The elephant house floor has geophones under it, because an elephant that goes down in the night is a dead elephant by morning and the sensors hear one lie down before a camera would show it. They log each animal's footfall as a pattern. There are four patterns in the day. There are five at night, and the fifth is heavier than any animal we have.
I am the elephant team leader, and the floor sensors were fitted at my request after we lost an old bull in the night in 2018.
How the sensors work.
Geophones set into the floor slab read vibration. An elephant walking is a series of impacts and the software identifies each animal by the spacing and the weight of them, because a cow of three tonnes and a young bull of four do not sound the same through concrete. It logs which animal is moving, where, and when, and it alarms if one lies down and does not get up.
It was trained on our four. It knows them the way I know them.
At night it logs five.
The four, moving the way they move at night, which is not much. And a fifth pattern, heavier, slower, that walks the length of the house between two and three in the morning and stops at the far end, by the old door that was bricked up when the house was extended, and stands there.
The software calls it unknown adult, and gives its weight from the impacts as about five and a half tonnes.
Five and a half tonnes is a big bull. We have never had one that size. The old bull we lost in 2018 was four and a half at his heaviest, and the fifth pattern is not his. The software was trained on him too, before he died, and it does not recognise this one.
What has been checked.
The sensors. Recalibrated by the company that fitted them. Their engineer ran the night's data through a fresh copy of the software and it found five animals.
The cameras. The house has four, on infrared at night, and they show four elephants, asleep or standing, and the length of the house empty, while the floor logs a fifth walking down it.
The four. They react. On the nights the fifth walks, the four stand, at two in the morning, and turn to face the far end, all of them, the way they turn when a keeper comes in, and hold it until the fifth pattern stops, and then lie down again.
The far end. The bricked up door. Beyond it is the old part of the house, which is a store now, and I have been in the store at three with a torch and it is a store.
The dust.
An elephant house floor has sand on it, and the sand shows prints, and the prints are raked flat at the end of the day.
In the morning, on the nights the fifth walks, there is a line of prints down the house to the far end. Not the four's, which are in the sand where they slept. A single line, bigger than any of ours, rounder, to the bricked door and stopping, and nothing coming back.
I have photographed them with a scale beside them, and I have shown the photographs to a colleague at another collection who has bulls, and he looked at them and asked what I was feeding it.
What is done about it.
The fifth pattern is in the log under unknown adult, and the alarm is set to ignore it, because for a month it woke the duty keeper every night with an animal down at the far end that was not there when she arrived.
I have not put it in the licensing return, because the return asks for animals held, and I do not hold it.
What I do.
I run the team.
The one thing I have changed is the raking. The sand is raked flat in the morning now, not at night, so the line of prints is there when I come in, and I look at it before anybody else does, and I rake it myself.
The reason is a morning in the winter when the line of prints went to the bricked door, as it does, and there was a second line beside it, smaller, ours, one of the young cows, going with it to the door, and stopping, and her prints came back, alone, and the log had her at the far end for eleven minutes standing beside a weight of five and a half tonnes, and she had been, when I went to her, shaking.