Zoo Hospital: What Do You Weigh?

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Kirstin weighs a sun bear cub in 2009.

“Hey, Hospital Keepers! Can you get a weight on this animal, please?”

Most of us try to avoid the scale, so it is not surprising when animals do the same. For this reason, hospital keepers have developed many creative ways to convince our patients to put their reservations aside and step up on the scale voluntarily.

Weights are one of the most important and noninvasive observations that we can attain to monitor an animal’s health. There are numerous stories of how keepers have been able to get weights on their animals without handling them or stressing them in any way. From elephants to hummingbirds, everyone must get on a scale once in a while.

Think about when you’ve taken a pet to the vet. Have you seen how your animal is weighed? If the animal is calm, one can simply walk it up onto or plop it into a scale of some sort. Your pet stays still and looks up at you like it’s no big deal. But what if they have other ideas? Maybe the animal doesn’t want to sit still and keeps walking off the scale. Or maybe they stay on the scale but are doing a tap dance the whole time. Or maybe they are scared of it and try their best not to touch it. Or maybe the animal’s first instinct is to destroy it. So many scenarios…

What we try to do here at the San Diego Zoo’s Jennings Center for Zoological Medicine is the same as what the keepers do out on Zoo grounds. Our common goal is to get each animal familiar with the presence of the scale and comfortable enough to step on it. This desensitization process is a special time that could last a few minutes, a few hours, or a few days. When we can’t share the same space with an animal, whether for their safety or ours, physically putting them on a scale is not an option. And because the act of being restrained can be stressful, a hands-off approach works best.

There are many factors to consider when weighing an animal: determining which type of scale set up will work best; getting the animal comfortable with the sight, smell, and feel of the chosen scale; identifying their most active time of the day, their “happy place,” or their favorite food. We also have to consider the animal’s reaction to the keeper’s presence and their level of food motivation. Another thing to factor into the equation is how much your subject weighs. Larger animals are fine to weigh in kilograms, while smaller animals must be weighed on a “gram scale.”

Some scales are built into the floor of the room or tunnel area that the animal travels over on a regular basis. This is the ideal set up for keepers and animals. But most of the scales are portable, with many different components. Platforms are placed across specialized load bars with long cables that attach to an indicator. A metal box is placed over a one-piece base scale with attached cables and reader. Other scales are “all-inclusive” with the platform, weighing mechanism, and read-out wrapped up into one hand-held unit.

The most routine way to get a weight is to set up the load bars, cover them with a platform, and then drop a favorite food item on it. The animal then walks over, steps up onto the scale, and settles in to eat the treat. Once we know that all four feet (and tail, too!) are on the platform, we glance at the indicator and record their weight before they walk away. Sometimes an animal won’t approach the scale until the keeper leaves the area. This is when our hospital’s surveillance camera system comes in super handy. We can focus the camera directly on the scale’s reader and then leave the room. Once the animal thinks it’s alone, it comes over for its treat and stands on the scale. Using our cameras, we can see them and their weight without them seeing us. Pretty sneaky, right?

When food is not a motivating factor, sometimes we’ll place the scale in their favorite corner or we’ll set up their “happy place” directly on the scale. For example, marmosets can’t resist a nest of any kind, so when we’ve had to weigh these little guys, we’ve set up a gram scale in their room, balanced on all four corners to avoid any rocking or extra movement that would compromise the accuracy of the weight. Then we put their favorite basket of hay on the scale, pushed the “tare” button to put the indicator back to zero, and gave the marmoset access to that space. Within seconds we’ve got a marmoset in the nest and ultimately an accurate marmoset weight. It’s like someone slipping a scale under your favorite reading chair at home to get your weight and you’d be none the wiser!

A tinkerbird sits on a perch on a scale.

The same concept goes for other species: get a size-appropriate and secure scale set-up, and then identify what motivates them to hop on. When weighing birds, we place a perch on top of a scale in a high-traffic area close to their favorite spot. Larger birds sometimes take their time to perch; hopefully, not too long or else the scale’s indicator goes to “sleep”! Once the bigger guys are on, they’re on and not going anywhere, so we know we got a solid weight.

For the little itty-bitty birdies, like a hummingbird, keepers have to stand close by to view the “flighty” animal and keep an eye on the scale in hopes of “capturing” the weight when the bird lands on the weigh perch for a split second. We hope to catch a stable number a few times so we can then take an average weight of the “snapshots.”

Monitoring weights is just one of the many labors of love we do as keepers to make sure our animals are thriving. Most of the time it takes longer to set up the scale then it does to get the weight itself. But it is very satisfying to know we have gathered valuable information on our animals, and all without having their day skip a beat!

Kirstin Clapham is a senior keeper at the San Diego Zoo. Read her previous post, Zoo Hospital: A Panda Patient.