I last left you with a description of the nesting and egg laying process. It is time to continue describing the rest of the story.
So, the eggs are laid, the nest is covered, and we wait. The wait can take several months in a natural setting, as the eggs take their time incubating, hatching, and the babies emerge from the nest.
In the meantime, why don’t I go over some neat facts about those little eggs and the hatchlings that result.
And yes, today we have baby pictures! Prepare to squee, folks.
The size and shape of Eastern box turtle eggs can vary slightly with some box turtles laying rather elongated eggs, and others laying rounder eggs, but by and large they can all be considered elliptical in shape. It is fascinating to see how much variation there can be from one individual to the next. There is one female here that lays a small number of very long eggs in each clutch (4cm long!): another females here lays more eggs in each clutch, each of which are smaller and rounder (just over 2cm long).
Here's a photo of eggs in a nest (I removed the covering soil)
Here are eggs from two different nests, laid by two different females. note the size differences:
Eastern box turtle eggs aren’t leathery the way, say, sea turtle eggs are. It takes a while after laying for the eggs to ‘chalk’: that is for the thin calcium layer on the outside of the egg to harden. Even after the eggs have chalked, the calcium layer is so thin that the eggs retain a degree of flexibility. In fact, I have found that in the course of positioning the eggs within the nest, it is not unusual for the female to dent some of the eggs slightly. These dents will pop back in to shape on their own.
Incubation temperatures vary within the nest, because, of course, external temperatures fluctuate, however temperatures within the nest do not shift as rapidly as the outside air temperature.
The length of incubation varies as well, and depends on temperature. Higher overall temperatures translate in to a shorter incubation period, while lower temperatures will cause the incubation time to increase.
Here’s an interesting fact: Eastern box turtle. Like many other reptiles, have no sex chromosomes. So, you may ask, how does a genderless egg develop into a male or female turtle? By something called Temperature Dependent Sex Determination. The temperature during incubation, specifically at a certain critical stage of incubation and embryonic development, determines gender.
The ideal range of temperature for incubation of Eastern box turtle eggs is about 72F at the low end, and 93F at the high end.
The result of much research and testing over the years has resulted in the finding that Eastern box turtle eggs which incubate at temperatures at the lower end of the ideal range, say 75F-80F, produce more MALE hatchlings, whereas incubation at the higher end of the ideal range, abut 84F or higher produce more FEMALES. Those eggs that are incubated at temperatures in the middle of that range have about a 50-50 chance of being male or female.
So, those slight differences on temperature within the nest may be critical to determining the gender of the hatchlings.
Average incubation time under natural conditions is approximately 70 days. Under laboratory conditions (artificial incubation) the time is somewhat shorter, most likely due to the fact that temperatures are more closely controlled in a man-made incubator.
I have artificially incubated quite a few Eastern box turtle eggs by now, and based on my own observation and data, eggs incubated at 85F-87F will ‘pip’ or begin to hatch after just 55 days.
Unlike bird eggs, box turtle eggs (or reptile eggs) are NOT rolled. In fact, rolling can destroy them, by breaking very fragile tiny developing blood vessels.
Prior to hatching the eggs swell slightly, which is possibly due to the final growth and shape of the baby inside. Another very interesting thing occurs frequently within 24 hours of ‘pipping’ (when the hatchling first breaks through the eggshell. The eggs may begin to ‘sweat’, as excess fluid is literally pushed out through the thin shell.
Here is a photo of some pipping hatchlings, with droplets of 'sweat' still visible on the eggshells:
Hatchlings are equipped with a caruncle, or egg tooth, at the tip of their beak in between the nares, or nostrils.
You lookin' at me?
Hatchlings can take 2 or even 3 days to fully emerge from the eggshell, or occasionally the process can take just a few hours. This process can be critical for the young turtle, as it still has a considerable amount of yolk to absorb: the eggshell appears to serve the purpose of supporting the yolk with gentle pressure, and of course protecting it as well. A baby that slips out of the eggshell prematurely runs the risk of not being able to properly absorbing the yolk, or even rupturing it, either of which are potentially fatal.
When the baby does emerge from the eggshell, there is still yolk sac exposed but it is typically no larger than a pea in size and it is rabidly absorbed via the action of smooth muscles in the abdomen.
This photo shows a yolk which is already mostly absorbed:
Here is what the yolk would look like the following day:
The baby turtle can survive on the contents of that yolk for several weeks or even longer.
Hatchlings may not emerge from the nest for several days or weeks (or even months…over-wintering in nest) after hatching. This may lead casual observers to believe that actual incubation takes longer than it really does. The hatched babies may have to wait for rainfall to sufficiently soften the soil in order for them to dig their way out.
When they first emerge from the eggshell, they still have the (somewhat) rounded shape of the egg itself. Within a day or two, the baby’s ribcage will spread out a bit and take on a slightly flattened shape.
The hatchling on emerging is typically grayish-brown in color, and frequently has a yellowish spot on each large scute: on the vertebral scutes, the spots are usually found on the raised section, which is referred to as the ‘keel’.
The hatchling’s plastron is typically yellowish, with a dark blotch on it. I find these blotches to be useful identifiers when the hatchlings are very young, but alas, the blotch pattern changes and grows with the turtle, so after a year or so the plastron pattern will have changed quite radically.
At this stage, the tiny turtle has no functioning hinge, its ribs have not yet fused together, and so its carapace is somewhat pliable. Hatchlings are incredibly vulnerable at this stage, as they are bite-sized morsels for a huge variety of predators, including some species of snakes and even chipmunks! It is estimated that mortality rates for hatchlings is in the neighborhood of 95%, although to be honest, I am not sure if that figure includes egg predation.
Once emerged from the nest, these tiny little turtles will spend most of their time hiding and feeding on whatever biota is available to them. They are primarily carnivorous at this stage in their lives, eating mainly pill bugs, small crickets, worms, slugs, and other small prey that utilize the same environment under clumps of grass and leaf litter. They will also feed readily on mushrooms. My own observation has been that they simply do not attempt to eat any living prey that is much wider or larger than their own mouth. Apparently they are well aware of what they can handle.
This explains why it is extremely unusual to spot hatchling box turtles in the wild. This hiding behavior will continue for several years, until the turtle reaches a size which enables it to be more predator proof (usually around 3 inches long), has a fully functioning hinge, and has developed a considerably hardened shell.
Once they make it to this stage, mortality rates drop considerably. Their diet by this age is far more varied, as they venture forth and sample a wider variety of food items.
