位於荃灣的城門水塘，除了出名的白千層樹外，還有十分惹人注目的獼猴群。城門郊野公園一帶是香港野生猴子主要棲息地，可看見大群獼猴出沒。
1913年-1936年間，水塘依山而建，周邊山上生長有毒植物馬錢子。馬錢子是香港的四大毒草之一，其果實含有毒性強烈、可致命的生物鹼 。馬錢的果實如果不幸掉進水塘，會令食水帶毒性，污染食水，影響香港市民健康。政府的相關部門因此採用生物治理，引進獼猴食馬錢子。獼猴對馬錢的毒性免疫，漸漸成了水塘的主人。遊人厭惡獼猴有時搶食，其實是守護我們食水安全的恩猴 !
Visitors to Shing Mun Reservoir in Tsuen Wan will be greeted by macaques, besides the famous Paperbark trees. Shing Mun Country Park is the main habitat for wild monkeys in Hong Kong, where large groups of macaques can be seen.
From 1913 to 1936, Shing Mun reservoir was built on the hillside, and the poisonous plant strychnine tree, aka poison nut, grew on the surrounding hills. Poison nut is one of the 4 major poisonous weeds in Hong Kong. Its fruit contains alkaloids that are highly toxic and could be fatal. If the fruits fall into the reservoir, it will pollute the water making it toxic, thus affecting the health of Hong Kong people.
The relevant government departments therefore adopted ecological treatment and introduced macaques to eat the plants. Macaques are immune to the toxicity of poison nuts, and gradually become the masters of the reservoir. Some people consider food-grabbing macaques as pesty, when in fact they are safegarding our drinking water.
