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August 19, 2019

By Ethan Riepl, Primate Keeper

The Saint Louis Zoo is home to three female black-handed spider monkeys: Jenny, Patty and Sammy. The most identifiable thing about spider monkeys, aside from the happy little noises they make when eating food, is arguably their impressive prehensile tails, which is a tail that can grasp and hold onto things.  A common misconception about primates is that they all have fully prehensile tails; however, only a handful of monkey species native to Central and South America possess this unique attribute,  including spider monkeys, which we have here at the Zoo. 

The tip of the tail is hairless and textured for gripping, rendering it almost as functional as their hands. Essentially they have a tail-version of fingerprints, complete with patterns unique to each individual.

While capable of picking up objects as small as a sunflower seed and occasionally being used to hold food when moving around, the tail often seems to have a mind of its own and is largely focused on searching for something to grab onto. Spider monkeys live in the highest level of the rainforest canopy and this prehensile tail gives them a safety line.

Visitors to the Primate House will likely notice the variety of complex feeders and toy-like objects in all primate habitats. These are common examples of the enrichment that is provided on a daily basis, specifically appealing to primates’ natural curiosity and problem-solving abilities; however, there’s a lot more going on when it comes to enrichment, both in plain view and behind the scenes. To start, environmental enrichment is all about providing opportunities for animals to engage in natural behaviors. While the aforementioned feeders and objects elicit some fairly conspicuous behaviors from the animals, such as foraging, problem-solving, investigation and manipulation of objects, they don’t represent anywhere near the full range of behaviors typical of most primate species. To identify and address those behaviors, keepers start by creating a detailed profile of each species concerning all aspects of their natural history. The questions range from the straightforward (What does it eat? Where does it sleep? How does it move around?) to a little more complex and obscure (How does it identify and communicate threats? Does it maintain a territory and, if so, how does it defend and utilize it?), and it’s not all that uncommon to encounter things that science just doesn’t know. This compilation of all available information about each species serves to paint a comprehensive picture of an animal’s lifestyle, the challenges it is equipped and expected to deal with, and ultimately its behavioral needs. With this information in hand, keepers regularly and repeatedly evaluate how well the current environment provides opportunities for the animals to engage in each of their respective behaviors. Wherever things are found lacking, keepers focus on creating those opportunities via new enrichment, perching and habitat decoration and even making adjustments to the daily management and routine. Environmental enrichment is a perpetual cycle of planning, implementing, evaluating and readjusting, and is always improving the standards of animal welfare.

A few years ago, the spider monkeys were the focus of this reevaluation process and the arboreal nature of the species was brought up.  Keepers do a great job of creating perches in the animals’ habitats and the spider monkeys were no exception; the habitat was a cobweb of ropes and branches and gave the animals ample opportunities to use their climbing abilities.  But there were a number of things that were brought up during the in-depth discussion: keepers could only hang food up to 7 feet off the ground; the animals were limited to the obvious pathways of the ropes and branches when getting around; many of the branches were more suited for walking across rather than hanging from; and aside from the attachment points in the ceiling that perching was suspended from, the very top of the habitat was severely underutilized. Spider monkeys are capable of supporting their entire body weight just from their tail, and keepers wanted a solution that strongly encouraged that behavior, in addition to addressing the other issues that were raised. Brainstorming ultimately led to the idea of creating a rainforest-like canopy in the habitat; a structure as high off the ground as possible, designed with hanging and swinging in mind and offering the animals the freedom of choice to move around it in whatever direction they pleased. Factoring in ease of construction, animal safety, and weight, the final design called for a massive cargo net that spanned the entirety of the habitat, built from seat belt material and pop rivets, and bolted to the ceiling. Constructing such a thing is no small feat, but fortunately there exists the Volunteer Enrichment Build Team, which meets monthly to tackle large, complicated, or intensive enrichment projects that keepers around the Zoo may not have the time or resources to do themselves. The volunteers were able to make quick work of it and it was soon ready to be installed by the Zoo Facilities Management crew, along with a system of pulleys that allows feeders to be raised up to the level of the canopy.

Now, these three spider monkeys are quite seasoned individuals and have had plenty of time in life to establish what they do and do not like.  One thing that has always held true for all of them is that they view new and novel things with significant skepticism, which can be a very useful natural behavior when it comes to encountering potentially dangerous situations. But as novelty is the spice of life and an important behavioral need to be addressed, keepers maintain a healthy regimen of new and unfamiliar things for all the animals and even for the spider monkeys, in spite of their staunch disapproval. So when the day finally came to introduce them to their newly decorated habitat, the old adage ‘You can lead a spider monkey to a cargo net, but you can’t make her climb on it’ never seemed more true. Cautiously entering the habitat huddled together for safety, they soon launched into a chorus of alarm barks and set about angrily shaking branches and ropes, as spider monkeys are known to do when they encounter someone or something they view as a threat. It took a full day for them to finally settle down, another two before anyone came close to approaching the cargo net, and almost two weeks before any of them got comfortable enough to actually hang from it.  They’ve since overcome all their fears and can now frequently be seen exhibiting all the natural behaviors of a canopy-dwelling primate, even deciding that it’s occasionally a nice place to grab an afternoon nap.

Click the photos before to see a larger image. Photos: Ethan Riepl