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Colobus on Colobridge

Diani Beach road is tarmac, single lane in either direction with no sidewalks.  Colobridges are set five meters above the tarmac and span the width of the road stabilised on either end by either trees or poles.  These bridges are used by primates and other tree-dwelling wildlife such as squirrels.  


Colobus on Pole of ColobridgeThe Use of Elevated Road-Spanning Ladders by Primates in Diani, Kenya

The use of Colobridges spanning a busy road in Diani was investigated, in relation to overall primate road crossing activity. The chain-link ‘ladder’ is suspended five meters above the ground between two platforms on trees or supported by telegraph poles, one on either side of the road. We consider the colobridges in Diani to be a useful and effective means of reducing primate road traffic injuries and mortalities. The colobridges are used by those species that are more arboreal.  In our context, the bridges are used more often by colobus and sykes monkeys and less by the terrestrial vervets.  Baboons do not use the bridges which is likely because of their size and terrestriality.

When considering use of colobridge-type structures, factors which will influence crossings are type of road, length of bridge, habitat type on both sides of the road, general foraging behaviour of the animal as well as its size and the amount of human activity in the area.

Two surveys of the use of colobridges have been done. The first in 2004 and the second in 2011. The report will be published on this website shortly.

Colobus Conservation has been involved in sharing our bridge building experience in other places and for other species, including in Jozani Forest, Zanzibar (Tanzania) for the endangered red colobus monkey.


How to Build a Colobridge

In 2003, Colobus Conservation compiled a short guide with illustrations to share this conservation measure with others.  An important tip in building such bridges is that it is far less expensive to position such bridges between existing trees, than to construct new bridges using poles dug into the ground. Poles entail more cost (poles, pole treatment, cement, etc.) and time for preparation of the poles and cement foundation. [See this guide online here].

A number of considerations should be undertaken before deciding if an aerial bridge will work to prevent road-induced mortalities of a target species:

  1. Due to financial and time obligations to build and maintain bridges, would speed bumps be more effective as a long-term method of slowing traffic?
  2. Does the ecology and locomotor patterns of the target species suggest that it will use an aerial bridge? In Diani, colobus monkeys, an arboreal primate, have low rates of bridge use. Arborealism is not the only factor to consider.
  3. How heavy is the target species? Baboons were not recorded as crossing the bridges in our 2011 survey perhaps due to their large size.
  4. Does the target species need to cross the road to access daily or seasonal home range attributes such as sleeping sites or dry season feeding locations? Individuals may not be at high risk of road-induced mortality if home ranges are small and are not bisected by the road.
  5. What is the conservation value for each individual crossing on the bridge? For example, in a highly endangered species, the value for reducing road-induced mortality even at low rates of crossing may be beneficial for species survival. For a non-threatened species, low rates of bridge use may not justify bridge costs.
  6. Is adequate staffing and finances in place to provide regular checks and maintenance of each bridge? At Colobus Conservation, each bridge is checked every three months to ensure that it is properly tensioned and that there are no broken parts. Support poles are examined for rot and termite damage. Any work required is carried out immediately.

If the circumstances warrant the use of aerial bridges, the following steps are recommended for their installation:

  1. Research should be undertaken to verify sites of animal road crossings as well as information on the locations of road kills of the target species.
  2. Contact Ministry of Public Works and Housing (or respective government agency) for permission to build bridges.
  3. From a legal perspective, determine the distance for placing the poles from the roadside, if they are being used for additional support of the bridge. The height that the bridge must be above the road surface must also be known. Minimum height of the bridges in Diani is seven metres from the road surface to allow for safe clearance of trucks and five meters from the edge of the tarmac.
  4. Where possible, connect the bridge to existing trees. Using poles as additional support on the road’s verge adds costs for example, additional material and labour are needed for the poles themselves, termite proofing the poles and digging pole holes. Cement is required to anchor poles in the ground. Poles also require extra transport needs, gear and staffing for transporting and erecting poles safely.
  5. The habitat around the selected site needs to be carefully considered. For example in the case of monkeys, there must be an aerial route leading to the bridge as they will not climb a tree specifically to cross the road using the bridge. Monkeys also will not change their route to access a bridge.
  6. Do a trial bridge. New bridges are used within days of being installed in Diani.
  7. Local publicity should be paramount throughout the planning phase to ensure local interest and support.