The process of preparing certain animals for release into the wild can be very lengthy, sometimes taking several years for some species. This is often the case with howler monkeys, which typically arrive at rescue centers orphaned and between a few months and a year old. Assessing the behavior of these animals during such a long process is essential to understand if they are progressing in the right direction.
Rehabilitation of howler monkeys
Wildlife rehabilitation for reintroduction into the natural environment is a complex process whose success depends on multiple factors. In this regard, the work done in captivity to prepare animals for survival in the wild is crucial. This is particularly decisive for socially complex and behaviorally intricate species, especially when it comes to individuals raised from their early life stages in captivity (Drews, 2002). In these cases, the environment provided under human care significantly influences their behavior and learning as they mature and prepare for their future release.
The mantled howler monkey (Alouatta palliata) is a species that, like the vast majority of primates, has a very prolonged infancy, which is crucial for its physical and mental development. This is a key factor during the raising of orphaned monkeys for release, as is the case with the majority of those that arrive at rescue centers in Costa Rica, such as Wild Sun Rescue, where I currently work.
According to Balcells and Veà Baró (2009), the infancy of howler monkeys lasts around 14 months, during which they are entirely dependent on their mother for both movement and nourishment, as they are still nursed even beyond this period. From 15 to 29 months, they can be considered juveniles, who continue to be nursed at first while the mother rests, eventually weaning them, although they progressively learn to consume solid food. Juveniles become independent from their mother in terms of movement but remain strongly bonded to her, especially during rest periods. After approximately 29 months of age, and up to 35 months for females or 48 months for males, they become fully independent of their mothers. The characteristic mantle of the species, as well as the sexual organs, begin to become visible, and males start to become more solitary and begin to howl. From this point onward, females experience their first estrus, marking the transition to adulthood (around 3 years of age), copulations begin, and, in the case of males, they become more solitary or increase threatening and fighting behaviors with other males.
With such a long period before reaching adulthood, the importance of the management practices carried out during this time becomes evident for the proper future performance of the animals. During the earliest stages of rearing, the mother-infant bond is crucial for the development of social skills (Harlow and Suomi, 1971). Therefore, it is important to provide the orphaned infant with a "substitute mother" to avoid imprinting on humans and to promote the grasp reflex. In this case, a fabric toy (such as a teddy bear or similar) can be useful.
However, as these monkeys grow, their social development needs become more demanding, requiring learning and interaction with other individuals of the same species. For this reason, as soon as the infants are capable of feeding themselves, social integration with other age-matched conspecifics becomes essential.
Throughout the entire rehabilitation process, up to sub-adulthood or adulthood (2.5-3.5 years of age), when these animals are ready for release, a series of progressive modifications to their captive environment must be implemented (in addition to those mentioned in the previous paragraph) so they acquire the necessary physical, mental, and behavioral characteristics and skills to successfully establish themselves in the wild.
Basic ethology of the species
Throughout the preparation process for release, which for orphaned howler monkeys usually lasts between two and almost three years, special attention must be given to the evolution of their behavior. This is essential both for monitoring the rehabilitation process and for learning and establishing best practices in the rehabilitation of these animals (Llorente, 2019). The study of behavior is a key tool through which the rehabilitation team can gather behavioral data that, once properly analyzed, will provide invaluable information.
A crucial part of the wildlife rehabilitation process is ensuring that animals receive proper psychological and behavioral preparation to adapt to life in the wild. This involves, among other things, ensuring that the animals learn and assimilate the behavioral patterns and specific behaviors used by their wild counterparts to survive and thrive in nature. To achieve this, it is essential to first understand the basic ethology of the species in its natural habitat, both through direct observation and by studying existing literature on the subject.
