A small New World primate (family Cebidae) from tropical forests of the Amazon basin. Adults weigh 0.6–1.1 kg with a slender body (25–35 cm) and a long, non-prehensile tail (35–42 cm) used for balance. Their fur is olive-gray to yellowish on the back with bright yellow-orange arms and a white face with a dark muzzle ("Roman arch" pattern). They have the largest brain-to-body ratio among primates. Squirrel monkeys are exceptionally gregarious, living in multi-male/multi-female groups that can number 20–300+ individuals—among the largest of any primate. They are diurnal and arboreal, foraging actively in the middle canopy for insects and fruits. They frequently associate with capuchins for foraging benefits and predator detection. Gestation is ~150–170 days with single offspring. They have been widely used in biomedical research, including neuroscience, pharmacology, and space biology (sent to space in 1958–59). Lifespan is ~15–20 years in the wild and 25+ in captivity. IUCN: Least Concern.