Arnold was six years old when he arrived at Jungle Friends on November 21, 2007 along with four other marmosets named Gizmo, Ernie, Marvin and Tuque. These boys were originally purchased by a research laboratory for iron toxicity studies; however, the lab found that marmosets were not good models for this study. Another investigator then wanted to 'use' them in schizophrenia studies! These boys narrowly escaped that fate. Fortunately for them, they were allowed to leave the lab and begin a new life at Jungle Friends.

Arnold lives with Gizmo and Ernie, and he likes to stick close to his two friends -- wherever they go, Arnold will follow. Although he's not the bold leader, Arnold is very inquisitive; he has inspecting every nook and cranny of his new home with great interest.