The St Louis Zoo in Missouri, US, has announced plans for a major US$11.5m (€10.28m, £9.08m) expansion of its Primate House, with the 35,000sq ft (10,668sq m) addition set to be completed in 2021.

The outdoor expansion, says the zoo, will be beneficial to the animals' health and wellbeing, providing eight new outdoor homes adjacent to the existing Primate House – home to around 40 primates from 14 different species currently.

New overhead mesh tunnels will connect the Primate House to tall sycamore trees and other outdoor habitats, allowing the primates, which include lemurs and monkeys, to explore different habitats at different times.

Many of the species currently have no access to outdoor habitats from the Primate House because they are too small or too strong for the mesh of St Louis' six existing outdoor habitats.

The zoo said that the increased number of habitats will give it the space to increase its participation in cooperative breeding programmes.

Visitors to the zoo will be able to explore using climbing structures, learn about the importance of preserving the world's forests, and walk along elevated boardwalks that enable them to experience the primates at treetop canopy levels. At one point, a clear acrylic tunnel will allow them to walk through a primate habitat.

Jeffrey P Bonner, Dana Brown president and CEO of St Louis Zoo, said: "Primate Canopy Trails allow us to improve animal care, health and well-being by providing access to enriching outdoor habitats, sunlight and fresh air. It also will allow visitors to experience primates like never before."