1 23 Animal Cognition ISSN 1435-9448 Volume 17 Number 3 Anim Cogn (2014) 17:755-765 DOI 10.1007/s10071-013-0707-y Quantity estimation and comparison in western lowland gorillas (Gorilla gorilla gorilla) Jennifer Vonk, Lauri Torgerson-White, Molly McGuire, Melissa Thueme, Jennifer Thomas & Michael J. Beran