Howler monkeys are a noisy bunch, and their deep hollering vocalizations (which seem to be associated with small testicles) can travel remarkable distances through the canopy. New research published in the journal Animal Behaviour has uncovered another way in which these roaring monkeys are perfectly adapted to rainforest life, as they found that howlers have remarkable navigation skills. Not only could they remember the location of their favourite foods (and when they were in season) they knew the critical locations for reminding neighboring troops to keep out of their territory with warning calls.
The research, led by Oxford Brookes University, studied black howler monkeys (Alouatta pigra) in Palenque National Park, southern Mexico. After analyzing over 3,000 hours of field observations, the researchers’ findings revealed that these monkeys were deliberately seeking out fruit trees that were ripening. The researchers express that the monkeys’ targeted navigation indicates that they’ve enjoyed these fruits in the past, and are able to anticipate when they will be available again.
Fruit is a precious commodity in such environments, coveted by many animals in rainforests. Howlers' diets contain a lot of mature leaves which is handy because they're everywhere, but they contain high concentrations of toxins and tannins which aren't ideal in large quantities. In order to get ahead of their fruit-loving competitors, the monkeys select a handful of trees with easy-to-remember ripening cycles and return to them when the eating’s good.
“Spider monkeys have been traditionally considered more sophisticated than howler monkeys in their spatial skills,” said lead researcher Dr Miguel de Guinea expert in Evolutionary Anthropology, Oxford Brookes University in an email to IFLScience. “This is because spider monkeys are highly frugivorous while howler monkeys' diet is largely composed by leaves: it is remarkably harder to locate fruits across rainforests (because they're highly ephemeral) than to locate leaves, which are highly abundant. However, here we found for the first time that howler monkeys do remember where and when to find fruits relating their cognitive skills to those of spider monkeys.”
Less of a howl, more of a gutteral death cry.
As well as cashing in on some premium produce, the study revealed that the howlers were moving through the rainforest in long straight lines until they reached a point where they had at some stage collided with a neighboring troop. Even in the absence of competitors, the monkeys were observed returning to these spots to vocalize, marking their territorial range with loud warning calls. While howler troop collisions rarely result in violence, winning or losing such encounters could be the deciding factor in whether or not they can continue visiting their favorite fruit trees so there’s much at stake when defending your patch.
The team's next move is to work on establishing the best tools to use when studying differt primate species in their natural environment, with hopes it will reveal new insights into the evolution of primate spatial cognition.
“Seemingly, black howler monkeys are able to optimize the structure of these route networks through time by taking into account not only the cost of traveling along certain areas but also the most beneficial set of connections among relevant locations,” wrote Guinea. “I have been working with Dr Sarie Van Belle and Dr Karline Janmaat (both co-authors in this study) together with a team of experts on developing a set of methods to compare the cognitive skills of primates across populations and field sites. Even though multiple primate groups are currently being tracked in the wild, most of that data is never analysed or constrained to descriptive metrics. We aim to provide a set of tools to generate a clear framework for the evolution of primate spatial cognition using naturalistic observations.”
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