Howler monkeys: Who coordinates their displacements when there is no hierarchy?

The black and golden howler monkey, whose scientific name is Alouatta caraya, belongs to the Alouatta genus and it is found further south in the distribution of this species. It inhabits in Paraguay, south of Brazil, north and east of Bolivia and north of Argentina, where it lives in jungles and in forest galleries on the banks of rivers and streams.

Since 2003, Vanina Fernández, biologist and CONICET fellow, has studied the behaviour of howler monkeys from the “Isla Brasilera” in the province of Chaco and in the Corrientes Biological Station, both in Argentina. The aim was to determine who coordinates the displacements of these groups and why. The first results of her work were published in the scientific magazine Primates.

All animals of the Allouata genus are sociable, that is to say that they live in groups of up to 21 individuals, including between one or four males, up to five adult females, then male and female juveniles and babies.

During the day they climb trees for different reasons. In some cases they climb in search of food -mostly leaves and fruits-, to monitor their territory or to rest. In fact, they rest 60% of their time.

Decisions about when or where to move are influenced by their feeding strategies, the source of food selection, defense of territory and the probability of encountering predators and other howlers’ group. The goal of Fernandez’s investigation was to analyze how sex, age, the females’ reproductive state and dominance affect patterns and displacement coordination in the context of possible meetings with other howlers that could compete for the same resources.

“An interesting point regarding this species is that these groups do not have a marked hierarchical structure with a dominant monkey, although they have a central male,” Fernandez comments and adds “This is important because the one that leads the movements is who decides what to do. During our fieldwork we could observe that suddenly one of the monkeys gets up and starts to move and although we did not register a level of voice communications within the group associated to this event, the rest followed this monkey. So the question is: why?”.

The results of Fernández study show that although any member of the group, even juveniles, could lead displacements, age is the factor of higher incidence. From the 262 reported displacements, almost 95% were led by adults.

According to the classical models of Primatology, the social structure and displacements are determined mainly by the resources’ availability: while the female are limited by the amount of food – because they need to be physically fit to bear offsprings – male just find themselves limited by the number of females because they have to copulate with most of them in order to produce a greater number of offsprings.

However, the biologist comments that there are cases in which these models do not apply: “We observed groups that move to eat something they had in the same place where they were, and we wonder why. My hypothesis is that they take advantage of these displacements to monitor the trees located in their territories and to check which available food they can eat on another day. These movements are coordinated by the adults because they know the land”, she explains.

The study shows that in the studied caraya groups, the availability of food is not the only key factor in the structuring of social relationships of the primates. Evidence shows that another important factor would be the competence for reproduction. This would be the reason why males guide more frequently the displacement when they meet other groups of howlers.

“Since males lead and decide when and where to go to fight, they make decisions to try to restrict the possibility of females to copulate with other groups”, the fellow comments and explains that these observations substantiate the hypothes that associate leadership with an attempt to monopolize copulations within the same group.

When the females are receptive, that is to say, ready to conceive, they try to copulate with all the available males. In fact, 53% of the copulations with other groups take place during fights among groups. “So for the males, it is not a good strategy to meet other groups when the females are receptive. However, when females from other groups are receptive, it is a good chance for them because they can copulate”, Fernandez says.

The biologist explains that this capacity of caraya female to copulate with several males would be associated to two causes: it could be a way to keep bonds which are essential in social animals – like monkeys – or it could be a strategy to avoid infanticide. This tends to happen when there is exchange of males between two groups and the females are not receptive because they are breastfeeding or conceiving. Consequently, the male that joins the group kill the baby to make the female receptive again.

“When the female copulates with every possible male of her same group or others, she eliminates the possibility of certainty of parenthood and so the male would not be able to recognize if an offspring is its or not, and therefore increase the baby’s chances to live”, Fernández concludes.