Path to Freedom: Howler Monkeys Release | Part 1

The process of preparing primates for release is highly complex and must be handled with the respect and professionalism it deserves. The same applies to the release itself, once the animals are ready, and the post-release monitoring. This post is the first in a series where I share my experience in the process of releasing a troop of mantled howler monkeys in Costa Rica (I have already covered rehabilitation and will continue to do so in other posts). In this link, you'll find a video where I explain and show this process.

Different Approaches Depending on the Rescue Center

The process of wildlife release, just like rehabilitation, can be approached in different ways. Each rescue center has its own particular characteristics, which means that their approaches and methods may vary. This doesn't mean that some work well and others poorly, but that they reach the same goal through different paths. Note: I'm not saying that all centers do well in rehabilitation and release, but we should judge this by their results in terms of release success rates, rather than by their methods.

The center I work at has small to medium-sized enclosures (approximately 15-30 m², 5 meters in height), with no vegetation inside. These enclosures are primarily used during the early stages of rehabilitation for the monkeys (once they no longer require intensive care in the ICU), as well as for introductions and troop formation.

Subadult howler monkey in the enclosures of the rescue center. The leaves seen in the image are collected and provided to them daily by the caretakers. - Photograph by Noelia Sánchez.

On the other hand, it has two large enclosures integrated into the forest (approximately 1,400 m² each, with canopy height), located nearly 2 km from the center. Accessing these enclosures requires at least 20 minutes by quad and about 10 minutes on foot. These spaces are used in the final pre-release phase of the monkeys, where they are moved once they are ready to face a wild environment, albeit still in a limited space with restricted interactions with wild monkeys. When it is considered that the animals possess the necessary skills to thrive in the wild, a "soft-release" process is carried out from these enclosures, creating a direct link to the forest.

For those unfamiliar with the term, a soft-release is a gradual process where animals are voluntarily released into their natural habitat. Unlike an immediate and total release (hard-release), the soft-release gives the animals time to adapt to the environment and learn to survive on their own, with monitoring and human support in many cases, such as food or temporary shelter provision, before their full emancipation.

View of one side of the field site from the outside. - Original photograph.

The advantages of working this way are evident. The release is gradual, allowing the animals to adapt to the environment they will be released into over a relatively long period. Other centers, with different infrastructures and capacities, employ other approaches. For example, in a center in Belize that has been working with howler monkeys for years, they rehabilitate the animals for an extended period in a large natural enclosure, but for release, they install small portable enclosures where the monkeys only stay for a few days before being released. In this case, the process is somewhere between soft-release and hard-release, which can make it more challenging to track and monitor the animals, but has the advantage of being able to release them in different locations depending on the individuals' characteristics and the ecosystem they are introduced to.

Preparing the Release Site

Working at a site like the field-site (as we call the release enclosures) is no easy task, especially because of its remote location and difficult access. To get there, we must drive a quad through rugged, slippery terrain, cross rivers and streams, and then carry everything we need to the site, ascending nearly 100 meters in elevation for more than 10 minutes through the jungle. After a few days without use, the path becomes practically invisible, as the forest grows so quickly that plants cover the trail in no time. So, one of the first tasks is to clear the path with machetes.

I won't deny that I love riding the quad on these trails :) - Photograph by Noelia Sánchez.

Before each troop is moved to the field-site, the area must be properly prepared. The animals must not be able to leave until the team deems it appropriate, and no other monkeys should be able to enter. The enclosure is fenced with a 2-meter high barrier, but these monkeys may eventually overcome such an obstacle. Therefore, we must clear all vegetation (small trees, shrubs, etc.) around the fence, both inside and outside, over an area of about 2 to 2.5 meters on each side. For trees that we decide to keep, we tie them to others to pull them inward, ensuring they don't grow towards the outside of the enclosure. This task is quite demanding, given the heat and humidity of the area, and the fact that it’s done manually with machetes. Additionally, to protect ourselves from snake bites, spider stings, ticks, and other animals, we wear tall rubber boots and long-sleeved clothing. All of this work requires several days, and we carry it out with the help of both staff from the center and internal volunteers, who are invaluable for carrying out these and many other tasks.

Konur, an intern at the center, clearing the perimeter of the field site with a machete. - Original photograph.

In addition to clearing the perimeter, we also need to check and prepare the fence, which is electrified at the top to prevent the animals from crossing it. This includes checking the solar panels, batteries, and connections, as well as removing climbing plants that could hinder the electrical system’s operation. All of this, in our latest release, took us about two weeks, as work at the rescue center continues as usual, meaning efforts must be divided between both tasks.

View of the perimeter once it is cleared. - Original photograph.

While the monkeys are at the field-site, they require supplementary feeding. Although they can mostly rely on the leaves provided by the many trees growing in the enclosure, the quantity or quality may not always be sufficient. The same goes for water, which, although they mainly get it from their leaf-based diet, it’s always good to supply it daily. To do this, we install pulley systems on some of the tallest trees, which allows us to provide food and water to the monkeys in the canopy (which is where we want them to learn to move) without the need to climb dangerous ladders. We also set up camera traps to monitor the behavior of individuals in areas or time slots that are inaccessible to us. Before each troop is moved to the field-site, we also check the pulleys to ensure they are not broken or blocked by the vines growing everywhere, and even install new pulleys if necessary.

Camera trap installed at the field site. We set up wooden structures to protect them from the elements. - Original photograph.

Food is provided in baskets with a cover that has a hole in the middle, preventing the food from falling easily, but allowing the monkeys to access it. For water, we use small buckets, though it’s not expected that the monkeys will use them frequently. These same food and water systems are also installed outside the enclosure, in the area where the animals will be released, to encourage them to leave and prevent them from staying too long in their comfort zone.

The baskets we use to suspend the monkeys. - Original photograph.

Finally, we use ropes to construct a hanging bridge that will allow us to “open” the enclosure at the right time. The bridge is installed on the outside of the fence; when we decide to begin the release, it will be extended toward the inside, connecting a tree inside with one outside, allowing the monkeys to exit the enclosure.

Hanging bridge installed, but not yet stretched. - Original photograph.

The path to freedom

Once everything is ready, including the animals, it’s time to move them to the field-site and begin their journey back to freedom! As soon as the transfer takes place, which will be soon, I will write about the process of adaptation for the troop in the new space. The period that the animals spend in the field enclosure is very interesting, as it allows us to make numerous behavioral observations, which we can then analyze and compare with previously released troops, as well as with their behavior along their rehabilitation process in the rescue center. Additionally, it’s the perfect time to start exploring the forest and preparing the environment to facilitate post-release monitoring.

One of the last steps is to introduce the field site to all the interns who will carry out the daily pre- and post-release monitoring and prepare them for such an important task. - Photograph by Noelia Sánchez.

If you’re interested in reading more about this process and other related topics, don’t forget to subscribe to the blog for the latest updates and follow me on social media. As always, I encourage you to leave your opinion, comment, or suggestion below. See you in the next one!

Roger Valls Martínez

PD.: I would like to thank everybody that has participate in getting the field site ready. I know it has been a hard task. Thank you so much Cam, Fran, Harris, Hannes, Konur, Mel, Noe, Poppy and Samantha. Also Albert and Dennis :)