthquake of January 17. Ten months later, while still an evacuee, she was advised by her building contractor that a small tortoise had been found. A thorough search of the yard revealed that it was one of a clutch of 15 leopard tortoise hatchlings. Given the absence of adult tortoises from the property since the earthquake the minimum time from nesting to emergence was over 300 days. This is considerably longer than the typical time of 120-130 days it takes to hatch leopard tortoise eggs when Sandy Chase incubates them artificially at 28.5° C, 83° F.

Obviously, deliberately leaving turtle eggs to incubate in the ground is a chancy business, and I'm certainly not advocating it as a productive method for a serious turtle breeding program. However, as Table 1 clearly shows, when one lives in a suitable location or within a species home range where natural incubation is possible it is definitely something that a turtle keeper should watch out for. One day you too may experience the pleasant surprise of discovering an unexpected arrival in your turtle pen.

* Reporters: WA, Walter Allen; SB, Sean Baker; HC, Helen Cain; BC, Betty Caldarelli; SC, Sandy Chase; JC, John Coburn; MC, Michael J. Connor; DD, C. Dee Dillon; PN, Peggy Nichols; DS, David Shaffer. "Many" indicates more than 6 reports.

Originally published in the Tortuga Gazette 33(5): 6-8, May 1997