S&J's group managing director, Crispin Reed, said the agency would develop a cinema and print campaign for the initiative, which is run by the UN's Environment Programme. This will run across Europe from the end of August, with the potential to break globally.
The project - dubbed GRASP - was launched in May 2001 with partners including the Worldwide Fund for Nature and the fellow UN organisation Unesco, in a bid to raise money from global governments, businesses and consumers to help save various threatened primate species from extinction. It has already gained funding from the governments of the UK and Norway.
The UNEP head of media, Nick Nuttall, said: "We are delighted that S&J has joined forces with us in this important cause. The company has a proven track record in handling sensitive issues with care and creative flair."
Reed said: "There is growing awareness of the plight of the great apes and UNEP has stepped in to co-ordinate the worldwide effort and to give the issue some much-needed political impetus."