Below is a compilation of the most relevant ethological and postural data, as well as habitat use, for the mantled howler monkey:
Feeding:
Howler monkeys primarily feed on leaves. However, although they have distinctly folivorous habits (Baum, 2005), they combine leaf consumption with other food resources in their natural habitat, especially ripe and unripe fruits (Kowalewski et al., 2015). It is crucial for their successful release that they are able to identify and properly utilize their primary source of nutrients in the wild. Native species' leaves in their natural form should also be their main food source during the rehabilitation process, and it is important to evaluate the proportion of time spent eating leaves. In the wild, overall, feeding occupies around 10-13% of their activity during daylight hours (Mendel, 1976; Muñoz et al., 2001).
In howler monkeys (Alouatta spp.), one of the most characteristic postural behaviors related to feeding is known as suspensory feeding or feeding in suspension (Hunt, 2022; Mendel, 1976; pers. obs.), which involves using their tail (and occasionally their hind limbs) as support to feed while hanging upside down, using their hands solely for eating.
They prefer to feed in the upper parts of trees (85% of observations), particularly in the sections farthest from the trunk (71% of observations). Additionally, they tend to feed from small-diameter branches (<5 cm), and in the case of suspensory feeding, evidence confirms that this occurs exclusively from branches with a diameter of <5 cm (Mendel, 1976).
Locomotion:
It can occupy between 10% and 24% of their daily activity (Muñoz et al., 2001; Mendel, 1976). In the canopy areas they frequent, mantled howler monkeys tend to move primarily using small-diameter supports (50% of observations) and medium-diameter supports (33% of observations), although the percentages may be even more favorable toward small branches for daily short-distance movements, as Mendel’s (1976) study also considered troop movements.
The most common forms of locomotion are walking (70% of the time), followed by climbing, descending, and jumping (18% of the combined time), often using their prehensile tail as a safety grip (Mendel, 1976).
Resting:
Due to their feeding strategy, which does not require significant effort or provide large caloric intake, the percentage of time these monkeys spend resting during the day is very high (Kowalewski, 2015; pers. obs.). This statement is consistent with the data obtained by Mendel (1976) and Muñoz et al. (2001), which suggest that the mantled howler monkey can spend between 61% and 75% of its time resting during the day.
As with other behavioral categories, resting primarily occurs in the upper third of the tree (75% of observations), especially in the more outer areas (45% of observations) (Mendel, 1976). The most commonly adopted resting postures are sitting (65% of observations) and lying down (25% of observations), most often on small-diameter branches (Mendel, 1976).
An important point in this regard is the use of the tail as an anchor to prevent dangerous falls while sleeping on such small supports. Mendel (1976) argues that it would be virtually impossible for these animals to rest in their usual locations (especially at night) without using their prehensile tails as a safety system, both in terms of risk and the energy required to maintain the proper posture.
Social behavior:
The mantled howler monkey is a social primate species with bisexual dispersal, meaning that both males and females leave their natal group to join their own group in adulthood. Additionally, this species lives in mixed groups with a low male-to-female ratio, which is why males often spend long periods of their lives as solitary individuals, as not all have "available spots" in the troops of the area they occupy.
This species exhibits a low frequency of social behaviors, if we consider as such those that are purely explicit, such as allogrooming, play, copulation, or aggression. This can be explained as part of the energy conservation strategy followed by this species. In this regard, Oliveira and Ades (1998) report that Alouatta fusca clamitans spends less than 5% of its daily activity on explicit social behaviors. Consistent with this, Mendel (1976) reports a prevalence of social-type behaviors (play, aggression, allogrooming, copulation, and vocalization) of between 2% and 8% of daily time, with a notable 7% vocalization in adult males.
However, apart from the displays and clearly interactive behaviors, howler monkeys communicate and organize their behavior as group members through indirect signals, such as approaches and withdrawals, as well as episodes of following and grouping. They also do this, both within the same troop and with other troops, through vocalizations, some of which are as loud as the famous howl (or howling), which can be heard up to 5 km away.
