Bamboo lemurs (Prolemur simus and Hapalemur spp.) survive eating typically deadly concentrations of cyanide in their natural diet of bamboo. This survival is an evolutionary novelty, likely shared only by giant pandas (Ailuropoda melanoleuca), red pandas (Ailurus fulgens), and some bamboo rats (Dactylomys spp., Rhizomys spp.). To understand the nature of this adaptation we must understand the biochemical and metabolic processes at work. However, the Critically Endangered status of bamboo lemurs and the Vulnerable and Endangered statuses of other bamboo eating mammals limit options for study. We used a novel approach by examining the form of cyanide excreted in urine. The form (cyanide ion or a detoxification product) is informative of the biochemical processes allowing survival. To determine the chemical structure of excreted cyanide compounds, we collected urine samples from wild (N = 46) and captive (N = 5) bamboo lemurs (P. simus, H. aureus, and H. griseus) from Madagascar and comparative samples from non-bamboo eating (non-cyanide consuming) lemurs at the Duke Lemur Center. We adapted gas-chromatography mass-spectrometry analytical methods to quantify cyanide and thiocyanate (the primary mammalian detoxification product) in lemur urine. Concentrations of both cyanide and thiocyanate in bamboo lemur urine, but not that of non-bamboo eating lemurs, were 10-20 times higher than concentrations in other mammals killed by acute cyanide poisoning. This implicates two co-acting mechanisms: 1) increased activity of the enzymatic detoxification pathway and 2) increased tolerance to residual cyanide. This provides the first evidence for the biochemical mechanisms behind this extreme dietary adaptation.