## 2 Results and Discussion
### Identification of Volatile Organic Compounds in P. tigris altaica Marking Fluid  
Thirty-two compounds were identified by chemical standards (except for 2-AP), peak area, odor detection, retention time, spectral matches with top five ion relative intensities (Table 2). An additional 48 unconfirmed unidentified peaks were determined to be present within _P. tigris altaica_ MF (Table S1). Identification of four of these peaks was attempted because they were characterized as having 'nutty', 'urinous', and/or 'corn-like' aromas by the odor panelists. These compounds (2-acetyl-1-pyrroline, (\(R\))-3-methylbutanamine, 3-hydroxybutanal, propanedionic acid, and (\(R\))-3-methylcyclopentanone) were considered to be 4 of the 7 characteristic compounds tentatively identified through spectral match with top five ion relative intensities, odor panelists' detection, and published odor descriptors. _P. tigris altaica_ MF was comprised of nine chemical groups. These include ketones (9), aldehydes (5), amines (1), amides (1), alcohols (7), acids (2), phenols (1), sulfur-containing compounds (2), and nitrogen-containing compounds (4). All of these compounds were matched with an MS NIST spectral library match of 80% or higher and with olfactory detection by a trained panelist.  
Fourteen of the total compounds had human-detectible aromas that matched their published odor descriptors (Table 2). Those compounds with no detectable odors were identified through retention time, spectral match with top five ion matching, and chemical confirmation (Table 2). An additional set of 21 odor events were detected by panelists, but the identity of the compounds was not confirmed with chemical standards, due to feasibility. Four of the 21 odor events were comprised of odorous compounds with 'characteristic' aroma notes.  
There have been few reports published on chemical constituents of tiger MF. The majority of them focus on the Bengal tiger (_P. tigris tigris_) and the Sumatran tiger (_P. tigris sumatrae_) [5; 11; 14; 20; 65]. Previous studies on tiger MF have identified the constituents based on the analysis of separated MF into two separate fractions, the \"lipid fixative\" and \"urine fraction\" [11; 20]. [PERSON] et al. [11] is the only study published on Bengal tiger MF that analyzes both fractions, but separately. Compared to [PERSON] et al. [11], the present study was able to detect equal number of sulfur-containing compounds in Siberian tiger MF (Figure 2). We also found five nitrogen-containing compounds in Siberian tiger MF, which is identical to the number previously determined in Bengal tigers [11]. Although the number of sulfur-containing compounds and nitrogen-containing compounds is the same, they were different in each subspecies. There were twice as many phenols in Siberian tiger MF than Bengal, but half of them were common to both. Aldehydes and ketones constitute similar numbers of compounds in tiger MF. Also, we determined the presence of 2-AP, previously undetected in Bengal tiger MF. The two groups with the highest number of common compounds were the alcohols and the aldehydes. Both studies identified 2-phenylethylamine as a constituent of tiger MF, albeit the identification in present study is preliminary (i.e., without chemical standard confirmation). 2-Phenylethylamine is found in the urine of carnivores and is one of the amine molecules that activates the trace amine-associated receptor in the epithelial tissue of the nasal cavity in bobcats and several other animals [66; 67]. 2-Phenylethylamine is found in highest concentrations in the urine of tigers and lions [28; 67]. Trimethylamine was identified and is a common compound identified in the MF of Bengal and Sumatran tigers, and African lions [19; 20].