Howler monkeys use mental maps for route navigation

Black howler monkeys navigate the forests of Mexico, Belize and Guatemala while visiting trees in search of fruit and other food. They move in a complex environment and need to adapt their route maps depending on which trees are fruiting as well as to cope with unforeseen changes in the forest.

Humans are understood to navigate using internal maps that lay out the routes between frequently visited locations. These simple, mental maps focus on specific routes and carry little information about where each route lies in relation to others. However, humans are able to use their knowledge of distance and direction in order to link up their mental maps and take short cuts, thus adapting their maps to their navigational needs. 

In a new study published by the Company of Biologists, scientists Miguel de Guinea (Oxford Brookes University, UK), Sarie Van Belle (University of Texas at Austin, USA) and colleagues investigated whether howler monkeys were also able to modify their mental-based route maps in the way that humans do.

The researchers followed groups of monkeys in the forests covering the Mayan ruins in Palenque National Park, Mexico. The groups were easy to locate because they called loudly at dawn. 

“We’d arrive at the study area where our focal group was expected to be found before sunrise,” said de Guinea. The team then followed the monkeys as they moved through the 50-hectare area. 

The trek was often very challenging for the researchers on foot, as the monkeys traversed the canopy, visiting their favorite fruit trees on high hills and in valleys. 

After a year of following five troops of monkeys, de Guinea and Van Belle analyzed 91 kilometers of route information and compared this with the simulated routes of randomly moving virtual primates. 

The experts found that the howler monkeys regularly travelled through the same sequence of trees and visited their favorite trees often, approaching these trees from a select few directions only. They clearly used mental maps to navigate along familiar routes through their environment. By contrast, the virtual primates seldom repeated a route. 

The howler monkeys were also able to adapt their route in the face of unforeseen changes. On one occasion, the monkeys encountered a 5m gap in one of their regular routes.

“A tree had fallen overnight,” explained Van Belle. “They stopped for half an hour and then travelled along the edge to reconnect with the second half of their travel path … as if they knew this was a new obstacle and they needed to consider their options on what to do next.”

Furthermore, when the researchers compared the distances traveled by monkey troops with the distances traveled by simulated primates, it was clear that the howlers used their knowledge of direction and distance to link familiar routes together in order to take short cuts and travel efficiently between locations. 

According to the researchers, the data from the study shows that howler monkeys can supplement their mental maps with knowledge of distance and direction, just as humans do. This is the first evidence which demonstrates that animals use the same cognitive skills as humans when navigating through their environment.  

The study is published in the Journal of Experimental Biology.

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By Alison Bosman, Earth.com Staff Writer