A reptile egg loses weight as it develops, because water goes out through the shell, and we weigh every egg weekly to a hundredth of a gram to know it is alive. One clutch of eleven has gained weight every week for three years, on a balance that is calibrated monthly, and candled they are full and moving.
I am the senior keeper for the reptile house, and the incubator is my responsibility because I am the one who reads the balance.
How eggs are monitored.
Each egg is weighed on the day it is laid and every week after, on a balance that reads to a hundredth of a gram. A developing egg loses between ten and fifteen percent of its weight before it hatches, through the shell, and the curve of that loss is how you know it is alive. An egg that stops losing has died. One that gains has taken up water from the substrate, which means the box is too wet, and you dry it.
You also candle them, with a torch against the shell in the dark, and a live egg shows a network of vessels and, later, a shape that moves.
The incubation period for the species is about sixty five days.
A clutch of eleven was laid in the spring three years ago.
They have gained weight every week since. Not from the substrate, which is dry, and not much: a few hundredths of a gram a week, steadily, on every egg.
Candled, they are full. Vessels, a shape, and the shape moves.
They are about eleven hundred days into a sixty five day incubation.
What has been checked.
The balance. Calibrated monthly with certified weights, and I have weighed the eggs on the veterinary balance across the yard, which is a different instrument, and it agrees.
The substrate. Dry, and changed, and the eggs weighed in a dry dish on the bench with no substrate in it. They gain in the dish.
The shells. A shell that has thickened would weigh more, and I have measured the shells with calipers where I can. They are the thickness they were.
The contents. This is what the vet and I did in the second year. One egg was opened, because an egg that is a year past its hatch date is not going to hatch and the collection plan needed to know what was in it.
It was a live embryo, at about the stage you would expect three weeks before hatching. Perfect, warm, moving, and not one day further on than it had been on the candling a year before, and it had gained a gram and a half in that year, and the gram and a half was not in the embryo or the yolk or the fluid, all of which the vet measured, and which came to what an egg of that age should contain.
The egg weighed more than the sum of what was in it.
What the vet wrote.
She is careful and she wrote a careful report. It says the embryo was viable and developmentally arrested, that the cause of the arrest was not determined, and that the discrepancy between the whole egg weight and the component weights was outside the error of her method and could not be explained.
She did not open a second one. Neither of us wanted to.
What is done about it.
The ten remaining eggs are in the incubator, weighed weekly, candled monthly, and recorded in the book as ongoing, which is a word the licensing inspector has read and not queried, because the book shows eggs being monitored to the standard and that is what he is there to check.
The species plan has them listed as failed to hatch. They are not failed. They are three weeks from hatching and have been for three years.
What I do.
I weigh them on a Monday and I write the numbers down.
The one thing I have changed is that I do not candle them alone.
Candling is done in the dark, with a torch, holding the egg against the lens, and the shape inside shows as a shadow that moves.
In the third year I was candling the fourth egg in the dark with the torch, and the shape moved, the way it does, and then it stopped, and it turned, inside the shell, so that what had been the curve of its back was something else, and it pressed against the shell on my side, flat, the way a hand presses flat against a window, and it held there for as long as I held the torch, and I put the egg down and put the lights on and I have had a colleague in the room every Monday since.