Field Notes #13: The other side of the world, Mexico and the monkeys

Text by Anaid Cardenas Navarrete

I vividly recall that day in April of 2020, sitting underneath the roosting family of black howler monkeys in the tropical forest of Palenque, Chiapas, Mexico, when I received a message from my former advisor urging me to halt fieldwork until the public health situation returned to normal. With a heavy heart, I left the field immediately, hoping to return within a matter of weeks or perhaps a couple of months. I even made a promise to the monkeys that I would be back soon—little did I know!

During the unexpected break, I had time to write my master's thesis, defend it, apply to graduate programs, and get accepted into Dr. Onja Razafindratsima’s lab at UC Berkeley. This marked the beginning of a new chapter in my life, in a new country and a new research adventure. Two years passed before I finally had the chance to reunite with the monkeys in the summer of 2022, setting up a three-month pilot study for my dissertation research. Then, in 2023, I embarked on my longest field season yet—a year-long data collection journey that was as daunting as it was exciting. As we drove the nearly 1,000 kilometers along the roads that connect Mexico City to Palenque, it felt like returning home after an extended absence.

The monkeys

The Yucatan black howler monkey (Alouatta pigra), known locally as "mono saraguato," is a diurnal primate that lives in cohesive social groups comprising 2 to 12 individuals. These groups typically consist of one or two adult males, two to four adult females, and their young. Feeding primarily on leaves and ripe fruits found in the tropical forests of southeast Mexico, as well as parts of Guatemala and Belize, they are famous for their loud calls, often emitted collectively, making them among the loudest terrestrial mammals. ​

Lumi, a female black howler monkey (Alouatta pigra). Photo: Anaid Cardenas Navarrete (ACN)

Unfortunately, like most primate species globally, they are endangered due to deforestation and habitat loss resulting from the conversion of tropical forests into cattle pastures and urban centers. ​

The site

Mayan ruins of Palenque surrounded by tropical forest. Photo: ACN

Palenque, a town perpetually full of tourists drawn to a rich Mayan heritage, has undergone rapid urban and agricultural expansion over the years. Since the 1960s, the implementation of policies by the Mexican government aimed at deforesting the southeastern part of the country for industrial cattle production, significantly diminished the potential habitats for primate populations in the region. Since my initial arrival in 2016, I've witnessed firsthand the detrimental impact of these development practices on the region's native habitat. Thus, my main interests are to understand how black howler monkeys manage to be resilient and to maintain their roles as seed dispersers amidst a rapidly changing landscape.

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My study sites typically encompass small forest fragments that serve as shelter for these primates and countless small critters within the ever-growing urbanization. This time, however, I’m also collaborating with Dr. Sarie Van Belle, a researcher conducting long-term studies on black howlers, to work within the protected confines of Palenque National Park (PNP)—a vital stronghold of local biodiversity.

Biodiviersity of Palenque, Chiapas

The people

Before going into the details of this adventure, it's important to acknowledge that none of my endeavors in the field would have been remotely possible without the untiring support of my research teams. Throughout this project, I've had the privilege of working alongside four teams, each composed of early-career biologists with a passion for primatology, ecology, conservation, or wildlife in general. Spending countless hours together in the field forged bonds that transcended simple companionship; in the harsh conditions we faced daily, trust and mutual respect became indispensable. 

Field research assistants during the field seasons from 2022 to 2024. Photos by ACN, DL, VCV, and IGA.

Among the people who accompanied me on this journey were Paulina, the eternal optimist; Irais, the tenacious apprentice; Alba, the seasoned field biologist; Erika, the meticulous explorer; Liz, the natural primatologist; Jorge, the embodiment of natural intuition; Nacho, the cheerful go-getter; Ulises, the unwavering wit; Valeria, the stabilizing force; and Dallas, the always-by-my-side.

​The fieldwork

During the pilot study in 2022, my team, comprising Paulina and Dallas, and I, ventured into various forest fragments across the transformed landscape of Palenque, securing permissions from landowners to conduct research on their properties. It was heartening to still be recognized as the biologist "following the monkeys" and to reconnect with the people of Palenque after such a prolonged absence. Their willingness to offer their land and assistance facilitated our surveys, enabling us to identify the groups of black howler monkeys for our research. Over the course of three months, we habituated the unfamiliar groups, experimented with new methodologies, and refined old techniques in preparation for the subsequent data collection phase. Despite the sweltering heat, relentless mosquitoes, and occasional encounters with venomous snakes, we remained optimist as we laid the groundwork for what was coming ahead.

​In May 2023, I returned to Palenque after being away for nine months. This time, I was accompanied by Irais, Erika, Alba, Jorge, and Liz, with whom we formed two field teams during the first five months of research. Later, Nacho, Ulises, and Valeria, joined me for the long final stretch of the project. Due to the harsh conditions of the highly degraded forest fragments, camping at the sites was neither safe nor logistically feasible. Thus, we all cramped in an apartment in the town of Palenque—our temporary base camp.

A group of sleeping black howler monkeys on a rubber plantation. Photo: ACN

Fieldwork for this research feels like assembling a thousand-piece puzzle that requires high coordination among team members to succeed. We usually arrive at the study sites before sunrise, with the hope that our study groups will wake up to a howling chorus. When the groups choose not to howl, we start a survey by spotting potential roosting trees from afar. After finding them, we spend all day with the group, following every step they take while collecting data on their activity, their collective movement, the tree species they feed on, and more. 

​The reason to continue...

While it is a privilege to observe primates in the wild, I found myself reflecting on their survival amidst disturbed sites on many days spent under the sleeping groups. It is becoming increasingly clear that the high rates of deforestation, site isolation, river pollution, road noise, and presence of domestic animals—all consequences of human activities—continue to escalate. It is sad and a reminder of the gravity of the situation when we observe monkeys caught in the crossfire of human activity, struggling to survive on plantations and navigating treacherous highways. It is my hope that my study will contribute to a greater understanding of the importance of respecting and protecting black howler monkeys and their habitat in an area that relies so heavily on them.

Me, observing the monkeys in the beautiful protected forest of Palenque National Park. Photo: Ignacio Guzmán