Substrate and space use:
Howler monkeys are distinctly arboreal primates (Ponce, 2018; pers. obs.). In the wild, they do not use the ground as support for any of their activities, except for moving in cases of extreme necessity or motivation (Serio-Silva et al., 2019). They also occupy almost exclusively the upper two-thirds of trees (95% of the time), regardless of their height.
As mentioned earlier, in the wild, they show a preference for the upper parts of trees (75% of the time), as well as for the outer areas, the furthest from the trunk (57% of the time) (Mendel, 1976). They also tend to occupy small- and medium-diameter branches, <15 cm (80% of the time) for resting, moving, and feeding, as they use their tail as an anchor to hold on while resting or moving, as well as when feeding in suspension.
Behavioral analysis as a tool for improvement
Taking the species' ethology into account is essential when working in wildlife rehabilitation, especially when the ultimate goal is the release of the individuals. Understanding behavior, in this case of howler monkeys, helps professionals working with them to identify the key skills and behaviors that we need to foster in them until the time of their release.
At the same time, and somewhat as a consequence of this, evaluating the animals' behavior is as important a part of the process as the previous one. After all, if we don't know how the animals are behaving at each moment of the rehabilitation process, we will never be able to understand their behavioral patterns, nor determine what the best management practices are for preparing them.
Understanding the species' ethology is important (and that's what I've been discussing in today's post!), but using behavioral analysis to assess the rehabilitation process and improve our management of it is just as essential. I would like to dedicate a full post to this latter topic, as I consider it both complex and fundamental. It is also something that currently occupies a significant part of my professional work with these animals.
I don’t want to go on for too long, so as always, I encourage you to leave your comments below so we can discuss and debate this topic. Thank you so much for reading once again—see you in the next post!
Roger Valls Martínez
Referencias
Baum, Jeffrey A. (2005). "Foraging Behaviors of Alouatta palliata, Mantled Howling Monkeys." Nebraska Anthropologist, 1.
Balcells, Cristina & Veà Baró, Joaquim. (2009). Developmental Stages in the Howler Monkey, Subspecies Alouatta Palliata Mexicana: A New Classification Using Age-Sex Categories. Neotropical Primates, 16, 1-8. doi: 10.1896/044.016.0101.
Drews, C. (2002). Rescate de fauna en el neotrópico: Iniciativas y perspectivas. Heredia, Costa Rica: EUNA.
Hunt, K.D. et al. (1996). 'Standardized descriptions of primate locomotor and postural modes.' Primates, 37(4), 363–387. doi: 10.1007/bf02381373.
Kowalewski, M.M. et al. (2015). Howler monkeys: Behavior, ecology, and conservation. New York: Springer.
Llorente, M. (2019). Primates: Biología, comportamiento y evolución. Barcelona: Lynx Edicions en colaboración con IPRIM.
Mendel, F. (1976). 'Postural and locomotor behavior of Alouatta palliata on various substrates.' Folia Primatologica, 26(1), 36–53. doi: 10.1159/000155728.
Muñoz, David & García del Valle, Yasminda & Franco, Berenice & Estrada, Alejandro & Alejandro, Miguel. (2001). Presupuestos de tiempo en una tropa de monos aulladores (Alouatta palliata) en el parque Yumká, Tabasco, México. Universidad y Ciencia, ISSN 0186-2979, Vol. 17, Nº. 34, 113-123.
Ponce, B. (2018). Preparing The Yucatan Black Howler Monkey for Its Return to The Wild: An Assessment of Wildtracks’ Approach to Rehabilitation and Reintroduction. Thesis.
Serio-Silva, J.C. et al. (2019). 'Terrestrial locomotion and other adaptive behaviors in howler monkeys (Alouatta Pigra) living in forest fragments.' Movement Ecology of Neotropical Forest Mammals, 125–140. doi: 10.1007/978-3-030-03463-4_9.
Suomi, Stephen & Harlow, Harry. (1972). Social rehabilitation of isolate reared monkeys. Developmental Psychology, 6, 487-496. doi: 10.1037/h0032545.
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