The mantled howler (Alouatta palliata) is a large, fruit-eating monkey found across Central America. It is famous for being one of the loudest land mammals on Earth, with a roar that can be heard up to three miles away. Their fierce call belies their gentle nature, as these monkeys live in nurturing family groups and tend to do well living around human settlements.

However, human expansion into primate habitat has had devastating consequences for the mantled howler. In 2015, the International Union for Conservation of Nature (IUCN) listed it as vulnerable and placed it on its Red List of Threatened Species, and there it remains.

The forests they inhabit are being increasingly lost to deforestation for agriculture, while roads and telephone wires, both critical elements of human economic development, pose their own particular threats.

“Howler monkeys are highly arboreal – they live in the trees and are hesitant to travel on the ground,” says Dr McKinney, a biological anthropologist at USW with an interest in human-primate interactions. 

Dr McKinney has been working in Costa Rica since 2006, at which time mantled howlers were listed as ‘least concern’ in the IUCN Red List. 

“Howlers often solve the problem of coming to the ground by using electrical lines to cross the road. While this helps them avoid the dangers of vehicles and dogs, they are at risk of electrocution on the wire or of falling into the road.”

Now, thanks to a grant from the Primate Society of Great Britain, there could be a solution.

Dr McKinney is part of a group of conservationists who plan to build monkey bridges to reconnect the trees and reduce fatalities. A monkey bridge is a simple rope bridge, placed near existing travel pathways where monkeys currently cross on electrical wires.

The project is due to start later this month, and the team plan to install four bridges in high-fatality areas in the Puntarenas province of northern Costa Rica.

Dr McKinney on field work in Costa Rica

"Bridges must be designed with the primate species’ size, temperament and travel requirements in mind,” said Dr McKinney, who is the consulting primatologist for the project.

Working with Dr McKinney is Carolina Orozco Zamora, a forestry engineer for Sistema Nacional de Áreas de Conservatión (SINAC), an agency of the Costa Rican Ministry of the Environment. Ms Orozco Zamora and her colleagues at SINAC will build the bridges, and Instituto Costrarricense de Electricidad (ICE), the national electricity company, will help with the installation.

“Our bridges will be designed after the ladder-style rope bridge which has been successfully used for other howler species in other parts of South America for the last 15 years.”

The bridges will be monitored using camera traps to ensure that the monkeys are using them. Camera data will help the team recognise if the bridges are not being used, so they can then move them to a more appropriate location.

The bridges may have benefits for other species too. “While designed with howler monkeys in mind, these bridges may also provide safe passage for other arboreal animals in the region such as squirrels, kinkajou, iguanas and capuchin monkeys,” said Dr McKinney.

“I am excited to be involved with this practical, important work, led by local organizations, to help protect these amazing primates,” said Dr McKinney. “My research specialises in how wild primates deal with human disturbance, including habitat alteration and ecotourism so it’s an area I am both fascinated in and passionate about.”

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