Ndume Weighs In by Adrienne Mrsny

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Using operant conditioning to motivate Ndume to cooperate in health-related tasks is an enjoyable and challenging job that I feel lucky to have. I have worked with him on big tasks (sitting in the Gorilla Bungalow building and letting me close him in, to prepare him for spending the night there) and small tasks (opening his mouth on cue so we can inspect the color of his gums and condition of his teeth).

Recently I was presented with this challenge: obtain Ndume’s cooperation with being weighed on a portable scale. I was given a large platform scale that Koko used to use, and placed it at the mesh in the caregiver area of his building. From here, Ndume was able to inspect the device by putting his fingers through the mesh to touch it. As this point, I began researching similar situations and how zoo keepers have approached comparable challenges. I came across an article where staff at the Baltimore Zoo trained a white rhino to approach and be weighed with a mock scale that was later replaced with the real scale (Pill & Hange, 2000*). Their approach seemed to be the perfect fit for working with Ndume. He would be introduced to the mock scale through the mesh and then, when he was comfortable, it would be placed in his room. After getting him to sit on the mock scale on cue, the steps would be repeated using the real scale. In this way, any initial curiosity or nervousness would not damage the real (and expensive) scale.

His initial response to the real scale indicated how he might behave when given full access to it. When Ndume wanted to gain attention from a caregiver or make a point that he was upset about something, he would position himself against the mesh where the scale was on the other side, poke his finger through the mesh and shove the scale with all his might until it moved an inch or 2 away from the mesh. He would then immediately gallop away.

A mock scale was made out of birch plywood and nontoxic silver metallic spray paint to replicate the real scale. This scale was then swapped with the real scale and placed against the room’s mesh in the same spot. At first Ndume seemed to notice the physical difference and treated the mock scale with less interest. However, one day in a moment of frustration, he put his finger through the mesh and shoved the scale. Still not the desired attitude, but it was a good sign that he was viewing the mock scale in the same way he viewed the real one.

When Ndume’s somewhat mischievous attitude towards the mock scale (which I’ll henceforth refer to as the ‘scale’) seemed to pass, it was time to introduce the ‘scale’ into his room. I chose a rainy day when I knew Ndume would be likely to stay inside for much of the day. I placed the ‘scale’ at the same spot along the mesh but inside his room and placed his lunch in the far room. I gave Ndume access to his rooms for lunch and closed him inside. He calmly entered the room and walked past the ‘scale’ to his food, only glancing at it out of the corner of his eye. After 30 minutes of eating, Ndume had finished looking for bits of food and seemed to suddenly notice the ‘scale.’ He walked towards it and sat next to it. The first thing he did was look at the underside of the ‘scale.’ He sniffed this area, scratched at the posts, licked it and tried to gently bite it, testing its texture. Then he proceeded to test its strength. He stood on it with all of his weight on his knuckles. Then he tried to pry the sides apart. Once he was satisfied with this investigation, he walked away.

After a few moments of circling his rooms to search for overlooked food, his interest in the ‘scale’ resumed. Ndume returned to testing its strength, this time with his full body standing on it. Ndume then sat down on the scale, modeling perfectly what I planned to train him to do. Before I could comment and reward him, he tucked the ‘scale’ under his arm like a newspaper and walked upright into the far room. Here he began to roll it on his back, cleaning it with blankets, and in a moment of pure pleasure throwing blankets into the air and onto his head announcing his pleasure with loud purring and smiling the whole while. When I moved to the window closest to him to get a better look, he collected the ‘scale’ up again and moved to the original end of the building. Ndume spent he rest of the day playing and resting on his ‘scale.’ When dinnertime arrived, I placed his meal in a separate room and was able to close the door and safely move the ‘scale’ back to the caregiver area overnight.

The following day I placed the ‘scale’ in his room again. This time I left it in even longer in hopes that he would lose interest in it with extended exposure. Unfortunately, it had the opposite effect. The ‘scale’ had become his favorite plaything – the entire rainy day spent by him shoving it between rooms with a pile of blankets.

I had the following two days off, which allowed me some time to reassess how I would approach the next phase of Ndume’s scale conditioning. At the outset, I had been prepared for him to be either aggressive or bored with the scale, not for him to see it as his new toy or favorite place to sit. I finally concluded that the only way to proceed would be by desensitizing him to the ‘scale.’

Upon my return to work, I placed the ‘scale’ in his room again during his lunch. However, this time I joined him sooner than normal, and he was still eating when I entered the caregiver area of his rooms. Ndume had just finished eating and the ‘scale’ was where I had left it. I called Ndume over to the mesh and without asking, he sat perfectly on the ‘scale.’ I praised him and thus began our formal ‘scale’ training sessions together.

We have been working steadily for over a week now and Ndume no longer carries the ‘scale’ around. If he does happen to move it from its original placement, he will push it back at my request. I do miss seeing him carrying the ‘scale’ around, but I am glad we are on our way to getting him to work with us on the real scale for health and diet-related weight checks.

* Reference:
Pill, L. & Hange, B. (2000). Using Operant Conditioning to Weigh 11 Southern White Rhinos. Animal Keeper’s Forum, 27, 10.